Asymmetric outrigger hulls or amas

Designs (and general discussion) about catamarans, proas, trimarans, and other craft that do not need ballast to stay upright
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george_dimsdale
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Asymmetric outrigger hulls or amas

Post by george_dimsdale »

I'm planning to build a drop in outrigger assembly to convert a canoe or similar small boat to a trimaran for sailing. I've looked at the designs offered by Chesapeake Light Craft and Storer and want to learn some more before I start cutting wood. I understand, with my limited knowledge, that asymmetric outrigger hulls may control leeway, rather than using a daggerboard. Can anyone direct me to information on asymmetric hull design and construction, especially as applied to plywood construction?

I live in southern Yukon in Canada, where our long lakes have long fetches for the frequent winds, so I will be sailing in chop up to 2 ft most of the time.

Thanks in advance for any help.

AlexQ23
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Re: Asymmetric outrigger hulls or amas

Post by AlexQ23 »

Hi Georges,
There are few things to be aware of: Construction: I thing foldable plywood would be great for an outrigger. The principle is fully describe in the Gougeon brothers book and could be design by folding some hard paper as template.
Profile design: A good start will be to get inspiration by the Proa dagger board profile known as Proa 1 to 3 noted P20110, P20212 or P30212.
Lateral area: Normally 4% of the sail area would be sufficient but in the case of a canoe I will go a little bit further and take 6 or 7%.
This surely will help to go up wind but I'm really not sure that will be enough to make it through! Good luck !

Robert Biegler
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Re: Asymmetric outrigger hulls or amas

Post by Robert Biegler »

george_dimsdale wrote:
Thu Jun 25, 2020 2:24 am
I'm planning to build a drop in outrigger assembly to convert a canoe or similar small boat to a trimaran for sailing.
I have tried drop in assemblies, though not yet a trimaran. My additional constraints are that I have previously used an Ally 17 folding canoe, with a skin over and aluminium frame, and I didn't want to put any additional bending loads onto the frame, and also that I have no car, must transport everything by bike, and want quick assembly. I have failed to achieve the quick assembly, and I never made the boat tack steering with the paravanes only. I tried, because that saved me the structure needed for a rudder.

I uploaded a few pictures of one attempt, using the Paul Ashford anchor dog design of paravane, at https://www.flickr.com/photos/78112151@N06/.

I now have a Klepper folding kayak, which, with some practice, should be faster to assemble than the Ally. For the rig, I still have that same surfboard mast, and a lug sail. I may make a split junk later. For now, I design also for minimal building effort. The plan is to use a ladder for a cross beam, in time-honoured AYRS tradition, and make a pair of low aspect ratio (about 0.5) foils, or asymmetric hulls. Low aspect ratio because I want them clear of the water while paddling. That ladder will also serve as a bike trailer, and if the foils are small enough, I can leave them attached.
george_dimsdale wrote:
Thu Jun 25, 2020 2:24 am
I've looked at the designs offered by Chesapeake Light Craft and Storer and want to learn some more before I start cutting wood. I understand, with my limited knowledge, that asymmetric outrigger hulls may control leeway, rather than using a daggerboard. Can anyone direct me to information on asymmetric hull design and construction, especially as applied to plywood construction?
I think with my planned aspect ratio of 0.5 I am pushing it in terms of still getting a benefit from the asymmetry. One boat that has used asymmetric hulls, apparently successfully, is the Windrider 10: https://www.youtube.com/watch?v=4N1gfQr8PU8. The aspect ratio is about 0.5. I also plan to incline my foils, so they act as Bruce foils. I want to use a trapezoid planform with asymmetric foil profile, which can be built from plywood. Of course, Pacific proas have used asymmetric hulls for centuries, but I don't know whether that really gives a better lift to drag ratio, or whether that is more about counteracting the drag of the ama well to windward of the rig.

For ease of building of the foils, it would be hard to beat the foils of Gerald Holton's Foiler 21: https://foils.files.wordpress.com/2014/ ... oto-dr.jpg. Also look up "Windlass" in AYRS Airs 1, published in 1971.

If you want more conventional hulls, something resembling more a Hobie 14 hull, I have seen that in either stitch and glue or in strip plank construction. You could have a look at Gary Dierking's designs: https://www.duckworks.com/gary-dierking-s/128.htm

I haven't seen asymmetric hulls in tortured ply. I expect getting the symmetry of the panels right could be a bit tricky. If you are content with one side being completely flat, I suppose you could put a symmetric pair of panels into the deck jig, then laminate two flat plywood panels just barely either side of the centreline, build two separate decks, then saw the hull panels apart along the centreline. I don't know of anyone who has done this, though, so I don't know how the flat panel may distort once it takes the stress from the bent panel.

Dick Newick tried to use asymmetry in amas of conventional length and depth, first with the half moon ama. He abandoned that in later designs, but I have not seen any explanation of his reasoning. I had some trouble finding a picture: https://i.ytimg.com/vi/L_UIokPATTc/hqdefault.jpg

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