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Request for collaboration

Posted: Mon Dec 10, 2018 1:00 am
by Robert Biegler
I think I know how to work out the passive stability of the elevation angle of an airship used as a sail. I had the idea oever a year ago, but I have other projects, both professional and boat related, with higher priority, so nothing has happened. Is anyone interested in a collaboration? The project requires some knowledge of trigonometry, and in case there is no closed form solution, some programming, though that could possibly be done with an Excel spreadsheet.

I did consider buying a small aerostat for practical experiments, but abandoned that idea the moment I checked the price of helium. When I searched for hydrogen, I only found sites about hydrogen fuels, but none telling me where I could get bottles, and at what price. Also, I doubt the cooperative that owns the house which contains my flat would be happy with me storing a bottle of hydrogen.

I also have two new designs for a hapa that should go well with a sailing airship. My employment contract with my university contains a standard clause which states that the university gets a share in anything I might patent. The Office for Technology Transfer was a bit surprised when someone working in the department of psychology came to them with an idea in marine engineering. They concluded that only one of the designs was patentable, and that the potential market was too small for the university to take an interest.

Being an academic, it would be useful for me if this could be published in an academic journal as well as in Catalyst. The International Journal of Small Craft Technology (https://www.rina.org.uk/ijsct.html) seems suitable. Referencing work previously published in Catalyst might also generate a bit of publicity for AYRS, and I could state as affiliation both my university and AYRS. The likely problem is that academic journals take over copyright, and take a dim view of essentially the same stuff being republished elsewhere. One possible solution is to submit the airship part alone to IJSCT, and publish information on both airship and hapa in Catalyst.

Is anyone interested in getting involved?


I also had an idea regarding hull slewing, as instantiated in Jon Montgomery's Quatrefoil (Catalyst 15). Quatrefoil's beams rotate around vertical axes. Instead, rake them aft, a little, perhaps 5 to 10 degrees. As wind pressure tries to lift the windward hull, that rake in the rotation axis means the hull will swing aft until constrained. It would not be necessary to actively slew the hulls, only to stop them from going too far. In case of capsize, some buoyancy in the masts (I assume a biplane rig) will also make the hulls slew. Remove the limit to let the boat fold up until the hulls touch, and it should come up at least on its side. If the hulls then have a step above the waterline, possibly only on the outside, it might even be possible for the boat to right itself without doing anything more.

What would worry me is a sailor getting caught between beam and hull. The best cure for that which I could think of is to put the net between the hulls up high, at the level of the hulls' decks. My other worry is torsion. As I understand it, resistance to torsion comes partly from the distribution of material in each indicvidual beam, as represented by the moment of inertia around the beam's long axis, and partly from the moment of inertia that comes from the distance between beams. Letting the boat fold up to right itself after a capsize means reducing the distance not only between hulls, but also between beams, at a time when waves will twist the structure. How much extra material would be needed to prevent the beams from breaking?

I am an extremely slow builder, and have other projects before this one. Does anyone want to build a (model) cat with raked rotation axes, and report back on how it works?

Re: Request for collaboration

Posted: Thu Dec 13, 2018 12:46 pm
by John Perry
Presumably you are aware of the work done by Didier Costes and the airships Zeppy 1 (1985) and Zeppy 2 (1992) and Zeppy 3 (2008). Zeppy 2 was an amazing project to cross the Atlantic ocean using an airship connected to a hapa (paravane) with pedal powered airscrew as a second means of propulsion. They did manage to get part way accross the Atlantic but weather conditions were not ideal and after some days they were forced to ditch in the sea. Amazing story which Didier himself told us during one of the AYRS meetings back around year 2000.

I would have thought that the elevation angle of a tether connecting an airship to a paravane would naturally be stabilised by the downward component of the tension in the tether reducing as the height of the airship reduces. So when the wind blows stronger the airship will tend to descend, which might perhaps be good in the sense that wind strength is usually less at lower altitude, so the effect of gusts might be mitigated.

It is not clear from your message what you have in mind, or what problem you are seeking to resolve, if a bit more information were available perhaps you might at least get helpful sugestions from members of this forum.

Re: Request for collaboration

Posted: Thu Dec 13, 2018 3:05 pm
by John Perry
and partly from the moment of inertia that comes from the distance between beams.
Robert: Quoting part of a sentence from you message, I think the above is a misconception. Considering two hulls joined by two cross beams, it is true that as the craft as a whole is twisted (i.e. one bow and an opposite hull stern rising relative to the other bow and stern) then each individual beam is subject to torsion producing shear stress in the cross beams that can be calculated from the second moment of area of the cross beam section and the angle of twist per unit length of beam. However, no additional torsion results from the fore and aft spacing between the two beams. After all, the angle of twist along the length of each cross beam is the same and does not change if the beam spacing is changed (assuming that any bending of the hull along its length is negligible). What does happen is that each cross beam bends into an 'S' shape with a point of inflexion at mid length and this causes bending stresses (tension and compression) in the beams that are superimposed on the stresses caused by the torsion of the beams. Both the bending stresses and the torsion stresses in the beams serve to resist twist of the whole structure, buy, unlike the torsion stresses, the bending stresses in the beams do change if beam fore and aft spacing is altered. The regions of the hulls between the beam attachment points are also subjected to both twist and bending, but in many cases the hulls are so ridgid by comparison with the cross beams that the deformation of the hulls can be neglected, at least for an approximate calculation.

It seems to be a common misconception that twisting of a multihull structure is resisted primarily by the torsional stiffness and torsional strength of the cross beams. For many practical designs, indeed I would say most practical designs, it is the bending strength of the cross beams that matters more than the torsional strength. Many multihull cross beams are made from aluminium tube which has a cross sectional form that one would normally select to resist torsion rather then bending, so this selection of material is unlikely to be optimum from a structural point of vew. However, aluminium tubes are readily available, require minimum work to make into a multihull cross beam, they dont have corners to bash your knees on and although they are not wonderful aerodynamically, there could be worse cross sections from that point of view. So I am not saying that it is nessasarily wrong to use aluminium tubes as cross beams, just that it may not be structurally optimum and something like an 'I' beam or channel section beam is likely to be better. An 'I' beam (or channel section beam) with strong top and bottom (the cap-spars in structural terminlolgy) is poor in torsion but makes good use of material in bending.

So far, I have considered only stresses due to twisting of a multihull structure but there are also superimposed stresses due to mast compression load, mainsheet load etc. and since these tend to be mainly bending stresses this again favours a cross beam section that is strong in bending rather than torsion.

