Hydrofoil Profile Question

Rick: thanks again, the Clark Y profile and others will be useful to me.

Tcubed: "In the case of your daggerboard, why are you aiming for 12% thickness? It seems needlessly thick. Myself, i'd favour 10 or even 9 per cent"

-my intent is still evolving, it is now a Bruce foil which, for a given depth requires greater aspect and also greater force than a daggerboard, both of which lead to greater moment (double at 45 deg) to be supported by the structure, hence the increased thickness which was originally 9%. I thought the fore-aft symmetry would allow me to flip it to either side; it's doable but it doesn't look safe. I'll use 2 foils. A foil with lateral symmetry will work on either lee- or weather side but I've done that already.

Honesty forces me to admit that I still haven't quite made up my mind on the final outcome, but I have some ideas on sailboats that I want to put together. Don't we all?

It's good to know about the incompressible fluid thing; it seems logical now but so do a lot of things when someone points them out.

The gliders I was making were super-lightweight, barely strong enough for outdoor use. Sink rates were ridiculously low. I lost all of them to thermals, even a few that kept going up after the dethermaliser popped up. I didn't care as they were cheap and I could turn them out at a great rate; I was doing what I do now I guess, varying one parameter at a time and looking to see what happened. It kept me off the streets.

 

That makes sense. You were into the minimum sink speed thing, and consequently keen for extreme lightness. Myself, i did slope soaring and a lot of time would use reflex in the ailerons/flaps as well as ballast to get my flight speed up, so could build in a lot of stuff. In fact needed to. The last wing i made i tested on the ground to withstand up to 30G, and still worried sometimes about breaking it. Previously i had bust a wing, being a bit too enthusiastic in my manoeuvring...

By the way, i'm still not sure what kind of boat you're intending this foil for. It's not a proa?

 

Not a proa. I was planning on a symmetrical profile for the kayak sailing kit I am designing for next season, to build over the winter. I mentioned proa because I didn't know how to define what I was looking for until Tom defined it correctly as fore-aft symmetry. I no longer think the idea of using the board on either side is workable, although I know a single leeboard can be deployed on either side using a tether, but it has been great exercise that has taught me a lot about profiles I didn't know before. I am going back to an earlier concept with a Bruce foil on each side. The Bruce foils will cancel the heeling moment of the sail, so I don't need the lifting sail I was considering using. These are the threads that address what I am thinking:

http://www.boatdesign.net/forums/sailboats/articulated-sailboat-24598.html
http://www.boatdesign.net/forums/sailboats/sailing-kayak-project-24541.html

I haven't given up on wild ideas althogether though, I am trying to fit a wing sail into this concept somehow, so the profiles may get more than one application.

 

I used a reflex profile once on a tailess Jetex model which flew really well. Or did you mean undercamber? I used that a lot on gliders and once on a performance power plane. That also disappeared through the clouds at about 4,000 ft despite only having enough gas for 15 seconds; I thought I wouldn't need a dethermaliser on its first powered flight. Damned thing probably reached 500 ft or more in that time under power, and then kept going up. I Got it back months later.

 

Interesting.
I would adjust reflex into the trailing edges just to get sufficient penetration on windy days. Normally i would avoid that of course, because it's not so efficient. But when you have that much lift you can afford a drop in efficiency just so you can keep up with the wind...The only plane i designed and built with a reflexed camber line (zero pitching moment foil) as the normal operating mode was a three Metre flying wing, which handled amazingly well.

The sailing kayak is very interesting. I personally would research kite sails for that. These seem the ideal-no heeling moment and ultra stowable. The foils then can just be straight up and down. Front foil/daggerboard (you probably already have a pedal operated rudder?) can just be "clipped" on to one side. That way no holes in hull. Don't have to switch it over, the slight asymmetry won't make a bit of difference, especially since there will be no heeling. (there are other very neat and efficient arrangements as well) The only drawback to kites i can see, is you must wear, not tack, always. I don't think that's a big deal, unless you're in narrow water, in which case you'de be paddling anyways.

 

My model was a flying wing too, straight wings with lifting fuselage. Tiny, about 18 inches, but flew well and very stable. I was very careful to get the profile exactly according to instructions on that one. It seemed quite efficient although that always surprised me.

I have not been able to get a kite sail to work. Where I kayak is surrounded by tall trees and full of tree-covered islands. The air currents are too chopped up close to water level for a kite.

My last design had one Bruce foil only and a lifting sail offset from center and also offset from vertical. Conventional foot-operated stern rudder. Sail folded umbrella fashion; the whole thing could be put up or taken down in 30 seconds. Before that I was experimenting with a square-rigged sail and leeboard.

As the current design is evolving, there will be two foils which will rotate to steer with foot pedals, and a stern skeg to keep the hull in line. The whole sail/foil assembly will be removable from the hull, a belly band around the forward hull will hold it down, taut but not tight, and a much tighter line will pass around the cockpit coaming. A quick release on the pivot will allow the rotating part of the assembly to detach and the cockpit band will have a backup quick release so the assembly can just slide off the side if I get into a situation.

I will probably build it in two versions, one with a lifting lateen sail and one with a sail wing as mentioned previously. The hardest part will be to make it rigid enough and still build it in what is just a woodshop; I can cut and drill metal but no welding. I think I have all I need to go ahead now, except for some more Aluminum tubing.

