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Atlantic Proa COE : CLR Balance

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If one of the main challenges of a shunting proa is getting the center of effort of the sail far enough forward to counteract the whether helm caused by the sail and windward ama drag, what happens in an Atlantic proa? I’m envisioning a 20’ light weight beachcruiser with the ama permanently to leeward and a single balanced lug hung smack in the middle of the vaka. I want to know if the whether helm produced by the rig’s COE sitting aft of the hull midpoint would be mitigated by the ama drag to leeward? I’m not interested in ultimate speed. I am interested in great stability and a true minimal design for transport and setup. Please shoot holes in this theory.

Thanks all,
Eric Owsley

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Hi Eric, the biggest difference between the atlantic and pacific proa, rig wise that is.
is that the pacific rig hangs overboard turning the boat up. the atlantic rig hangs between the hulls putting the CE in the middle.
that already is an advantage for you. as the driving vector in the atlantic proa is driving both hulls forward, where the pacific proa the driving vector is turning the vaka round the ama.
  so….. better CE in the middle aft, then CE aft and to leeward.
now, you know that 2 hulls moving in water will turn towards the shorter one….. so your balance comes from the length of your ama. shorter is better for you.
But you dont want that ama sinking I think. then the way I see things, would be a short beamy ama, flat bottom if conditions allow for it.
  You probably gonna still end up with some weather helm, but that’s what your rudder is for…....
that’s my opinion anyway. good luck man.

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ericjayowsley - 23 December 2013 07:56 AM

... what happens in an Atlantic proa?... I want to know if the whether helm produced by the rig’s COE sitting aft of the hull midpoint would be mitigated by the ama drag to leeward?

I think theoretically it should be possible to balance weather helm with the drag force of the leeward ama. As wind strength increases, so does weather helm, but more force on the ama would cause more drag. Figuring out how to keep those forces in balance is the big question.

But on a small proa with a big rudder, maybe it’s not that big of a problem.

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ericjayowsley - 23 December 2013 07:56 AM

If one of the main challenges of a shunting proa is getting the center of effort of the sail far enough forward to counteract the whether helm caused by the sail and windward ama drag, what happens in an Atlantic proa? I’m envisioning a 20’ light weight beachcruiser with the ama permanently to leeward and a single balanced lug hung smack in the middle of the vaka. I want to know if the whether helm produced by the rig’s COE sitting aft of the hull midpoint would be mitigated by the ama drag to leeward? I’m not interested in ultimate speed. I am interested in great stability and a true minimal design for transport and setup. Please shoot holes in this theory.

Thanks all,
Eric Owsley

Eric,

An Atlantic style proa can be made to balance quite well, without the need to slide the rig fore and aft. In this respect an Atlantic proa can be simpler than a Pacific proa, the disadvantage being that the leeward ama of an Atlantic proa must have sufficient volume to counteract the heeling moment and needs to be long to reduce drag when loaded (usually as long or longer than the vaka), so more hull material is required for an Atlantic proa than for a Pacific proa. If you want high performance, then an Atlantic proa is the way to go (higher maximum righting moment), but if you want economy by minimal design, Pacific is the way.

[ Edited: 26 December 2013 07:31 AM by Mal Smith ]

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so more hull material is required for an Atlantic proa than for a Pacific proa. If you want high performance, then an Atlantic proa is the way to go (higher maximum righting moment), but if you want economy by minimal design, Pacific is the way.

Would not a pacific proa made from the same amount of material as the atlantic be built much longer and leaner and with longer akas and thus sail faster then the atlantic proa?

Cheers,
Johannes

 

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Johannes - 26 December 2013 08:38 AM

so more hull material is required for an Atlantic proa than for a Pacific proa. If you want high performance, then an Atlantic proa is the way to go (higher maximum righting moment), but if you want economy by minimal design, Pacific is the way.

Would not a pacific proa made from the same amount of material as the atlantic be built much longer and leaner and with longer akas and thus sail faster then the atlantic proa?

Cheers,
Johannes

 

It depends. For windward performance, generally any point of sail with the wind forward of the beam, there is no substitute for righting moment. Atlantic proas have the best righting moment/total weight ratio (RM/D) of any configuration. On the other hand, the RM/D for a Pacific proa is actually not that good, hence Pacific proas often use movable ballast or water ballast to augment righting moment. The prime advantage of the Pacific proa is, as you say, length/total weight (L/D). The high righting moment of the Atlantic proa allows it to carry a lot more sail area for it’s length than an equivalent Pacific proa (whatever that means), so even downwind the Atlantic proa may have more sail area available to drive it’s relatively shorter hulls, so may still be faster than a pacific proa.

The problem with Atlantic proas is that they are not very forgiving. To take advantage of the available righting moment, the large sailplan makes them prone to diagonal capsize and nosediving and they are also prone to backwards capsize when taken aback.

A Pacific proa’s operating speed/length ratio will be relatively lower that that of other configurations. The relatively low RM/D or more to the point, RM/L, means that Pacific proas have a relatively small sail area for their length. These characteristics make for boat which is seakindly and forgiving, whilst still having reasonably good performance due to its length. The goal of the pacific proa is economy, not outright speed.

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Thanx Mal Smith! That was a great explanation!

Since I have never given the Atlantic Proa much though I must ask some (maybe stupid) questions regarding the larger sailarea and greater righting moment.

Can one reliably fly a hull on the atlantic proa/has it been done in race-conditions? I get the impression that the pacific proa is much easier to control when flying a hull, and with a good race-crew could fly the ama with control for extended periods of time.
Would a longer pacific proa with longer akas (greater width, less stresses and less windage) be faster flying a hull compared to the atlantic proa with more sail-area -  that might have to reef the sail earlier due to risk of capsize?

Cheers,
Johannes

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Johannes,

There is probably too little data on racing proas to to draw any firm conclusions. However, an Atlantic proa is like a trimaran with the windward ama removed and racing trimarans can and do fly the vaka on occasion, so we must assume that it would be possible on an Atlantic proa. Pacific proas with a lee sponson are more forgiving at large angles of heel, but sailing with large angles of heel is not fast. The attention required to fly a hull optimally, just above the water, will be similar regardless of whether it is an Atlantic or Pacific proa. Furthermore, the speed advantage of flying a hull is not that great. It makes a difference for two identical boats if one flies a hull and the other doesn’t, but in the case of Atlantic vs Pacific, it’s probably not going change the performance margin enough for the Pacific proa to have the advantage.

Cheers,
Mal.

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Thanks all for the thoughtful responses.

Malcolm, every time I see your little ArcSail avatar it sets my mind reeling. The curved foils I believe are out of my league construction-wise, but as I understand it, the curved foils were designed to counteract the lifting forces of the rig and keep the proa flat at all times. Thinking about Atlantic proas has got me thinking about this rig again. With the ama to leeward keeping the boat flat, could not a simpler foil plan be used with the ArcSail to achieve a lifting, shunting sail with no change in COE?

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The foils don’t need to be curved. They can just be canted such that the sum lift vector points at the desired COE/CLR intersection point. The same can be said for the sail i.e. rather than a complete arc, you could just have two canted elements, or a series of flat elements at varying cant angles. A smooth arc is in effect just an infinite series of flat elements. The nice thing about using curved foils and/or sails is that the trust vector always points in the right direction regardless of the loading distribution. The loading distribution could vary, for example, due to wind sheer effects for the sail or wave or immersion depth effects for the foils. For ease of construction though, using flat elements is certainly a viable option. There may be some theoretical loss of efficiency but this can be more than made up for by the practical benefits.

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