An 'I' beam or a channel is not good aerodynamically but a non structural fairing can be added to the front of it which will reduce air drag and, perahaps more importantly, will reduce drag caused by wave tops and spray hitting the beam. That gives a sort of 'D' section with the flat side aft, many modern multihulls do indeed have this kind of cross beam section. Until fairly recently no one seemed to bother much about aerodynamic fairing of the aft side of cross beams, but I note that in the last few years fairing in this area is becoming more common. I am rather surprised that it has taken so long for this step in the evolution, I think I first saw canvas fairings on MOD70 trimarans (forward beam only) and of course on America's cup catamarans.

Re: Request for collaboration

Posted: Sun Dec 16, 2018 1:41 am
by Robert Biegler
John Perry wrote:Presumably you are aware of the work done by Didier Costes and the airships Zeppy 1 (1985) and Zeppy 2 (1992) and Zeppy 3 (2008).
I have read what was Costes published in Ultimate Sailing IV (I think), and whatever I could find on the net. I got the impression that Costes now is an advisor, and that the most active person and cotact is Stephane Rousson. I did write to him. The background is that I work at a university, and my contract includes the standard clause that the university shares rights in whatever I may patent while I work there. There is no distinction between stuff that is work-related and stuff that isn't (I am a biologist, I work in psychology, now, and sailing aerostats have nothing to do with either, except as a good platform to study marine life). I don't mind, because the help they can offer is worth it. So when I thought a new hapa design might be patentable, I went to the Office for Technology Transfer. To make their job easier, I contacted Rousson to ask whether the Costes-style hapa he was using both with the latest Zeppy and with kites was based on a more recent patent than the one I already knew. He refused to tell me, on the grounds that I was going to be his commercial competition. I pointed out that, first, a patent granted a temporary monpoly in return for publication, so not even telling me whether there was a patent went rather against the spirit of the thing; and second, that I would happily let him build and market my design if it even turned out to be better than what he had. That didn't change his mind. I found the relevant patent, it just took more work. Costes might think differently about collaboration, but I have no contact details for him, and Rousson seems to think only in terms of competition. I don't think I'll be getting any help there.
John Perry wrote:I would have thought that the elevation angle of a tether connecting an airship to a paravane would naturally be stabilised by the downward component of the tension in the tether reducing as the height of the airship reduces.
That is what I expect for an aerostat straight downwind of the anchor point, whose angle of attack is determined by the attachment point below the envelope, at least once there is enough wind that you can neglect the effect of buoyancy. If the aerostat has a wing attached to the envelope like in Bernard Smith's book, and is at the edge of its flight window, I think it has to be treated more like a kite.

Then there is the question whether the line attaches to a single point, as in an aerostat and in Smith's drawing, or whether you set up a bridle, say to the root and tip of the wing. That changes the caculation of the equilibrium, because now the angle of attack when the aerostat is straight downwind should increase as elevation decreases, like an old-fashioned single line kite. The exact function relating angle of attack to elevation depends on the length of the bridle, and on how lift and drag of the envelope and the elevator surfaces change with angle ot attack. The only way I could do this is by an iterative procedure that calculates these torques for some angle, finds out which way the whole thing would pitch, adjust the angle a bit, and calculates again. Repeat until torque reaches a small enough value. I found a paper on pressure distributions along an airship hull. That would need to be converted first into overall lift and drag, and then into a lookup table for a range of angles of attack. The lookup for elevator surfaces could skip the first step. Still, doing this would take quite a bit of time. Enough that I won't do it this side of retirement, unless my programming and maths skills improve enough that I can speed up the process a lot from the weeks it would take now.

So depending on the configuration, the calculations can be quite tedious. Then I have to describe the case for when the whole thing is at the edge of its flight window. I think I can make the simplifying assumption that the relative contribution to total forces from what happens when the aerostat flies straight downwind versus what happens at the edge of the flight window are proportional to the sine and cosine of the angle between the line to the anchor point and the total horizontal aerodynamic force. So if I can describe each limiting case, I think i can describe what happens anywhere in between. But I am sure the forces at the edge of the flight window are not as simple as you described. Your description applies to one of the possible configurations flying straight downwind.

I don't know how clear that is without drawings, but I won't have time to make any during the next few weeks, which is exam grading time.

Re: Request for collaboration

Posted: Sun Dec 16, 2018 2:03 am
by Robert Biegler
John Perry wrote:
and partly from the moment of inertia that comes from the distance between beams.
Robert: Quoting part of a sentence from you message, I think the above is a misconception. Considering two hulls joined by two cross beams, it is true that as the craft as a whole is twisted (i.e. one bow and an opposite hull stern rising relative to the other bow and stern) then each individual beam is subject to torsion producing shear stress in the cross beams that can be calculated from the second moment of area of the cross beam section and the angle of twist per unit length of beam. However, no additional torsion results from the fore and aft spacing between the two beams.
Not in either individual beam. And my understanding was that the torsional strength of individual beams makes a negligible contribution to the torsional stiffness of the whole structure.

My memory of what I read also is that if the hulls of a catamaran can be treated as rigid, then the two beams can be thought of like the two sides of a single I beam. I don't know whether that works out the same as your account when quantified. The two accounts have in common that the structure is stiffer in twist the more the two beams are separated.

If the my interpretation of what I read is valid, then the two beams getting closer together when hulls slew should make the whole structure less resistant to twisting. In your account, what happens when supporting the whole boat by one bow and one stern when the beams are at 90 degrees to the longitudinal axis versus some other angle? Do the beams or any other part of the structure need to be stronger to deal with that?

Re: Request for collaboration

Posted: Wed Dec 19, 2018 11:13 pm
by John Perry
Robert Biegler wrote: Not in either individual beam. And my understanding was that the torsional strength of individual beams makes a negligible contribution to the torsional stiffness of the whole structure.
Robert, I guess that what you meant to say is the the torsional strength of individual beams makes a negligible contribution to the torsional STRENGTH of the whole structure. Strength and stiffness are different properties! (and both are important in boat designing)

But, leaving that quibbling point aside, I do agree with you - both the strength and stiffness of the whole structure under twisting loads are more dependent on the bending properties of the cross beams than on the torsional properties of the cross beams. However, I am not sure I would go as far as to say that the torsion has negligible effect, I think that I would want to include both both bending and torsional effects in a design calculation, at least until I was sure that I could neglect the torsional effects. Another point is that there have been a few multihulls built with only a single cross beam, for example the Hydropterre trimaran and the trimaran that Peter de Savery had built with the America's cup in mind. For a multihull that has only one cross beam, the torsional strength of the cross beam is obviously critical!

