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Old 28-03-2012, 08:14   #1
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Lightbulb Hyrdofoil on Cruising Sailboat

I've got this idea to develop a hydrofoil for my 26' reinell. The math can be figured out fairly straight forward, with water being 30 times denser than air. I could determine wing size and lift capabilty to reduce the wet surface by 50% or so at 6kts (dispaclement hull speed).
The boat has a fixed shoal keel, so i figured the foils could be afixed to the keel. Perhaps a bi-foil would keep the foil profile from protruding beyond the freeboard.

Couple unknowns:
can the boat only plane when running?
how to compensate for a relatively reduced keel profile while reaching?
How much lift is too much? I dont imagine that lifting the hull completey out of the water would be practical or even benneficial, with loss of leateral resistance and steerage.
Is there a need to be able to adjust the hydrofoil while underway? ie Flaps, angle of attack.

Im looking for some feedback or hear from sailors that have done this or have seen it done.
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Old 28-03-2012, 09:39   #2
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Re: Hyrdofoil on Cruising Sailboat

I admire your imagination; I think of stuff like this often as well.

I just hope you don't get too black and blue in this forum from being touched with all the ten foot poles !
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Old 28-03-2012, 09:51   #3
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Re: Hyrdofoil on Cruising Sailboat

If the foils angles aren't adjustable, when the boat heels much of the lift will go to moving the boat sideways, to the lee. The whole reason for the keel is to prevent that.
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Old 28-03-2012, 09:54   #4
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Re: Hyrdofoil on Cruising Sailboat

What is the expected L/D of a hydrofoil? About 20? So if you have a 10,000# boat you will need 500# of excess thrust. ABove and beyond what it takes to get to hull speed. Good luck with that. You need to reduce both the wave-making ability and the wetted surface so just lifting her 50% will be worse than all the way out of the water. But give it a try and post pics.
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Old 28-03-2012, 10:39   #5
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Re: Hyrdofoil on Cruising Sailboat

My first calc suggest that 28 sqft foil at 3' bellow the water and 6knts water speed will generate 1,680 pounds of lift. That is about 43% of the vessels displacement. the 28 sqft is as large as i can practically build due to keel shape and vessels beam. However, at 9 kts the foil can lift the full vessel displacement. The Big question now becomes...
Can I get to 9 knts waterspeed? Would reduceing the displacement by 40% allow me to get to the magical 9 knts for 100% lift.

Also @ daddle: L/D is 133 lbs:sqft
I've seen planing hulls make a tremndous wake just below planning speed.
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Old 28-03-2012, 10:48   #6
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Re: Hyrdofoil on Cruising Sailboat

You'll need 200# of thrust to power that foil. A really big spinnaker might do that. Try to keep the rudder in the water. Heh.
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Old 28-03-2012, 10:52   #7
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Re: Hyrdofoil on Cruising Sailboat

If you're able to accomplish this hydrofoil idea that lifts you up out of the water like that... You must go on and name your boat "Wheelie" or something similar. Its just obligatory.
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Old 28-03-2012, 10:52   #8
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Re: Hyrdofoil on Cruising Sailboat

Start by calculating how fast the wind has to blow to generate the power to achieve foiling (partial).

Next how much drag you're creating and how much it slows the boat when not on the foil for using the boat in nonfoiling conditions.

Hobie Trifoiler and Moths have a wand as a sensor to control pitch of the foils. Hydroptere is a bigger foiling boat reasonably successful to look at for ideas.


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Old 28-03-2012, 14:11   #9
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Re: Hyrdofoil on Cruising Sailboat

Danosimp,
You made a slight mistake: water is about 800 times denser than air...

There can't be such a thing as too much lift. The lift generated by a foil is proportional to the angle of attack. Then, you just have to adjust the foil setting or the boat trim to ride ath the desired height.

If the keel and rudder are still partially immersed, you don't risk to lose guidance and steerage (provided you keep ventilation in check). Also remember that lift is proportional to the square of speed. Then, the faster you sail, the less immersed keel and rudder area you need.

