edt wrote:they might be supercavitating not sure. Remember the sailrocket has a record of 65 knots with non-cavitating style hydrofoils and that whole boat was mostly built in a garage.
from the sailrocket page:
"Cavitation is the main problem faced in hydrofoil design. This is the phenomenon in which bubbles of water vapour spontaneously form on the foil surface due to the pressure reduction. Subcavitating foil designs such as the Vestas SailRocket foil, attempt to avoid cavitation, although it is generally accepted that cavitation is unavoidable at speeds significantly above 50 knots."
So we should get our foils small enough to cruise around at 50 knots before we have to worry about cavitation. Let's do it.
In many respects, you are probably right .
A traditional, non cavitating foil should be able to approach 50 kts w/o cavitation.
But there are no guarantees your small foil will have enough lift to get you up to speed.
Or that your kite has enough thrust to accelerate you up to speed.
Unlike the example of the 1000hp ocean racer going 100 kts, there is little chance he can do it with only 50 hp.
It becomes a question of efficiency.
Makin the foil smaller, does nothing for the strut requirements.
The strut is still a significant chunk of wetted surface in a KBHF
All that surface area is still generating increased drag proportional to velocity squared.
Ultimately, you may need to get all that stuff out of the water.
James has already said it, but there is a reason that surface piercing foils are chosen for their ability to reduce wetted area as lift increases.
just to beat this to death for a bit:
assuming the basic equation for lift of a foil section : L = Cl * .5 * density*Area* Velocity^2
Coefficient of lift (Cl) can be changed with flaps or other camber adjusting techniques. But KBHFs don't do that yet.
You can change Area ( topic at hand) , but still no 'real time' adjustments happen here, either.
The rider can certainly change the pitch angle to increase Cl.
Typical low drag design Cl (ranging .1-.3) happen at small angles. if you increase AOA to increase Cl, the drag goes up and stall becomes a possibility and the inception of cavitation gets closer.
a practical limit for Cl is probably Cl=.5
so if you pick an area and low drag Cl to allow you to fly at 50 kts, what is the likely take-off speed for a small foil with no rider adjustments other than AOA?
if you can increase Cl by 5x you can maintain the same lift capacity @ sqrt (50*50/5) = 22kts takeoff speed and 50 kts top end.
... if true that would seem like a really specialized application and not appealing to most folks
(similar to what sail-rocket does)
It will be interesting to see where the design for fast foils ends up.