The torsional stiffness of a shaft can be calculated from the second moment of area of the shaft cross section, the torsional modulus of the material and the length of the shaft, the relevant forumlae can be found in any textbook on stress analysis. However, any such text book will also tell you that this method of calculation is based on certain assumptions, one of which is usually quoted as 'plain sections remain plane'. That means that if we consider all the molecules of the shaft that lie on some plane transverse to the axis of the shaft when the shaft is not twisted, those molecules must still lie on a flat plane after the shaft is twisted. It is not correct to treat any normal pair of multihull cross beams as a single member under torsion with a combined second moment of inertia for the two beams together since the bending that occurs along the lengths of each beam when the whole structure is twiste, violates the assumption that plain sections remeain plane.

I have made a quick finite element analysis to illustrate these points. The first picture below shows a simplified catamaran structure - I haven't bothered to give it pointy bows or tapered sterns since the details of hull shape are not relevant for this purpose. If the hull centre-line spacing is taken to be 'L' I have made the overall length of each hull '2L' and the cross beam spacing 'L'. As Robert suggests, I have supported the craft on small supports located right at the bow of one hull and the stern of the opposite hull and I have allowed the weight of the craft to twist the structure - not a purely academic load case, I can somehow imagine that inadequate support for a catamaran during on-shore storage could result in this kind of situation.

As you see from the first picture, each cross beam bends into a slight S shape (the picture exaggerates all distortions to make the distorted shape clearer). The colors indicate the level of the Von Mises stresses, red being high, blue low. Clearly the beams are subject to bending stresses wjth highest stresses being towards the top and bottom of the beams in the regions of the beams where the curvature is greatest. At the midpoints of the beams there is no curvature, these being points of inflexion, so there are no bending stresses at the mid-points of the beams, but note that even at these points the Von Mises streess is a bit higher than it is in the main body of the hulls (where it is very low because I made the hulls solid, so they are very ridgid), this being due to the presense of torsional stress which applies uniformly right along the length of each beam.

The second and third picture shows what would happen with the same support/loading arrangement if this were a 'slewable' cat, as Robert describes, with the hulls totally slewed so that the cross beams lie parallel to the hull centrelines. Two pictures, one for supports at the hull ends that are futhest appart and one for supports at the hull ends that are in proximity. You see how the beams still bend into an 'S' shape, but the points of inflexion no longer lie at the beam midpoints and the bending stresses are higher towards one end of each beam, the highest stresses being greater than for when the hulls are not slewed (as in the first picture) - I hope this answers Robert's question, but if actual stress values are needed a lot more detail of the craft would need to be known.

Note that in the second and third pictures the cross beams are not twisted along their length (since the hulls have not rotated about their long axis) so at this slew angle the cross beams are not subjected to any torsional loading, only bending loading. Note that the stress color at the points of inflexion in the second and third pictures are a darker blue than in the first picture, about the same shade of blue as the hulls, indicating no torsional stress at these points as well as no bending stress.

I suppose I could repeat this analysis for an intermediate slew angle, say 45 degrees - this would show a stress situation intermediate between the the two extreme slew angles, zero degrees (first picture) and 90 degrees (second and third pictures)

BTW, if you are wondering what the red arrow pointing downward in each picture is, that represents the force of gravity directed downwards from the centre of gravity of the whole craft.
Slewcat_01.jpg
Hulls not slewed
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Slewcat_02.jpg
Hulls slewed 90 degrees one way
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Slewcat_03.jpg
Hulls slewed 90 degrees the other way
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Re: Request for collaboration

Posted: Thu Dec 20, 2018 12:53 pm
by John Perry
Rober Biegler wrote:
Is anyone interested in getting involved?
Yes, I am always interested in helping with other members projects and have done on a number of occasions in the past. One that springs to mind is the series of paravanes that I constructed for Slade Penoyre, past Treasurer of AYRS. The limitation on collaborative work is that I do wish to get on with my own AYRS style projects as well, and time seems so short, surprisingly even shorter when retired than when in employment! At the last Devon meeting of the AYRS I presented an idea for applying hydrofoils to a sailing boat in a slightly different way to the ways that others have done it. That presentation was now nearly three years ago and I still have not started any practical work on that potential project. Then, at the beginning of every winter I have been meaning to learn how to use CFD properly, but made no real progress on that. And within the last few days I have been thinking about ways to verify CFD work on sailing boat rigs by measuring the pressure distribution accross a sail using sensors attached to or built into the battens of a fully battened sail. Just the other day I was discussing this with a retired professor of civil engineering who lives locally, between us we came up with a concept for a simple kind of pressure sensor to measure the low pressures involved, say in the range +/- 30Pa gauge. I would really like to make one and see if it might work.

BTW, this discussion seems a bit disjointed, there are bits about a 'slewable' catamaran and bits about an airship/paravane combination, two completely differnent concepts. Perhaps the discussions should be separated.

Re: Request for collaboration

Posted: Fri Dec 21, 2018 1:54 pm
by John Perry
Robert Biegler wrote:
That is what I expect for an aerostat straight downwind of the anchor point, whose angle of attack is determined by the attachment point below the envelope, at least once there is enough wind that you can neglect the effect of buoyancy. If the aerostat has a wing attached to the envelope like in Bernard Smith's book, and is at the edge of its flight window, I think it has to be treated more like a kite.
I had to look up the definition of 'aerostat' - apparently it means any aircraft using lighter than air gas foy bouyancy, e.g. powered airship, balloon, hot air or gas. I had not appreciated that you were thinking of wings attached to the tethered aerostat - depending on the size of the wings, that could certainly make it more like a kite than anything else.
Then there is the question whether the line attaches to a single point, as in an aerostat and in Smith's drawing, or whether you set up a bridle, say to the root and tip of the wing. That changes the caculation of the equilibrium, because now the angle of attack when the aerostat is straight downwind should increase as elevation decreases, like an old-fashioned single line kite. The exact function relating angle of attack to elevation depends on the length of the bridle, and on how lift and drag of the envelope and the elevator surfaces change with angle ot attack. The only way I could do this is by an iterative procedure that calculates these torques for some angle, finds out which way the whole thing would pitch, adjust the angle a bit, and calculates again. Repeat until torque reaches a small enough value.
Re. the attachment of the bridal, do you mean to the leading edge and trailing edge? - wing root and wing tip does not seem right.
If you neglect any curvature of the kite string and a bridle arrangement is used to fix the angle between the kite string and some datum line on the aerostat then surely the function relating angle of attack of the aerostat to elevation of the aerostat is very simple and not dependent on lift and drag. However, I wonder if that is really what you want to know. I am guessing this is to do with a kite and paravane combination as so often discussed within AYRS circles. If so, then you ultimately need to know the overal lift and drag of both the airborne and water borne components, all resolved to a horizontal plane, in order to predict performance. Would also be nice to know whether the kite will be stable in elevation rather than zooming down into the sea/land as kites sometimes do!