I suggest you google "foiling moth" for ideas.

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Old 29-03-2012, 07:19   #10
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Re: Hyrdofoil on Cruising Sailboat

Hello everyone, more calculations...I have determined that with my given sail area, speed to plane off, and maximum practical foil size, I will need 19+ knts of wind to overcome my wake and plane the vessel. 15 knts is typically my first reffing point. Being a shoal keel and a light crusier, if I didnt reef and maganged to get onto a plane, my intuition tells me the boat will slice like a poor golf swing or the lift on the windward foil would increase the heeling force.
My intial idea was to build a fixed foil and mount it to the keel. With the assistance of your input, I realized I will need to control both dihedaral angle and angle of attack to counter heeling moment and lateral drift. Getting into some complicated control systems now. I read that modern hydrofoils are computer controlled with an array of sensors and servos, basically fly by wire.

@Hydra...what was i thinking? I had planed on using airfoil formulas and adjust for water density initially, but manged to find the formulas for hydrofoils. Foiling moth is tiny. Looks like hiking out is the foil control system. Looks fun!

@Cal40John: Wow, is an amazing machine. It turtled at 60knts!
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Old 29-03-2012, 07:49   #11
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Re: Hyrdofoil on Cruising Sailboat

From experiments with outlandish hydrofoils years ago the figures are nearer to 100 times the sail area to keel area (which applies over a wide speed range of course).
Biggest problem is stopping the air getting down to the foil. Easy with a keel but on a thin (rudder like) support for horizontal foils it's easier to use a few horizontals on the leading foil so the angle of attack on the rear foil is set by the depth of the fore foil (ie determined by boat speed).
The model I developed used three hull, pod like to provide boyancy, and a tilting mast so the wing leant top towards the wind, lifting the boat by a factor too.
In another variation the foils were inclined too, the two to leeward had the bottom end toward the centre line, the sail wing mounted rigidly above it. The single windward foil was also inclined, tip toward the centre line, with the centre of effort directed to the centre of effort of the sail.
Steering proved to be it's downfall. That and getting it all balanced out.
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Old 29-03-2012, 13:55   #12
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Re: Hyrdofoil on Cruising Sailboat

Quote:
Originally Posted by Danosimp3 View Post
Hello everyone, more calculations...I have determined that with my given sail area, speed to plane off, and maximum practical foil size, I will need 19+ knts of wind to overcome my wake and plane the vessel. 15 knts is typically my first reffing point.
I didn't redo your calculations. But keep in mind that on a hydrofoil boat, there is no hydrostatic stability when flying, for the reason that there is almost no buoyancy. For this reason, no ballast is needed.

Then, you could lighten your yacht by 1350lbs / 612kg. Assuming 200lbs of crew, the weight should go from 4000lbs to 2650lbs. Since lift is proportional to STW squared, the take-off speed should be reduced by almost 20%.

And, neglecting the wavemaking resistance, STW is roughly proportional to TWS. Then, your yacht without ballast should fly with 80% of 19kts, that is ~15kts.
Quote:
Originally Posted by Danosimp3 View Post
Being a shoal keel and a light crusier, if I didnt reef and maganged to get onto a plane, my intuition tells me the boat will slice like a poor golf swing or the lift on the windward foil would increase the heeling force.
My intial idea was to build a fixed foil and mount it to the keel. With the assistance of your input, I realized I will need to control both dihedaral angle and angle of attack to counter heeling moment and lateral drift. Getting into some complicated control systems now. I read that modern hydrofoils are computer controlled with an array of sensors and servos, basically fly by wire.
Now you see that the main problem isn't lift but stability, on pitch and roll axes. There is a good study of this problem in this document: http://ftp.rta.nato.int/public//PubFullText/RTO/MP/RTO-MP-015///$MP-015-15.PDF (a bit slow to download, but worth waiting).
By the way, the author was behind the technical design of Alain Thébault's Hydroptère in the 1990s. Also note that until now, this boat has no "flight control computer": all is controlled by the crew.

Alain
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