Asymmetrical hulls - basic questions
The idea of an asymmetric hull that will crab to windward is for me of the key attractions of the Proa. What is the current wisdom on this? Is there a downside? What does one lose in putting energy into moving the boat sideways to windward? Is it a zero sum game? What is the hydrodynamic theory behind it? If good, what are the guidelines for hull shape? Is it completely nuts to imagine a dead flat, maybe even vertical, side to the hull? (It would certainly make for an easy build.) For an Atlantic design, should one go for asymmetric main hull, ama, or both?
thanks!
Simon (this is my first real post here 😊
Hi Simon the answer is of course a very certain “it depends”. 😊
It is not strictly true that an asymmetric hull shape will make the boat “crab to windward”. Any sailing boat going upwind has a large sideways component of sail force and a smaller forward component. The sideways component is balanced by the boats “lift” and the forward component by the boats “drag”.
For a symmetric hull with symmetrical foils, when the boat is going straight there can be no lift. Therefore the boat starts to crab sideways, creating the leeway angle. Now the foils start generating lift.
An asymmetric hull will generate lift even without any leeway angle. However, in practice going upwind this lift will not be enough to balance the sail sideforce. So once again the boat starts to crab sideways, increasing the leeway angle or “angle of attack”, so increasing lift until it matches the sail sideforce.
The issue is that the hull is not a very efficient foil, which means that generally a greater leeway angle is required with the asymmetric hull than with a symmetric hull with good foils.
What about comparing an equivalent symmetric and asymmetric hull without additional foils? The difference is probably not huge. And the “it depends” becomes very big. This is the kind of stuff that is impossible to calculate without a supercomputer. And experimental data is always with huge margins of error. And then there is the point that when you’re not going upwind, the shape may be a hindrance rather than an asset.
Where I see the big advantage is that having an asymmetric hull can help balance the ama drag, and when coupled with an oceanic lateen can help with steering the boat.
That’s my view (and some science) on the subject.
Thomas
Original Hobbie cats used asymmetry, it gave a very (overly?) slim hull
It has not been seen for years, so the assumption must be that it is not worth the bother.
Mark
Hi Simon, welcome to the forums.
The idea of an asymmetric hull that will crab to windward is for me of the key attractions of the Proa. What is the current wisdom on this?
Asymmetrical hulls were popular on the Hawaiian and California cats in the ‘60’s - designs by Woody Brown, Choy, and Alter. The main benefit was no foils to worry about when sailing off the beach. Legend has it that Woody Brown saw the asymmetrical proa hulls while in the Pacific during WWII, and being a glider pilot, saw the asymmetry as a form of underwater wing, which he incorporated into Manu Kai, his brilliant Hawaiian beach cat. The hulls were very long and slender, gave a great ride and were fast for their day, however over time, they fell out of favor because round bilge hulls could be better load carriers for a given LOA, and in racing, the daggerboard cats were faster both upwind and down.
This does not mean that they are never a good idea - it is still safer, simpler and cheaper to gain lateral resistance from hull shape alone, and do without troublesome boards, Wharram still uses deep V hulls for his boats - just don’t expect to be first to the windward mark.
Where I see the big advantage is that having an asymmetric hull can help balance the ama drag, and when coupled with an oceanic lateen can help with steering the boat.
With asymmetrical proa hulls, the center of lateral resistance (CLR) is not actually centered, but will shift far forward, as much as a third or quarter boat length. This can raise hell with sail balance if not accounted for. Traditional rigs do shift forward, so they balance well.
Thankyou all for your thoughtful replies. First I want to clarify something - Thomas - when you say foils, do you mean dagger and rudder, or lifting foils? I’m assuming the former.
I’m not a racer, more of a builder/cruiser type, and my watchword is parsimony - reliability, simplicity in build and functioning, and low(er) cost are positive values for me. I’m liking Claudio’s Sweet Aloha approach. So the deep V without foils makes sense.
My question then, slightly refined, is - symmetrical deep V or asymmetrical, flat one side, one chine the other. This is for a hypothetical ~34ft Atlantic with a 20’ ama. (many more questions on aspects of this project coming 😊
Simon
Yes with foils I meant rudders/daggerboards etc.
With asymmetrical proa hulls, the center of lateral resistance (CLR) is not actually centered, but will shift far forward, as much as a third or quarter boat length. This can raise hell with sail balance if not accounted for. Traditional rigs do shift forward, so they balance well.
