Proa Questions: HarryProa vs Atlantic

[Inquisitor]

Ok... I'm exploring... not judging!

Setting my sights on a heavy, passenger, windward hull AND a light, longer leeward hull. AKA... HarryProa or Atlantic. I am also only considering free-standing masts. So... this question is merely about sail placement... on the windward or leeward hull?

Here is what I think I know... please correct or add other issues.

(1) If the base of the mast is at the same height, the tipping moment is the same... (ref: terhohalme's excellent paper)

(2) However, on a HarryProa, the sail can be lower (and have lower COE) so it should be able to carry more sail for the same tipping moment.

(3) On a HarryProa, the sails are closer to the water and on the "spray" side of the boat. If one is considering a schooner rig, they will be getting even more spray. Extra drag??

(4) On an Atlantic, the air will be cleaner coming into the sails as it has not crossed the windward hull. This is probably not an issue going up wind, but on a beam reach, it might be significant.

(5) On a HarryProa, getting it out of the passenger compartment makes it easier to place accommodations.

(6) Structurally, I'm not sure I see an advantage one way or another. A prudent engineer is still going to make the passenger hull strong enough to be lifted clear of the water by the cross beams. If anything, the greater height in the passenger hull will make it easier to support a free standing mast.

(7) Also, note the bending moment diagram of the two designs. When the passenger hull starts flying, the beams act purely in cantilever. In the Atlantic the highest bending moment (and resulting largest cross section) will be at the passenger hull. In the HarryProa, the highest bending moment (and resulting largest cross section) will be at the leeward hull where it is harder to conceal cosmetically and probably has somewhat more drag from spray and diving (if its low).

Are there other issues to consider? I appreciate any help you can shed on this.

 

[tspeer]

Think of the directional balance of the boat. I assume you will have the boards/rudders in the lee hull of either configuration. With the Atlantic, there is a large lateral separation between the rig and the foils, but with the Harry, the rig and foils are in a line.

Getting the right balance is one reason for using a schooner rig. Not only doe it give you the ability to adjust the balance by differentially trimming the sails, the shift in the combined center of effort from one shunt to the other is less.

You can use a footprint plot like this one to look at this issue.

The pitching and heeling moments from the rig are represented as an equivalent shift in the center of gravity (black lines). The blue lines are the combined center of buoyancy from the two hulls, as a function of heel and pitch trim. Where the two sets of lines cross gives you the boat trim for that condition.

You can see that the peak force from the rig points forward as well as outboard. So the effective lead is going to be different for the Atlantic vs Harry proas, even if the foils and rig have the same relationship in the sailplan drawing.

 

[Inquisitor]

Tom,

I'm trying to understand you're point. I'm not sure that I'm quite there yet. I guess, I'm trying to gauge is this a first order effect or far down the list. Basically, the BMW/Oracle boat is an Atlantic (with a spare appendage sticking into the wind )... the sail's on the large windward hull and the rudder/dagger board is on the lee hull. It seemed to do pretty good.

P.S. Although, I'm leaning toward a HarryProa placement, its because of the COE rise required. It alone drops the driving force by 20% to keep under the same heeling force.

BTW - I finished the buoyancy program... Flotsam ... Maybe it'll be useful for you although, I'm sure you have Professional grade programs that'll do the same thing.

 

[tspeer]

As I see it, the heeling moment from the rig will be the same for the Harry and Atlantic proas. However, the fact that the Atlantic rig is offset to windward, means the variation in yawing moment will be greater, making it more difficult to balance the boat directionally. Or at least that the different balance has to be taken into account.

With the Harry proa, the foils and rig are lined up, so the balance is not as affected by the change in apparent wind angle or wind speed. Its balance will be similar to a monohull, except for the drag of the windward hull.