You haven't mentioned aerodynamic and gravitational forces on the kite string which would considerably complicate the calculation if you intend to include them. The kite string will be curved in a plane that may not be vertical, due to both gravitational and aerodynamic effects and this curve will change the gradient of the string at the kite end and hence if the kite is connected through a bridal its angle of attack will be directly affected. The aerodynamic lift (downforce actually) and the drag of the kite string will also come into the lift and drag calculation for the airborne system as a whole, and this may be quite significant.
I found a paper on pressure distributions along an airship hull. That would need to be converted first into overall lift and drag, and then into a lookup table for a range of angles of attack. The lookup for elevator surfaces could skip the first step. Still, doing this would take quite a bit of time. Enough that I won't do it this side of retirement, unless my programming and maths skills improve enough that I can speed up the process a lot from the weeks it would take now.
I could take a look at this to at least see what would be involved in those calculations. I take it you have the pressure distribution along the airship hull for a range of angles of attack, not just for zero angle of attack? Of course the pressure distribution on its own does not give you the total drag, there is also skin friction. I dont know about airships, but for seaships, both surface pressure and skin friction are significant in determining drag. Do you also have lift and drag curves for the wings that you are thinking of attaching to the airship?

I do wonder where this kind of investigation is intended to lead. I can see that a kite and paravane combination, or aiship and paravane combination, or glyder and paravane combination, could be a contender for the world sailing speed record. All these combinations actually have some similarity to Paul Larsen's Sailrocket - Sailrocket replaces the kite string with a streamlined carbon fibre pole, but the principle is much the same. So is a kite sting going to be better than a streamlined pole - I think that is quite an interesting question.

A kite string can probably be made a lot longer than it is really practical to make the pole and that does have a big advantage in that it raises the wing (or whatever aerodynamic object is taking energy from the wind) to a greater height where wind velociy is higher and wind turbulence is less. Against that, replacing the pole with a string means that a human located in one of two objects, that are linked only by a string, has to maintain control over both objects, and for it to be a sailing craft I would say this control has to be maintained without stored energy and powered actuators. Surely that is likely to complicate the control problem and it is actually control difficulties that seem to have been the limiting factor for at least some of the record contenders, Sailrocket may be no exception. The aerodynamic properties of a string versus a pole are probably a bit less important but, depending on its length, a string may have less aerodynamic drag than a pole, however, if the pole is well streamlined the string does not need to get all that long before it becomes aerodynamically inferior to a streamlined pole.

Re: Request for collaboration

Posted: Fri Dec 21, 2018 6:47 pm
by John Perry
Just one further point: I recall that when Didier Costes last attended a meeting of the AYRS, which was quite a few years ago now, I clearly remember him saying that the best lift to drag ratio of an airship hull used as an aerofoil is about 4:1. Perhaps this one piece of information might enable you to do some rough calculations to see whether whatever it is that you have in mind is worth persuing in more detail? Does anyone know if Didier is still contactable? My impression was that he was very ready to share information and help others with projects.

Well I had better leave this topic now so that others here can get a word in edgeways!

Re: Request for collaboration

Posted: Sun Dec 23, 2018 7:41 pm
by Robert Biegler
John Perry wrote:Robert, I guess that what you meant to say is the the torsional strength of individual beams makes a negligible contribution to the torsional STRENGTH of the whole structure. Strength and stiffness are different properties! (and both are important in boat designing)
Probably. I had assumed that if the structure is designed to impose greater stress on an element, say by closer spacing of the beams, there would be more strain.
John Perry wrote:The second and third picture shows what would happen with the same support/loading arrangement if this were a 'slewable' cat, as Robert describes, with the hulls totally slewed so that the cross beams lie parallel to the hull centrelines. Two pictures, one for supports at the hull ends that are futhest appart and one for supports at the hull ends that are in proximity. You see how the beams still bend into an 'S' shape, but the points of inflexion no longer lie at the beam midpoints and the bending stresses are higher towards one end of each beam, the highest stresses being greater than for when the hulls are not slewed (as in the first picture) - I hope this answers Robert's question, but if actual stress values are needed a lot more detail of the craft would need to be known.
This is exactly what I had hoped to see. Am I correct in interpreting the larger red areas as representing greater stressed when slewed? Such that a slewing catamaran would need stronger beams?

I don't think we need to separate the thread for his. For the time being, that is all I want. The only innovation in this is to rake the rotation axes back a little, so that sail pressure will slew the hulls. I still don't know whether it is worth trying out. My three worries are crew getting caught between moving parts, that highly loaded moving parts might wear more than non-moving parts, and that the noise when wear makes them sloppy would take all the fun out of sailing. Mostly, having the whole sructure held together by hinges, each of them critical to the integrity of the whole structure, makes me a bit uneasy. But I admit that worry is not based on any actual engineering knowledge that tells me whether moving parts really do wear more, and if yes, whether the amount of extra material needed to compensate is enough to matter.

Re: Request for collaboration

Posted: Sun Dec 23, 2018 10:21 pm
by Robert Biegler
I just lost two hopurs of typing a reply, because the system logged me out before I clicked on "Preview", then didn't take me backl to what I had written after I logged in again. I have to see when I have time to write this again. Probably a week or so.

Re: Request for collaboration

Posted: Wed Jan 16, 2019 10:44 am
by John Perry
Hi Robert, Sorry you lost your typing, this reminds me that when writing more than a few words to an internet forum it is best to first enter the text into a separate app then copy and past it to the forum website! I like to use the free Notepad++ app for that kind of thing, I find it much better than a full word processing app when you just want to put text together and are not worried about the layout of the text on a printed page.

Anyway, you asked:
Am I correct in interpreting the larger red areas as representing greater stressed when slewed? Such that a slewing catamaran would need stronger beams?
Yes, with the loading arrangements we are considering the peak bending momeent in the beams - and hence stress in the beams if we are neglecting torsional effects - is indeed greater when the catamaran is slewed. And this is indeed shown by the larger red areas for the fully slewed diagram compared with the non-slewed diagram.