In response to the above quote, my own boat (small symmetrical wa’apa type thing) has the CLR way forward. In my experience it’s at about 1/3 or 1/4 LWL as you say, and others seem to have the same experience. I’m not sure there is much difference between an asymmetric or symmetric hull here. (But I’d like to hear more about this!)
The way I look at it (going on a broad reach now), the massive forward shift of the CLR causes weather helm. Sheeting out increases weather helm. Ama drag increases weather helm. So how to prevent the boat rounding up? Paddle / rudder / oar, works. But asymmetry can actually make the boat go straight without these.
Now combine asymmetry with a sail that works well at a wide range of sheeting angles. You can sheet in or out to alter the weather helm just enough to balance the lee helm from the asymmetry (without affecting drive much). Making the boat go straight on a range of courses. As I understand it this is how the traditional proas balance. And the more modern proa Te Wheke definitely does.
Depending on the amount of asymmetry, it may not work on all courses of course. The other concern is how the relative effects of the hull shape and sail position change with wind and boat speed. For example I think I remember that Te Wheke used to actually get lee helm from asymmetry to the point where the sail could no longer provide the counter moment and it would gybe.
Disclaimer: The above is the Truth as according to myself. It may be complete BS. Please let me know.
This topic has been relocated to General, because it is not presenting an original design or concept.
” Ama drag increases weather helm.”
this is for a pacific type I assume. How about for an atlantic?
Now for the really basic question - how does asymmetry work? I assume it has to do with the relative water line lengths ? How would you calculate the right amount of asymmetry? Is a continuous curve desirable, ie maximum beam amidships, or curvature at the bows then parallel sides?
thanks again
” Ama drag increases weather helm.”
this is for a pacific type I assume. How about for an atlantic?
Yes you assume correctly. For an Atlantic it would be the opposite. My canoe balances better at the moment with the ama to leeward for this reason.
Now for the really basic question - how does asymmetry work? I assume it has to do with the relative water line lengths ? How would you calculate the right amount of asymmetry? Is a continuous curve desirable, ie maximum beam amidships, or curvature at the bows then parallel sides?
thanks again
I don’t know and I’m pretty sure you can’t calculate it. So it’s not a very basic question 😊 There is a discussion on the forum somewhere saying that actually the details of the hull shape don’t have that much effect on resistance. There are many ways that can work, depending on what your constraints are.
You could consider the hull like a really low aspect ratio wing. The pressure distribution around the wing causes a lot of the forces acting on the wing.
http://avstop.com/ac/flighttrainghandbook/pressuredistribution.html
The lift of the hull acts through the centre of pressure. As the angle of attack increases (leeway) the centre of pressure moves forward. As a deep V type hull will have quite a bit of leeway the centre will be quite far forward. But this basic wing theory stuff doesn’t fully apply to a hull because the aspect ratio is so low.
Conclusion: models are probably the easiest way to test these things!
SimonP - 19 July 2014 11:25 PM
” Ama drag increases weather helm.”
this is for a pacific type I assume. How about for an atlantic?
Yes you assume correctly. For an Atlantic it would be the opposite. My canoe balances better at the moment with the ama to leeward for this reason.
ok, this is very cool for my needs, since I’m thinking of a 2 or 3 mast cat schooner. Such rigs will move the CE back, but the leeside ama might compensate. How does that work hydrodynamically? What are the factors involved? ie, what effect lengthening the ama or deepening it?
Probably mainly drag. Best way to get drag increase is probably to add weight (and more sail area). But do you really want to add drag? For an Atlantic proa I would make the ama the same length as the main hull, to minimise drag at high speeds. It’s probably more important to design it properly to prevent nosediving, rather than worry about sail balance.
With big enough rudders there is no need to worry about balance.
tdem - thanks
so a long ama is desirable on an atlantic - presumably with fine tapers and maximum buoyancy for surface. That would suggest a cylinder with pointy ends. But I guess since you want more buoyancy as the ama is pressed into the water, a wide V is better. However, for a foil-less design, deep is good. Where is the sweet spot between draw and beam? What about bows - contemporary wisdom seems to be vertical to below waterline, putting bouyancy fwd. Is there any drawback to ‘square’, i.e. keel and deck parallel? What about collision, isn’t overhang wise in that regard?
thx