LOWWWW speed hyrdrofoil?

[Bill Fish 6]

Hi all...

Been toying around with a pet project, which involves a boat of a certain size and carrying capacity....and most likely this project will never get past the "thinkin bout it" stage....but it is an interesting mental exercise (at least for me) when you are trying to juggle conflicting requirents by trying various technological/engineering combinations...

Anyhow, what I want in my virtual boat is the ability to go about 2 times hull speed with relatively good fuel economy, which seems to me to be a bit too fast for displacement/semidisplacement, and too slow for reasonably efficient planing. By the way, does anyone have an example graph showing miles per gallon versus speed for a theorectical/typical boat (preferably for a 1 to 2 ton)?. I ask because i am wondering how much efficiency you "get back" once you start planing since while you are guzzling fuel at a high rate, you are also travel a darn bit faster as well.

Well, back to the point of this post. I've always had the impression that hydrofoils came about because some folks wanted to go REALLY fast in boats. Even faster than those planing maniacs (and perhaps with better fuel efficiency as well).

And it just occurred to me, would hydrofoils make ANY sense for low speed boats (ie 1.5 to 2.5 hull speed)? Obviously the foils would be ALOT bigger than those on the high speed jobs. But would such a low speed hydrofoil, with say 10 to 30 percent of the weight of the boat still being supported by purely bouyancy forces (to help with stability) be more efficient than a "normal" boat at such speeds? Of course even if so, the foils would surely have some disadvantages as well (which might or might not be acceptable).

Doest this make any sense at all? Has anyone seen a low speed hydrofoil? Or even a technical discussion as to why it would make no sense at all?

take care

Blll

[tom28571]

Good question Bill,

I don't know the answer to that but I do know that it is perfectly possible to have a boat that planes at 2 x hull speed. It's a question of optimizing for speeds in what, for most boats, is the never never land of transition between displacement and full planing. The answer is to keep the bottom loading in lbs/sq ft low and keep the CG more forward than in the usual planing boat. Top speed will be more restricted because of increased wetted surface but the boat can perform well up to 3, 4 or more times hull speed.

[Wynand N]

Hi Bill Fish 6 :)

Nice scenario, why not calculate it yourself?

The lift produced by a foil depends upon the planform area (larger, more lift), the fluid (water produces more lift than air), the velocity (faster, more lift) and the form and porportions of the foil.
The shape of the foil is defined by its section, aspect ratio and plan form. The coefficient of lift (CL) is a fuction of these factors, together with the angle of attack.
By the way, coefficient of (CL) lift is a measure of the lifting effectiveness of the foil, that is, how well the foil operates as a lift producer.

CL can be very low or maybe as high as 3, depending upon the nature of the foil or foil combination and the angle of attack.

Suppose the coefficient of lift (CL) at an angle of attack of 3 degrees is 0.4 and we wish to calculate the lift in seawater (density 1024 kg per meter cubic) flowing at 15m/s (29.1 knots) of a simmilar foil having a surface of 0.8 meter square (8.6 ft sq).

Bernoulli's theorem goes something like this;

Lift Force (FL) = 1/2 x p x a x v2 (sq) x CL
= 1/2 x 1024 x 0.8 x 15sq x 0.4
= 0.5 x 1024 x 0.8 x 225 x 0.4
= 36864N
or 3758 kg (divide N by 9.81)
or 8284 lbs (divide N by 4.45)

Calculating drag is no "drag" at all. Instead of using CL, replace with CD (coefficient of drag)

Drag Force (FD) = 1/2 x p x a x v2 (sq) x CD

Happy calculating.

[tspeer]

Full flying hydrofoils really don't make sense below, say, 12 kt. If you're not going fast enough to plane, then you're not going fast enough to fly, either. The reason is simple: as the speed drops, the drag penalty of supporting the boat with buoyancy goes to zero. The drag penalty of supporting the boat with hydrofoils goes to infinity. So there's a crossover speed at which it takes more power to fly than it does to float. If you're interested in sub-planing speeds, you're below the crossover point.