Press release: AC50 catamaran will do 20mph SOG against a 10mph tide in zero wind?

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dave367
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Press release: AC50 catamaran will do 20mph SOG against a 10mph tide in zero wind?

Post by dave367 » Sat Feb 02, 2019 7:26 pm

Please see: https://goo.gl/zSe52R

Anybody care to refute the statement? Or perhaps discuss starting a wind-powered express delivery service on the Amazon River? :D

John Perry
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Re: Press release: AC50 catamaran will do 20mph SOG against a 10mph tide in zero wind?

Post by John Perry » Wed Feb 06, 2019 9:00 am

I don't think there is enough public domain information about the aero and hydro dynamics of the current F50 catamarans for it to be possible to calculate whether or not this claim by Artemis Technologies is realistic. However, perhaps we can do some guesswork analysis to at least get an idea.

Doing 20mph SOG against a 10mph tide in zero wind is equivalent in sailing performance to doing 30mph with a 10mph tailwind and no tide. To understand that just imagine that you are observing the yacht from a drifting boat that is moving with the tide and ignore any land that may be in sight since that is irrelevant to the performance of the yacht you are observing, only the wind, water and the yacht itself matter from that point of view.

So the question is, could the Artemis F50 catamaran achieve 30mph Velocity Made Good (VMG) directly downwind with 10mph true wind speed? Clearly, if it could it would be tacking downwind - sailing directly down wind faster than the wind is not an option with that type of yacht (I had better not re-start *that* discussion again!!)

Well we could look at the known performance for this general style of yacht. I think the highest speed claimed for an AC50 is 47.2 knots but I don't know what the true wind speed was at the time, perhaps someone here does? The F50 is a souped up AC50, so shall we guess its maximum speed to be about 50 knots?

Another piece of information is that in the 34th Americas Cup an AC72 class yacht was reported to have reached 44.15 knots in 15.8 knots of wind, a boat speed to wind speed ratio of 2.79. I know that's a bigger and probably more expensive yacht, but at least it's the same sort of design concept. Let us assume that the ratio of boat speed to wind speed is the same at 10mph wind speed as at 15.8 knots - for a more conventional yacht with hulls in the water one would expect that ratio to vary quite a lot with wind speed but less so for a hydrofoil yacht. Let us also assume that the guessed 50 knot top speed for an F50 is achievable in the same 15.8 knot true wind speed in which an AC72 achieved 44.15knots. Wild assumptions but we have to start somewhere. This would imply a boat speed to wind speed ratio of 3.16 for an F50. So, if an F50 could really achieve that boat speed to wind speed ratio in a 10mph wind, then, if it could also do that sailing directly down wind (which it can't) it could make 31.6 mph downwind VMG with 10mph true wind and the Artemis claim would then be believable. But for this kind of yacht, high boat speeds are not achieved on courses anywhere close to direct down wind. So what is the course relative to true wind that gives best boat speed for a hypothetical yacht capable of a boat speed to wind speed ratio of 3.16? Drawing a velocity triangle for boat speed and true wind speed indicates that course to be with the true wind only about 18 degrees aft of the beam and a boat sailing at 31.6 mph on that course would only be achieving about 10mph VMG direct down wind, not nearly enough to justify the claim. To try to achieve a higher downwind VMG the boat would have to bear away, but then it would go slower through the water. So, would the benefit of bearing away to sail a more direct downwind course make up for the resulting loss of boat speed? Well, the boat speed only has to drop by 1.6 mph and the required 30 mph is not achieved, never mind what the course to the wind then is. That leaves me feeling skeptical about the Artemis claim although considering the uncertainties in the above 'back of an envelope' analysis I would not like to say for certain that it is all porkies.

Ricsalop
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Re: Press release: AC50 catamaran will do 20mph SOG against a 10mph tide in zero wind?

Post by Ricsalop » Sun Feb 10, 2019 7:48 pm

A thought on the AC50 conundrum viewed from the basic lift equation.
2L= Cl. Ro.V^2.A
If ignor the piranhas and crocodiles and tentatively suggest that for maximum veloicity the foil in the water would need to maximise its lift as will the foil in the air. (Both foils utilizing the same foil profile. The ratio of the densities of water and air (800:1) being balanced by the ratio of the foil areas and pretending that the velocities also are in balance) This will occur when sailing at angle that splits the two streams. The slowest point of sailing will occur when heading directly into either source, the relevant foil then will be acting as a centre board. (whether it sticks up into the air or down into the water) If one of the media has no velocity, as in the AC50 case, heading directly up stream would average the media velocities to 5mph. It drops off is lifting foils at 7mph, drag increases and we are heading for the waterfall, crocodiles and piranhas we that we were trying hard to ignor.

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