# Modern hull shape vs ideal hydrofoil shape

Discussion in 'Multihulls' started by JohnGB, Feb 19, 2021.

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### JohnGBJunior Member

This question has been nagging me for a while, and I can't seem to find an explanation for it. I'm an engineer that has had to deal with fluid dynamics in the past. Theoretically the ideal shape for an object moving through water is more along the lines of a hydrofoil. Nature would seem to confirm this with the shape of dolphin fins, and any number of other examples following the theoretical ideal shape.

I understand why this wouldn't be the case for a monohull, as roll characteristics play a large part, but I don't understand why this isn't the case for catamaran hulls. I could possibly imagine a need for sailing catamarans to have more forward buoyancy to limit forward pitching when there are forward forces on the mast (via the sails), but this would be better with a theoretically ideal hydrofoil shape than what is typically used.

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### TANSLSenior Member

I'm afraid I'm not able to understand what you're saying. This is what a hydrofoil is to me.

Does this look like a dolphin?
The man, who is not too stupid, copies nature and makes a boat similar to a dolphin, when the boat sails underwater. Example, submarines. When the ship moves between two fluids, the man does something similar to, for example, a duck: the lower part is hydrodynamic and the upper part is aerodynamic (man continues to copy nature). And the union between both phases, liquid and gas, the interface, he manages the best that he can (the dolphin also avoids navigating the interface). This is a universal law that applies to both monohulls and multihulls.

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### bajansailorMarine Surveyor

Welcome to the Forum John.

If I understand you correctly, are you thinking in terms of how horizontal sections (ie waterlines) through the vessel should be more 'hydrofoil' shaped?

Or perhaps that the hull should be more torpedo shaped?

SWATH (small waterplane area twin hull) vessels have hulls that are shaped a bit like this, and they are essentially catamarans.
Wiki describes them reasonably well.
Small-waterplane-area twin hull - Wikipedia https://en.wikipedia.org/wiki/Small-waterplane-area_twin_hull

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### JohnGBJunior Member

@bajansailor I'm referring to the cross sectional area. In (probably non nautical) engineering terms an airfoil (e.g. NACA designs) is for a foil that passes through air, and a hydrofoil is for a foil in water. I'm referring to the horizontal cross sectional shape of a hull and comparing it to the horizontal cross sectional shape of a wing where it has a bulbous leading edge and narrow tapering trailing edge. Whether in water or air, the general shape that offers the least resistance. I'm not sure about the transition point between water and air, but given that the airfoil (it seems clearer to call it that) shape is efficient in both, it's not clear

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### JohnGBJunior Member

I'm referring to the 2-dimensional shape of a airfoil / hydrofoil's cross section, which is exactly what is used in aircraft wings. This is both the most efficient shape passing through water as well as air, so I'm not sure that I understand your duck reference. I hope this image clears up the confusion

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You're approaching this from a 2D flow point of view. A hull is 3-dimensional.
Chalk and cheese..

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### JohnGBJunior Member

So is an aircraft wing, or a daggerboard. Your answer doesn't address why most catamaran hull designs are more like a backwards airfoil in design. I'm sure there is a reason, but "it's 3D" isn't it.

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If you're so sure it isn't the fact that it is 3D...then why ask the question?

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### JohnGBJunior Member

If you asked why your leg is hurting, and I said it's because it's a full moon, upon which you replied that that wasn't the reason. Would it be fair for me to say "If you're so sure it isn't the full moon... then why ask the question?"?

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### philSweetSenior Member

Because an airfoil has lift that is a proportional to velocity squared, and a boat has constant lift regardless of speed. There isn't any lift to drag ratio on a boat because the two terms don't compare. There just isn't any efficiency measure that you can apply to a hullform that is comparable to the lift/drag ratio of a foil. You have to accept that and take up a different approach to designing a hull to do a job.

All boats are, effectively, supersonic near the surface. Faster boats have proportionally more "supersonic" surface area. Shock waves cause drag. Aero people call it shock drag. Boat people call it wave drag. It's the same thing though. Airfoils produce a 3D shockwave where the shock energy is concentrated in a spherical shell because that's the solution to the wave equation in odd-numbered dimensions. Boats produce a 2D wave system where the energy gets distributed over the entire surface within the boundary because that's solution to the wave equation in even-numbered dimensions. So at the surface and near the surface, think supersonic shapes, not raindrop shapes.

C1 canoe with diamond waterplane -

supersonic foil section -

Last edited: Feb 19, 2021
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### bajansailorMarine Surveyor

Re John's aerofoils in post #5, I just love that funky dragonfly wing - did somebody squash the poor dragonfly, or did it intentionally photobomb the aerofoils page?

And imagine what it would be like if you had a wonderfully assymmetric hull shaped like the turbine blade or the blackbird - or (heavens forbid) the dragonfly wing - the handling characteristics of these would be 'interesting' to say the least.

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### bajansailorMarine Surveyor

Have a look at the lines plan of the lovely Expannie designed by Chuck Paine.
The waterlines in way of the keel are classic aerofoil shapes - do these help at all?

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### philSweetSenior Member

I wasn't sure if you were asking about streamlining in the plan view to minimize drag, or foiling in the lateral plane to generate lift. I tried to deal with the former in the first post, here is the lateral view of things.

This is mostly a non-problem. Modern boats don't develop a lot of pitching moment. They are so fast they sail with the apparent wind forward of the beam even when making best speed dead downwind, and the foresail's nose-up moment entirely cancels the main's nose-down moment.

And regardless, it still doesn't work. Buoyancy is always upward and we sorta take that for granted. Lifting foils work in the body reference system. If you want to have some of each, you need a map of every operating condition. This really, truly, deeply sucks. The traditional, quasi static, linear methods of stability analysis that work well enough for most purposes with pure buoyancy simply aren't good enough to allow you to evaluate foilers. So the workload goes up by a factor of 100 or so.

The problem in this specific case is that the perturbation velocities due to hull dynamics and wave orbitals mean that you can't get an effective L/D over about 14 in that location. And that is for a perfectly clean foil. You could do better on really big boats, but they don't need them because RM scales as size to the 4th, and sail moment scales like size to the 3rd, and big hulls have lower drag figures anyway. We are learning to use foils on multihulls, but they don't go on the bow, they need to go nearer the CG to minimize perturbations and stay in the water. They also cause dynamic pitch instability at the bow (porpoising). That's why an airplane has the lift aft of the CG and uses the empennage to push down and cancel the main wing's nose down moment. If they don't, they porpoise. At the bleeding edge, some of the fastest boats do have the foils well ahead of the CG and use a lot of aero tricks and automation to control pitch instability. They also break stuff a lot.

Oddly enough, spoon bows show up on the exact opposite types of boats - inland monohulls with short length, wide beam, and usually a crew in trapezes. Those are the boats that can benefit from that idea. Some of the latest Mini's use a spoon bow. They do it for the RM gain, and are about the biggest boats I know of that work with a ski nose. Under some conditions, the benefits of the RM gain can exceed the penalties of the ski nose. But in the mini's case, this is probably a result of artificial rules constraints.

Last edited: Feb 20, 2021
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### DCockeySenior Member

Flow around a transport aircraft wing is very close to 2 dimensional. Flow around boat and ship hulls is 3 dimensional. Water goes under the hull as well as around the hull.

What do you mean by "a boat has constant lift regardless of speed"? Buoyancy?

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### JohnGBJunior Member

@philSweet: Thanks, thinking in terms of a shockwave boundary helps.

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