What is actually happening is that the beams are bending rather differently in those two diagrams. For the non-slewed diagram, if we consider each beam individually, one end of the beam is going up and the other end is going down, relative to the middle of the beam, but the topsides of the hulls remain parallel to each other so there is no rotation of the ends of the beam about a horizontal axis transverse to the beam. This means that each beam bends into a symmetrical 'S' curve with the same curvature and the same bending moment at each end of the each beam. Compare this with the second of the two diagrams. Again, relative to the middle of the beam, one end of each beam is going up and the other end is going down and if it were still the case that there were no relative rotations between the beam ends then the bending moments and bending stresses in the beams would be exactly the same as for the first diagram. However, in the case of the second diagram, the longitudinal tilting of the hulls is causing a relative rotation of the beam ends about horizontal axis transverse to each beam. This modifies the symmetrical 'S' shape bending seen in the first diagram by adding bending moment to one end of each beam and taking it off the other end. The result is still an 'S' shape bending of each beam, but one end of each beam is now carrying greater bending moment than the other end, so the maximum bending stress in the structure is increased.

So would a slewing catamaran need stronger beams? Based on my three diagrams it would seem so, but I think you also need to consider the situation(s) under which the boat would be significantly slewed. For normal sailing you probably dont want to use large slew angles because if you do you will significantly reduce lateral stability and hence sail carrying ability. If you intend to use large slew angles primarily to aid in righting the boat after a capsize this is a special situation which probably has quite different loading on the structure compared with normal sailing, so I dont think you would necessarily need stronger beams for that situation. I think you would need to analyze the sailing situation and the capsize situation separately.

Although this is a digression from the topic of 'slewable' catamarans, I think the loading situation for the non-slewed catamaran shown in the first of my three diagrams may be worth some further consideration. As I said before, this loading situation could result from uneven supports when the boat is laid up on shore. It also has some similarity to the situation when the bow of one hull and the stern of the opposite hull are both on wave crests. It is quite a simple matter to determine the maximum bending moment in the cross beams under this loading condition. If, to begin with at least, we ignore the torsional stiffness of the beams, then each end of each beam transmits only a vertical force to the attached hull. From the symmetry of the situation these four vertical forces are equal in magnitude but one diagonal pair is upwards and the other downwards. So let the magnitude of these four forces each be 'F'. We can then consider momnents about the mid length of one of the hulls. For one hull, half the total weight of the boat is applied upwards at the point of support which we assumed to be right at the end of the hull, so if the length of the hull is 'L' and the boat weight is 'W' that gives a moment about the hull midlength point of L*W/4. For our assumed loading condition this moment is resisted only by a couple generated by the two forces, magnitude 'F' applied to the hull from the two beam ends, so if the fore and aft spacing of the cross beams is 'a' this couple is F*a. Equating F*a to L*W/4 gives F = (L*W)/(4*a). If we are interested in the hull being supported by wave crests rather than by supports at the extremity of bow and stern then we need to make some estimate of the effective length between supports - as an initial rough guess perhaps about two thirds of L would be not too far wrong? Now that we have the magnitude of F we can consider the bending moment in the beams. F is applied vertically upwards to one end of the beam and vertically downwards to the other end, and there is no rotation at the beam ends so, if the length of the beam is 'b', the maximum bending moment is F*b/2, i.e. (L*W*b)/(8*a). From this we can select an appropriate beam cross section to provide adequate strength and stiffness. This is not a complete analysis of the beam loading, but a good start, for a more complete analysis you would also need to add in bending moment resulting from things like the mast foot load and the mainsheet load.

Coming back to slewable catamarans, the 'Dragonfly' range of trimarans has hinges that carry the full beam bending moment so yes, it could be done, but it would certainly add weight and cost to the boat. I can see that it would offer better resistance to a 'leebow capsize' since it would move the bouyancy of the leeward hull forward relative to the centre of gravity of the boat. As for capsize recovery, various schemes have been proposed by AYRS members and others but I know of no cases where such a scheme has been successful with a multihull larger than a beach cat and with an unaided crew in conditions severe enough to cause a capsize in the first place. Maybe this is simply because multihull capsizes are quite rare anyway. (Leaving aside the big racing trimarans which do capsize but are usually righted with the aid of large rescue boats)

Re: Request for collaboration

Posted: Sat Feb 09, 2019 11:33 pm
by Robert Biegler
John Perry wrote:
Fri Dec 21, 2018 1:54 pm
I had to look up the definition of 'aerostat' - apparently it means any aircraft using lighter than air gas foy bouyancy, e.g. powered airship, balloon, hot air or gas.
I came across the term when I looked up what to call something held aloft by gas, streamlined, and tethered. It was the tether that interested me. I was thinking about how the structure of an airship would need to be modified to attach a ventral wing, when it occurred to me that the wing could be suspended underneath what was already designed and engineered to be attached to a tether.
John Perry wrote:
Fri Dec 21, 2018 1:54 pm
Re. the attachment of the bridal, do you mean to the leading edge and trailing edge? - wing root and wing tip does not seem right.
The wing is supposed to be approximately vertical. Attachment has to be at the root and tip to contol angle relative to the vertical plane. Angle relative to the wind would be controlled by a rudder on a tail, like the Walker Wingsail.
John Perry wrote:
Fri Dec 21, 2018 1:54 pm
If you neglect any curvature of the kite string and a bridle arrangement is used to fix the angle between the kite string and some datum line on the aerostat then surely the function relating angle of attack of the aerostat to elevation of the aerostat is very simple and not dependent on lift and drag.
If I assume a wing hanging from the aerostat at one attachment point, I think I can assume that the aerostat has a constant pitch or angle of attack in a vertical plane parallel to apparent wind. In the horizontal plane, its angle of attack would be 0, and it would have drag only.

Looking at the wing, in the vertical plane it would have drag only. In the horizontal plane, its lift to drag ratio depends on angle of attack.

For that configuration, it may be a fair approximation to calculate forces separately for aerostat and wing, and separately in the vertical and horizontal planes.

If I rigidly attach the wing to the aerostat, then things get more complicated. I can calculate angle of attack in the vertical when the aerostat is parked straight downwind from the anchor point, though it will be messy, and I will need both lift and drag coefficients for both aerostat and any non-vertical control surfaces. I think I can assume that the effect of the bridle on pitch will be proportional to the cosine of the angle in the horizontal plane between kite line and the longitudinal axis of the aerostat.

