Why aren't sailboat hulls designed as airfoils?

[Charles Burgess]

In answer to your original question...

First of all my answer are based upon techniques used in wind tunnels and tow tanks.

Second, given your aeronautical background, the batwing shape of the B-1 bomber can illustrate how a sailboat sails upon the density interface where water and air join.

Basically, a sailboat flies a lot like an airplane: take a scale plastic model of a B-1 bomber you can get at any hobby store, and dip one wing in water up to the fuselage center-line. The fundamental difference between the wing in the water and that in the air is density of the fluids - otherwise all the factors are nearly the same. It is the interface of the density change that is the most interesting.

If you had made the model B-1 with some clay in one wing for ballast and glued it so that no water can enter, you could put the model in a pool (or your bathtub) and blow on the wing that is in the air, and the B-1 would sail. If you compare the whole assembly as an airfoil (which the wing and body of the B-1 is), and compare it with the sailboats of the first half of the 20th century you'd find a lot of similarities

Because the wing of a sailboat that is in the water is in a fluid of greater density than the wing that is in the air, thus the keel needs less area than the sail to equalize the pressures. Thus, the sailboat hull is an airfoil.

[edited to add: also do the same with a model of the X-15 airplane, and you'll gain some insight into the latest performance sail hull designs, the good and the bad.]

[markdrela]

Originally Posted by Ad Hoc

kerinin
Ive never heard of Xfoil before, i don't use CFD, so had a quick look. Confirms my thoughts above.
"..In the low Reynolds number regime the results are usually not very accurate if a laminar separation bubble or larger separated flow regions occur. This is a result of the integral boundary layer method, which simply cannot model separation (this would require some sort of coupling between boundary layer analysis and the calculation of the external flow). The code has a option to perform a displacement iteration in order to take the displacement effects of the boundary layer into account, but there is no direct interaction....Transition prediction is performed by testing the boundary layer parameters against a set of empirically derived transition relations, which work quite well for attached flow in a wide range of Reynolds numbers.."

Where did you find this quote? If it's describing Xfoil, then it's 90% wrong.

[Ad Hoc]

Ah my mistake, this is refering to profil when comparing against xfoil, misread it.

http://www.mh-aerotools.de/airfoils/methods.htm

[Paul B]

Originally Posted by Charles Burgess

If you had made the model B-1 with some clay in one wing for ballast and glued it so that no water can enter, you could put the model in a pool (or your bathtub) and blow on the wing that is in the air, and the B-1 would sail.

Except the lift of either the "sail" or the "keel" would be the wrong way around.

[Doug Lord]

Originally Posted by Paul B

Except the lift of either the "sail" or the "keel" would be the wrong way around.

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Wrong-assuming the foil is symetrical the lift in the water and in the air would be identical to the way it is on a "normal" sailboat.

[Paul B]

Originally Posted by Doug Lord

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Wrong-assuming the foil is symetrical the lift in the water and in the air would be identical to the way it is on a "normal" sailboat.

You continue to be an idiot. Does the B1 have symmetrical lifting surfaces?

[Doug Lord]

Originally Posted by Paul B

You continue to be an idiot. Does the B1 have symmetrical lifting surfaces?

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For the purposes of Mr. Burgess's illustration you are dead wrong.

[Paul B]

Originally Posted by Doug Lord

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For the purposes of Mr. Burgess's illustration you are dead wrong.

You never tire of being the biggest fool on internet sailing forums.

In fact, if anyone did perform the experiment noted by Mr. Burgess they would find that blowing air on the model from an "upwind" angle the model would simply blow backward due to the Cd/Cl.

I'm not sure if Mr. Burgess even understands what the B1 is. His reference to "batwing" seems to refer to the B2, not the B1. Note the B1 and B2 illustrations below.


Since we are talking about models, maybe you would like to share all your breakthrough designs in the One Meter and Marblehead Class R/C racers? Those are the benchmark classes for design in the community.

[tippo747]

if the airofoil is the perfect shape...........i think they reversed the airofoil......to give room......so you can operate the ropes....and stuff....