Hydrofoils

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All want learn about design a hydrofoil boat need read a thread in boatdesign.net Forum do by DOUG LORD : http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/sailing-foiler-design-foil-assist-full-flying-40894.html

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HYDROFOILS

In this page we will talk about the science of hydrofoils.

– APPLICATIONS

Today we have much hydrofoils applications, see:

http://www.youtube.com/watch?v=j_WKgcOiq2M

And in our segment:

Doug Lord F3

– BENEFIT

The goal is making the hull of the water. With this we subtract a large part of the frictional and residual resistance and the boat can reach speeds much faster than traditional boats.

-TECHNICAL PECULIARITIES

Today, we mainly use two types of hydrofoils:

surface piercer: is a inclined wing  in water

From Hidroptére site:

Surface piercer out of water

Surface Piercer Hidroptére - see the fins on the surface

The surface piercer is a wing inclined from the horizontal, so, as the boat increases speed and goes out of the water the same occur with the surface piercer  and with it the lift decreases until reaching a point where the surface of the immersed surface piercer is sufficient only to maintain the hull above the water, hence it is said that the surface piercer has a self control of lift generation.

The lift generated is due the differents pressures in the faces of the hydrofoil caused by the foil angle of attack.

However this difference in pressure can cause air intake on the surface of hydrofoil on the side of low pressure near the water surface diminishing the hydrofoil ability in producing lift. For this reason we put fins on the surface of hydrofoil as an attempt to stop the entry of air.

fully submerged foils: is a horizontal wing submersed in water

This feature takes this kind of hidrofoil go up until the hidrofoil leaves the water, it is not self-adjustable. To prevent the exit of the water we use flaps to reduce the lift generation .

The problem then, with this type of hidrofoil, is that we must have a mechanism to move the flap properly. The simplest mechanism is used, for example in the Moth forward hidrofoil, is a bar out on the edge of the hull having a float, as the hull rises, the angle of the bar is changing and this angular motion is taken to the mechanism that moves the flap. As the hull rises, the mechanism change the angle of the flap and the efficiency of hidrofoil generate the lift goes down.

In the above video you can clearly see the bar in action on the bow of the Moth.

But we have others mechanisms to move the flap, for example:

http://en.wikipedia.org/wiki/Hydrofoil

Flap movement

RC HYDROFOILS

Well, and for our RC sailing boats what we can do and what are the problems?

See the video above: Hidroptére RC a Casteljaloux 47.

What I see?

If the wind increases, the windward hull starts to leave out of water, either by the lift generated by hidrofoil as the moment of the wind in the sail. When the hidrofoil leaves the water, totally, all the weight of the boat is divided between the rudder hidrofoil and  the leeward hidrofoil.

My first observation would be that the weight supported by surface piercer hidrofoils must be more than a single statical weight division between the three hidrofoils, must be in minimum the total weight divided by the rudder hidrofoil and one single surface piercer, according theirs relative positions with hull CoG. But here we have also the dynamical aspects that we must see. The boat is moving, in waves, with velocity, how much we need increase the areas in these hidrofoils to taking into account these parameters?

Let’s see my HidroPET weights:

	Weight		Longitudinal CG	col D X col F
	g		m	gm
Longitudinal structure	81,0000		0,5750	46,5750
Transversal structure	80,0000		0,7900	63,2000
Lateral Foil platform (2)	40,0000		0,7900	31,6000
Rudder Platform	20,0000		0,0400	0,8000
Bow PET	40,0000		1,1500	46,0000
Stern PET	40,0000		0,1500	6,0000
Port/Stb PET	80,0000		0,7900	63,2000
			0,0000	0,0000
Hydrofoil (2)	100,0000		0,7900	79,0000
Rudder	80,0000		0,0000	0,0000
Battery	200,0000		0,1000	20,0000
Rudder servo	40,0000		0,0400	1,6000
Sail servo	70,0000		0,0900	6,3000
Mast and sail	350,0000		0,7900	276,5000
				0,0000
				0,0000
Total weight   g	1221,0000			640,7750
CG Longitudinal   m	0,5248

Let see a statical and horizontal situation for weight distribution on hidrofoils:

Fr – Force on the rudder hidrofoil;  W = total weight;  Fsp = Force in the two surface piercer

Fr x 0 + W x 0.5248 – Fsp x 0.79 = 0

Fsp = 0,811 kg  (Each surface piercer need develop a minimum of 0.4055 kg)

Fr = 1.221 – 0.811 = 0.409 kg

But for me we can add 3 times 0.811 kg for each surface piercer. Why 3 times? Intuition, 3 times in minimum.

This value was idealized when I was seeing the video and in the worst situation, (windward hull out of the water, boat with velocity) the hull not leave the water. There was a deficit in the area in hidrofoil. And about this we have other problem, the righting moment.

When the boat is out of the water we have a up vertical force in the rudder submerged hydrofoil , another in the port side surface piercer , another on the starboard surface piercer – they are vertical forces up to balance the weight force, vertical down.
When a piercer leaves the surface out of the water driven by the moment introduced by the wind in the sail, we have the vertical force of the leeward surface piercer and the  rudder submerged hydrofoil   balancing the weight of the boat. These forces provide a restorative moment, which opposes the moment created by the wind = Weight x d, where d is the horizontal distance of weight, applied at the center of gravity of the boat, to the vertical plane passing by a line joining the point of application of forces in hidrofoils.
As the slope increase d decrease diminishing the restorative moment. If the efficiency of the sail does not decrease more, for sure, the boat will capsize.
Then the trimaran hydrofoil equilibrium   has to be very well studied and that leads, inevitably, to the movable ballast as warns our friend Doug Lord.

Doug Lord also gave us valuable information:

a -the beam at least 1.36 X’s length-like Hydroptere

b –  foil loading on the F3 (hidrofoil boat made by Doug – several years ago – see fig above)  was about .176 lb per square inch(at takeoff) and much more as the RM was developed

c – the foil section doesn’t really matter that much-just keep it at about 9% t/c max

d – on the F3 the main foils were set at +2.5 degrees angle of incidence relative to the static waterline-with the rudder foil at zero degrees

Using the load .176 lb per square inch lets see what area we need for the surface piercer:

0.176 lb/in² = 0.012374 kg/cm²

Condition – one surface piercer out of water:

a) Without margin to dynamic forces

Area = 0.811 kg / 0.012374 kg/cm² =65.54 cm²

b) With a dynamic coefficient = 2 for dynamic forces:

Area = 2 x 0.811 kg / 0.012374 kg/cm² = 131.08 cm²

c) With a dynamic coefficient = 3 for dynamic forces:

Area = 3 x 0.811 kg / 0.012374 kg/cm² = 196.62 cm²

If I use a surface piercer with dimensions; length = 35 cm and width = 6 cm; we have a area = 210 cm² and aspect ratio = 5.83 and we are attending this last supposition.

I will use a t/c = 6%.

CONTINUE….