The wings influence on elevation angle should depend on its angle to the vertical, and the sine of the angle in the horizontal plane between kite line and the longitudinal axis of the aerostat.

Calculating tese two compnents separately ignores any interactions between aerostat and wing

Then I have to work out the effect of elevator surfaces on the pitch of the aerostat, and how that changes between straight downwind and the edge of the flight window.
John Perry wrote:
Fri Dec 21, 2018 1:54 pm
I am guessing this is to do with a kite and paravane combination as so often discussed within AYRS circles. If so, then you ultimately need to know the overal lift and drag of both the airborne and water borne components, all resolved to a horizontal plane, in order to predict performance.
Only the airborne, because at this point, I am not trying to predict performance.
John Perry wrote:
Fri Dec 21, 2018 1:54 pm
Would also be nice to know whether the kite will be stable in elevation rather than zooming down into the sea/land as kites sometimes do!
That is the only thing I want to predict.
John Perry wrote:
Fri Dec 21, 2018 1:54 pm
You haven't mentioned aerodynamic and gravitational forces on the kite string which would considerably complicate the calculation if you intend to include them.
That is why I intend to leave them out.
John Perry wrote:
Fri Dec 21, 2018 1:54 pm
I could take a look at this to at least see what would be involved in those calculations. I take it you have the pressure distribution along the airship hull for a range of angles of attack, not just for zero angle of attack?
Figure 6 in Freeman (1932), attached.
John Perry wrote:
Fri Dec 21, 2018 1:54 pm
Do you also have lift and drag curves for the wings that you are thinking of attaching to the airship?
I scanned some figures in Marchaj, but I can't find the files now.
John Perry wrote:
Fri Dec 21, 2018 1:54 pm
I do wonder where this kind of investigation is intended to lead. I can see that a kite and paravane combination, or aiship and paravane combination, or glyder and paravane combination, could be a contender for the world sailing speed record.
I want a drone that has better speed than Saildrone, that can stay at sea for months, and that can loft a camera at 50 meteres into the air. I thought this would be a good idea when someone at an ethology conference said he observed dolphins only near to shore, because following them offshore was too expensive.
John Perry wrote:
Fri Dec 21, 2018 1:54 pm
Just one further point: I recall that when Didier Costes last attended a meeting of the AYRS, which was quite a few years ago now, I clearly remember him saying that the best lift to drag ratio of an airship hull used as an aerofoil is about 4:1. Perhaps this one piece of information might enable you to do some rough calculations to see whether whatever it is that you have in mind is worth persuing in more detail?
It does help. It's probably enough to do all the calculations for the simple case of a thethered aerostat with a wing suspended underneath. Though I don't know when I'll get around to it. I just started a new project at workd that needs a lot of reading and thinking.

Re: Request for collaboration

Posted: Sun Feb 10, 2019 3:16 pm
by AlexQ23
Hi Robert!
Could you thing at a blimp/kite that could be shaped like an thick hollow square box inflated with hydrogen or helium. Horizontal surfaces could be your wings and the vertical surfaces could serve to maintain the direction. You can also curve them (10% of C) to outside at an angle of about 165° to obtain a maximum drag. That could be validated on a CFD program. How are you going to pursuit some dolphins with this contraption, that's another story! Interesting project, indeed. Alex.

Re: Request for collaboration

Posted: Wed Feb 13, 2019 10:45 pm
by Robert Biegler
AlexQ23 wrote:
Sun Feb 10, 2019 3:16 pm
Could you thing at a blimp/kite that could be shaped like an thick hollow square box inflated with hydrogen or helium. Horizontal surfaces could be your wings and the vertical surfaces could serve to maintain the direction.
I must be misinterp0reting something, because my mental image now is of a cube, and I don't see how a cube's horizontal surfaces could be wings, or the vertical surfaces maintain direction.
AlexQ23 wrote:
Sun Feb 10, 2019 3:16 pm
You can also curve them (10% of C) to outside at an angle of about 165° to obtain a maximum drag.
I think drag is undesirable for this application. This is supposed to be a sailing vehicle that can go upwind.
AlexQ23 wrote:
Sun Feb 10, 2019 3:16 pm
How are you going to pursuit some dolphins with this contraption, that's another story!
If you can fix the azimuth angle of the kite line relative to the boat that is being pulled, the whole thing becomes inherently self steering relative to the wind direction. So I thought communicate by sattelite phone, set a course relative to the wind that is approximately in the direction you want to go, and include hydrophones and an analysis package that alerts you when any species of interest calls. The watch stander then asks for video, and takes control. If you have a collaboration with labs in different enough time zones, nobody has to stand watch during their night.

I originally thoght most of the payload would be carried in the air, but I think that would be more expensive than carrying it in a boat. Then the aerial part could be just large enough to carry camera, perhaps radar (for example if protecting a fishery), a radar reflector, and perhaps AIS. All of this would be well clear of the water and so not be obscured by waves.

Re: Request for collaboration

Posted: Wed Feb 20, 2019 4:37 pm
by John Perry
The picture below is an interpretation of the aerostat with (approximately) vertical wing. I say wing, although I would really prefer call it a sail since it performs the duty of a sail on a yacht rather than the wing on an aircraft.
Aerostat-01.jpg
(24.38 KiB) Not downloaded yet


I can see that you could use a bridle attached at wing tip and wing root but I think the attachments would need to be on the leading edge of the wing, otherwise tacking is not possible, at least not without two bridals. Instead of a bridal I have sketched a semicircular track with a free running traveler to which a single line is attached, the traveller taking up a position such that the tension in the line is directed to a point just forward of the center of lift (lift being a horizontal force in this case) of the whole aerostat and wing combination, thus reducing the torque needed to control the wing angle of attack. The track and any supporting structure could be streamlined so that air drag would not be much different to having a bridal. Gybing is not possible with this arrangement, nor with a bridle, but is also not necessary if tacking is possible, provided that plenty of sea room is available.

I have drawn the wing as a two element aerofoil with the front element attached to the main body of the aerostat. This wing could perhaps be quite closely based on the wing sail design used for the AC50 catamaran and the subsequent F50 catamaran since that is now proven technology and it gives a significant advantage in lift coefficient compared with a single element symmetrical section aerofoil. By having the rear element (the flap) of the wing in two sections that are individually controlled it is possible to vary the position of the centre of lift along the span of the wing (i.e. vertically) and so make small adjustments to the roll angle of the aerostat. Adjusting the roll angle will in turn give some control over altitude above the sea.