 

I'm sure you'll come up with viable design solutions, just make sure you let us know what you achieved..

I don't understand what your conditions are, but surely kites are good for catching steady wind above the surface turbulence caused by geological and biological perturabations?

 

http://www.foilkayak.com/

Have you seen this page...

 

Yep. The flyak. Bin told about that. Needs at least 225 watt just to beat the regular kayak. To do that you would need to look like the guy in their picture for a start, and have olympic level performance. I doubt if I ever got close to that output; right now at 69 years old I would guess I'm a 75 watt kind of guy. At that output their own numbers show that I can go twice as fast in a regular kayak. However, it's still an amazing piece of engineering.

My intention at this point is a home-made sail rig for a kayak, I need the profile, subject of this thread, for a Bruce foil which will give me a non-heeling rig since a kayak is not stable enough for a regular centerboard or leeboard. However, I was also thinking of a lifting foil as a down-the-road exercise, so mounting the sail rig on the flyak is a fascinating possibility.

The flyak would have a speed of 10k at 225 W which translates to about 11 lb thrust from the sail if I have done my arithmetic right, a bit less as the inefficiency of the paddles is no longer a factor. However, the Bruce foils would create additional drag so I should allow for their drag. A sail of the size I have planned (25 sq ft) should provide enough power. Of course, there would be the question of weight; the displacement of 90 kg given in the web site would need me to lose some flab.

I have another thread going in which I am discussing the shape of hull that would be best. From its shape the flyak is at it best in a straight line, like the kayak I will be using. I have yet another thread discussing a rig in which the Bruce foils and sail rotate separately from the hull for steering; this allows the possibility of almost instant tacking; anyone who has tried to tack in a sailing kayak knows this can be a challenge since they are slow to turn and lack momentum, so one can easily run out of forward motion before the tack is completed. I thought a less directional hull would do a better job than a kayak, possibly a skimming dish shape, like an racing keelboat less the keel and with a flat aft for planing.

Lot of thinking and design work left to do ...

 

AK
I understand that if you buy one you get a training course thrown in and it seems to be quite extensive; not just a 5 minute get in and see how you go.

With hydrofoils it is not only power that counts. It is also weight. Hence you need to look at both aspects of the equation. Maybe a fellow weighing 65kg could keep it flying at 225W but if he weighs 80kg he will need proportionally more power.

The flyak should outperform a single scull over a 2000m course as the power levels required to keep it flying are within the potential of an Olympic athlete for the 4 to 5 minutes it would take to cover 2000m.

I have tried foiling with pedals and can just get the boat to lift clear of the water but there are lots of control factors that need to be considered and I do not have the power reserves to be bothered with the effort. It simply takes all my concentration to get enough power to fly. You can fly at lower power levels but you go slower than a displacement boat. The break even for better speed is around 250W if you can build it really light and make really good foils.

Rick W

 

I agree.

I don't have human power levels for activities of a few minutes but according to a Wikipedia article an elite athlete can achieve 6W/kg for more than an hour. You can half that for a regular, in-shape male off the street. The Flyak figures are given for 90 kg displacement so deducting 15 kg for the boat we have a 75 kg paddler achieving 225 kw. At that output the Flyak's speed is given as 3 m/s or about 6 k, not enough to beat sculls or a decent kayak; but our paddler could certainly do better for only 4 minutes and the flyak's speed increases linearly, according to the graph given.

I doubt if the foils I would be able to build would be good enough, but I can get more power from a sail so foiling is still a possibility; not my prime objective though.

 

Ancient, What sort of kite did you use? There are big differences in low wind usability and stability. The development rate for them in the recent past has been insane. It is ofcourse a downer if you have to pull back the kite when the wind dies for a minute. If you can paddle during that minute you may keep it aloft on the apparent wind. The bridled soft foil kites are the lightest and can have a high CL together with efficiency. Dropping them in the drink and relaunching without pulling them back in is also possible with some models (closed leading edge), though they will be wet and a bit heavier.

EDIT: By the way, can't you please make a rigid wing kite, like a sailplane? You said your planes all dissapeared in thermals, if you can make them strong enough too, it could be the most efficient kites yet.

"The only drawback to kites i can see, is you must wear, not tack, always. "

It's not necerily true.

 

I need inrormations aboat design of high speed craft.tanke you

 

Sigurd: it was a very small kite, fabric over a rigid frame. The real problem was I had to launch from inside the boat rather than on the beach as I believe is normal, due to wind patterns. It got dunked first time and just didn't want to go back up there again. For me kiting was an interesting theory and an unsatisfactory practise, something I don't really want to do again. I accept that a better kite, with proper training would do a better job but ...

Tohid: I think you are in the wrong thread. I see you joined recently, welcome to the forum. To start a new thread for your topic click on the "Boat Desing" link at the top of this page then click on "New Thread" at the top of the new page. State the topic clearly in the title to attract attention from the memebrs.

 

I watched a fellow trying to launch a kite from a canoe in flukey conditions. He only managed to get very wet and ended up on the lee shore with a long walk back carrying the canoe. He did not have a paddle.

It was entertaining.

Like many things it no doubt takes practice. Steady light breeze would be nice as well during initial attempts.

Rick W