The wing would almost certainly be heavier than air, even if filled with hydrogen, whereas the body of the aerostat is lighter than air, hence the whole thing will tend to float with the wing vertical - i.e. the wing will act like a ballast keel on a yacht. Possibly that alone would be sufficient to maintain pitch stability but I have sketched a nominally horizontal tail-plane with adjustable flap(s) to help with pitch control. I have shown a somewhat larger nominally vertical tail fin with adjustable flap(s), this would control the angle of attack of the wing and hence the lift (in a horizontal direction) developed by the airborne system. I have shown the control surfaces mounted from a pole extending behind the aerostat so as to give a greater lever arm but alternatively the main body of the aerostat could be extended and the control surfaces mounted directly on it, as is done on most airships.

If the wing is to be nominally vertical then it is necessary that the airborne system has significant positive buoyancy, it cannot be neutrally buoyant since that would result in it descending to sea level as soon as the wind blows. If the design intent is that the wing is always nominally vertical then the required positive buoyancy for the airborne system can be determined by deciding a minimum angle of inclination for the tether line, say 10 degrees, and also a design maximum horizontal aerodynamic force to be developed by the airborne system and opposed by the waterborne system. The minimum net buoyancy is then that maximum horizontal force multiplied by the tangent of the inclination of the tether line. The tangent of 10 degrees is 0.18, suggesting that with a vertical wing the aerostat may need quite a lot of positive buoyancy in order to cope with strong wind conditions. Having the airborne system positively buoyant has implications for the waterborne system since the waterborne system must at all times provide a net downforce to counteract the buoyancy of the airborne system, otherwise the whole craft could become airborne and would then rise uncontrollably towards the upper atmosphere and that would be the end of it.

It is undesirable to have to reduce the buoyancy of the aerostat by releasing gas, so the aerostat needs enough buoyancy to maintain altitude in strong winds and the waterborne system then needs to be heavy enough to stay on the sea surface in a flat calm, hydrodynamic downforce from a paravane not being available in a calm conditions. However, making the waterborne system unduly heavy would incur a lot of hydrodynamic drag. One possibility might be for the waterborne system to take up sea water ballast as necessary to balance the upward force component from the tether line but that is perhaps risky since maximum ballast would be needed in a flat calm so would it be feasible to take ballast on board quickly enough in the event of a sudden lull in wind strength?

Taking the above paragraph into account, rather than having a nominally vertical wing, it may be worth considering an alternative design having wings above and below the aerostat, these wings being inclined to the horizontal plane so that the aerodynamic force developed by the airborne system is at least approximately in line with the tether line. The aerostat could then have closer to neutral buoyancy, reducing the risk that in calm conditions it might lift the waterborne system off the sea surface.

Given that with a vertical wing on the aerostat, the waterborne system does need to have a certain amount of weight, it does make sense to place as much as possible of the electrical and electronic system, batteries etc., in the waterborne system rather than the airborne system. And, as you say, a boat is probably a more economical way to carry weight than is an airship.

I haven't sketched the waterborne system but I can can envisage various options. The waterborne system could reverse direction on tacking (i.e. it could be a proa) or it could tack in a conventional manner. There could be a curved paravane, like a hook in the water, as a number of AYRS members have experimented with in the past. Hook shaped paravanes can be made inherently stable in immersion depth, at least in moderate sea conditions. Alternatively there could be an inverted Tee shaped paravane pulling downwards as well as to windward. Robert, do I recall that at one time you made one of these with a planing surface mounted ahead of it to follow wave profiles and maintain immersion depth? As a more 'high tech' alternative, I am also envisaging stereoscopic cameras, perhaps located part way up the tether line, which focus on the sea surface and clever image processing software that determines the profile of the sea surface ahead of the craft so as to make appropriate rapid adjustments to underwater control surfaces. I am assuming that we are not trying to make a pure sailing craft, so we can have control surfaces actuated by electrical/hydraulic power, probably generated by solar cells, these possibly being located on the airborne system although that does mean more weight and air drag for the tether line.

A big question is how large (and hence expensive) does this whole thing need to be? Experience has shown that airships generally need to be big to have a useful payload. In this case we are not looking to carry cargo, crew, passengers or bombs and we don't need to carry engine(s) and fuel so we do not have the problem of maintaining constant net buoyancy as fuel is depleted. On the other hand the aerostat needs to support the weight of the sail and the tether line, this is effectively the engine. So how does the weight of an F50 wing sail compare with the weight of an equivalent engine, airscrew and fuel? I did try a quick internet search but could not find the weight of an F50 wing sail, perhaps someone here knows that?

Everything seems to point to this being quite a sizeable contraption - airships back in the 1930's were huge and that made them unwieldy and easily damaged (just like wingsails on AC50s and F50s, but probably more so). More recent airships have been less ambitious in terms of range and payload and have benefited from modern materials but even so they are not small. This is a sailing airship and needs to minimise drag. The buoyancy of the gas filled component(s) will tend to increase with the cube of linear dimensions whereas the aerodynamic drag will tend to increase with only the square of linear dimensions.

To take this further perhaps you could do with some estimate of likely dimensions. Perhaps start with an F50 wingsail - determine how much hydrogen is needed to support the weight of that together with tether-line, control surfaces, solar panels etc. and allow for the need to have positive rather than neutral net buoyancy for the airborne system. See whether that is viable and if not scale up or down as necessary, making some assumption for how the weight of structures varies with size - maybe just a cube law to begin with.

On the other hand, if you want an autonomous sailing vessel with good performance perhaps you should be talking to the Artemis Technologies people: http://www.artemisracing.com/en/technologies/home.html

LOOK - they have just what you want and, from what they say, it is available pretty well 'off the shelf' !

OK, you asked for a camera 50m up, the masthead on the Artemis remote controlled sailing boat is only 25m above sea level (which would still give quite a nice view I think), so you might need a small kite flying from the top of the mast. Alternatively you could scan the ocean with a whole fleet of electric remote controlled drones deployed from the Artemis remote controlled sailing boat.

Re: Request for collaboration

Posted: Mon Mar 04, 2019 2:28 am
by Robert Biegler
John Perry wrote:
Wed Feb 20, 2019 4:37 pm
The picture below is an interpretation of the aerostat with (approximately) vertical wing.
That was pretty much my starting point.
John Perry wrote:
Wed Feb 20, 2019 4:37 pm
I can see that you could use a bridle attached at wing tip and wing root but I think the attachments would need to be on the leading edge of the wing
That is the intention.
John Perry wrote:
Wed Feb 20, 2019 4:37 pm
Gybing is not possible with this arrangement, nor with a bridle, but is also not necessary if tacking is possible, provided that plenty of sea room is available.
The airborne part always tacks. Whether the waterborne part shunts like a proa or tacks and gybes is a matter of choice. Except that the waterbourne part may not have enough momentum to tack, and may have to wear around, like a square rigger.
John Perry wrote:
Wed Feb 20, 2019 4:37 pm
By having the rear element (the flap) of the wing in two sections that are individually controlled it is possible to vary the position of the centre of lift along the span of the wing (i.e. vertically) and so make small adjustments to the roll angle of the aerostat. Adjusting the roll angle will in turn give some control over altitude above the sea.
Research budgets being what they are, this thing has to be as simple as possible for the application I have in mind. The bridle is simpler. It is a passive system. Every actuator needs power to move, and more power still for the sensors and the processing that tell it how to move. An actuator is also another moving part that can fail. For other applications your solution may be preferable. For this one, simplicity is too important.
John Perry wrote:
Wed Feb 20, 2019 4:37 pm
The wing would almost certainly be heavier than air, even if filled with hydrogen, whereas the body of the aerostat is lighter than air, hence the whole thing will tend to float with the wing vertical - i.e. the wing will act like a ballast keel on a yacht. Possibly that alone would be sufficient to maintain pitch stability but I have sketched a nominally horizontal tail-plane with adjustable flap(s) to help with pitch control.
Where possible, I like to control attitude to a flowing medium by a surface in that medium. That way the forces to be controlled and the controlling forces stay in about the same ratio. So I think a horizontal tail-plane is advisable.
John Perry wrote:
Wed Feb 20, 2019 4:37 pm
If the wing is to be nominally vertical
I was thinking that if the roll angle relative to the kite line is roughly constant, then the wing's contribution to altitude is neutral. If the wing had a constant angle relative to the vertical, then it would push down when the whole kaboodle rises above the designed altitude, and push up when the aerostat falls below.
John Perry wrote:
Wed Feb 20, 2019 4:37 pm
Having the airborne system positively buoyant has implications for the waterborne system since the waterborne system must at all times provide a net downforce to counteract the buoyancy of the airborne system
Given that research budgets are modest, and so most weight needs to be carried by water for economic reasons, and with the wing being vertical only when the aerostat is straight downwind, I don't think too much buoyancy will be a problem. Let's say the hull carries 100kg payload in the water, and weighs another 50kg. If we want up to 10kg payload in the air, and assuming, to be on the safe side, that the aerostat and wing combined weigh four times the payload, it would need buoyancy to lift at least 50kg. That leaves a lot to be added as safety margin without risking that everything goes flying.
John Perry wrote:
Wed Feb 20, 2019 4:37 pm
Taking the above paragraph into account, rather than having a nominally vertical wing, it may be worth considering an alternative design having wings above and below the aerostat, these wings being inclined to the horizontal plane so that the aerodynamic force developed by the airborne system is at least approximately in line with the tether line. The aerostat could then have closer to neutral buoyancy, reducing the risk that in calm conditions it might lift the waterborne system off the sea surface.
Just what I was thinking, except for having a wing above the aerostat. It is a complication that is not necessary if using a bridle. That is for a wing that will be vertical when directly downwind. If you were thinking of treating the aerostat like a kite, then it will pull up a lot more when parked directly downwind, and that could pull everything out of the water. The whole reason why I am not proposing a helium-filled kite like those Dave Culp experimented with is that I want the airborne part to stay at a constant altitude as it goes from one side of the flight window to the other. A kite doesn't do that, unless it is driven through the zone far from the edge of the window to develop extra power. I think Dave Culp called that dynamic sheeting. The aerostat is not intended to pull large loads, so dynamic sheeting is another complication that is to be avoided.
KiteROV Aerostat fixed wing 1.jpg
(63.7 KiB) Not downloaded yet
John Perry wrote:
Wed Feb 20, 2019 4:37 pm
Robert, do I recall that at one time you made one of these with a planing surface mounted ahead of it to follow wave profiles and maintain immersion depth?
Yes, a proafied version of Paul Ashford's anchor dog. I attach a picture of variants of what I expect to be an improved design. I built one, but haven't tested it yet. I have to build a better connection to the canoe.
John Perry wrote:
Wed Feb 20, 2019 4:37 pm
To take this further perhaps you could do with some estimate of likely dimensions. Perhaps start with an F50 wingsail
Far smaller than that. Anything that can carry an F50 wingsail is far beyond the budget for the use case I am thinking of. It would be appropriate for aerostats that carry radar system for the military and for border control. Those things currently seem to need a support ship of a few thousand tons.
John Perry wrote:
Wed Feb 20, 2019 4:37 pm
LOOK - they have just what you want and, from what they say, it is available pretty well 'off the shelf' !
It would need to keep moving to stay upright, and I am not sure that even the best software could do that in rough seas. If it did stay upright, sending this thing through a pod of whales would rather change their normal behaviour, and then what's the point? Not suitable for the job I have in mind.

Re: Request for collaboration

Posted: Fri Mar 08, 2019 1:13 am
by Robert Biegler
John Perry wrote:
Wed Feb 20, 2019 4:37 pm
On the other hand, if you want an autonomous sailing vessel with good performance perhaps you should be talking to the Artemis Technologies people: http://www.artemisracing.com/en/technologies/home.html
The use case I was thinking of would include this: https://www.nationalgeographic.com/anim ... iscovered/

Re: Request for collaboration

Posted: Thu Aug 08, 2019 7:13 am
by Dikshavarma
It is by all accounts a typical misinterpretation that winding of a multihull structure is opposed basically by the torsional solidness and torsional quality of the cross pillars. For some viable plans, without a doubt I would state most functional structures, it is the twisting quality of the crossbars that issues more than the torsional quality. Numerous multihull cross shafts are produced using an aluminum tube which has a cross-sectional structure that one would typically choose to oppose torsion rather than twisting, so this choice of material is probably not going to be ideal from an auxiliary purpose of vew. In any case, aluminum cylinders are promptly accessible, require the least work to make into a multihull cross pillar, they don't have corners to slam your knees on and in spite of the fact that they are not superb efficiently, there could be more terrible cross segments starting there of view. So I am not saying that it is necessarily off-base to utilize aluminum tubes as cross shafts, only that it may not be basically ideal and something like an 'I' pillar or channel segment bar is probably going to be better. An 'I' shaft (or channel area bar) with solid top and base (the top fights in basic terminology) is poor in torsion however utilizes the material in bowing.

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