23 replies to this topic

[BoardSail]

Hi, may be I am in a wrong forum, but I think it is a closest to what I want to do.
And I want to make a hydrofoil for my windsurfing board. (I am going to plug it into centerboard slot, and probably also into rear fin slot using a beam connecting both slots in this case, if it will be needed for a balance.)
The foil shold be with static AoA relative to board, without flaps. Also I am going to mount a canard on the bow for horizontal stability.
This way all should work automatically, canard should be always on a surface in planning mode,
the main foil should be submerged, if it is going down it increases AoA, going up decreases AoA even to a negative without going ventilated.
The board should go up with wind 5K and have minimal drag in normal position on different speeds. ( do I want too much? )

I tried to read this and that about NACA foils, but still not feeling comfortable enough to start calculating the foil.
My question is what do you think would be an optimal NACA (or other) foil for this, for fresh water?

many thanks!

[Doug Lord]

If you haven't already you need to do some research on foiling boards-one source foils.org ; another "Foilers!" a french website that has the latest in every type of foiling including boards. One consideration to keep in mind: the early Rush Randle foiling windsurfers used take-offs on the "air-chair" which is characterized by two foils on one strut. The main foil is very low aspect but the biggest characteristic of this type is the ability to jump and safely re-enter. The new French board has much higher aspect foils-still two on on strut- and probably won't re-enter as well but is likely to take off earlier. The Rich Miller foiling board(very low flying altitude) sounds similar to your idea. Good Luck!

PS- there are a number of good sections for foils but the planform of the foil-like the Rush Randle type or like the new French type-is of far greater importance and sets the tone for the whole design. As would choosing the Rich Miller low altitude configuration...

google: "foiling windsurfer", if you haven't already

"Foilers!" http://foils.wordpress.com/


foils.org http://www.foils.org/sailbord.htm

[BoardSail]

If you haven't already you need to do some research on foiling boards-one source foils.org ; another "Foilers!" a french website that has the latest in every type of foiling including boards. One consideration to keep in mind: the early Rush Randle foiling windsurfers used take-offs on the "air-chair" which is characterized by two foils on one strut. The main foil is very low aspect but the biggest characteristic of this type is the ability to jump and safely re-enter. The new French board has much higher aspect foils-still two on on strut- and probably won't re-enter as well but is likely to take off earlier. The Rich Miller foiling board(very low flying altitude) sounds similar to your idea. Good Luck!

PS- there are a number of good sections for foils but the planform of the foil-like the Rush Randle type or like the new French type-is of far greater importance and sets the tone for the whole design. As would choosing the Rich Miller low altitude configuration...

google: "foiling windsurfer", if you haven't already

"Foilers!" http://foils.wordpress.com/


foils.org http://www.foils.org/sailbord.htm

thanks for reply!
Of course I searched google a lot and familiar with Millers model (canard type) and with Rush Randle "airplane" shaped one.
I just think the "canard" type is more pitch-stable. It is automatic in this regard. Comparing to "airplane" shaped design where a surfer should keep pitch-balance very well. I just do not have that much experience.
This is why I've selected a canard type. I read Miller about disadvantages of static canard, (why he has invented the hinged one), but I hope to mitigate this issue by using a special U-shape of canard.

Currently I am looking for a foil section, which is optimal for the purpose, fast lift-up and stability on different speeds, so the foil does not jump out of water on higher speeds.

[IC Nutter]

With any foil section, you may need about 10 degrees angle of attack (AOA) at take off and say 2 degrees AOA or less at full speed, so the board needs to be able to pitch down about 8 degrees over the full speed, so you need to look at the overall geometry to make sure this is possible. For stabiliy, the canard needs to have a couple of degrees more AOA than the main foil, whch makes the canard very draggy at the low speed end. To reduce the canard drag and overall pitch change you need to compromise on the max lift from the main foil (less AOA), which means higher take off speed or more foil area. An alternative may be to use a surface piercing foil as the main foil. Then the pitch change is unnecessary.

The actual foil section is probably not that critical at this stage, but have a look at Tom Speers' H105 foil section and read what he has to say about it. Tom Speers' foil section

[kprice]

As IC says the angle needed at takeoff can lead to a lot of hull drag unless you set up the main foil for the right AOA. Miller's board got around that by putting the foil at the back of the board, so that pitch up to get more AOA did not put a lot of board in the water.

Miller's setup worked nicely except the complication of the hinged front foil. A U-shaped foil might have trouble getting enough lift. Another idea is a freely hinged flat foil ( think of a little surfboard on a ball joint).

Should be a fun project.

KP

[BoardSail]

With any foil section, you may need about 10 degrees angle of attack (AOA) at take off and say 2 degrees AOA or less at full speed, so the board needs to be able to pitch down about 8 degrees over the full speed, so you need to look at the overall geometry to make sure this is possible. For stabiliy, the canard needs to have a couple of degrees more AOA than the main foil, whch makes the canard very draggy at the low speed end. To reduce the canard drag and overall pitch change you need to compromise on the max lift from the main foil (less AOA), which means higher take off speed or more foil area. An alternative may be to use a surface piercing foil as the main foil. Then the pitch change is unnecessary.

The actual foil section is probably not that critical at this stage, but have a look at Tom Speers' H105 foil section and read what he has to say about it. Tom Speers' foil section

Reading Tom Speer ...

As IC says the angle needed at takeoff can lead to a lot of hull drag unless you set up the main foil for the right AOA. Miller's board got around that by putting the foil at the back of the board, so that pitch up to get more AOA did not put a lot of board in the water.

Miller's setup worked nicely except the complication of the hinged front foil. A U-shaped foil might have trouble getting enough lift. Another idea is a freely hinged flat foil ( think of a little surfboard on a ball joint).

I think to place the main foil not to very end of the board, rather somewhere in between the centerboard and fin. This will distribute load mostly on the main foil, with some little load on the canard. So the canard should not have big drag and lift issue. I hope.
I am thinking to make it a flat bottom, round top section, so it can work as a piercing foil while semi-submerged, and as a planning foil on full speed.
During start the canard should go up first, to the surface, this will create additional AoA for the main foil, let say 10-12 dgr. I'll need to calculate height of the leg for proper angles.

U-shaped canard will be like a bottom section of cone , so it keeps about same AoA and vertical direction of lift force when rolled on sides.
I doubt that a simple free ball joint will work.
Another idea is a joint like skateboard's one, but set oppositely, so when the board rolls right, it will turn left, keeping canard's AoA and direction of lift force same relative to water. Something like that.
I think U-shaped one is just more simple to make.

[cal40john]

Just a single simple foil.



Well maybe I was wrong, looks like a canard but no adjustments.

[ortegakid]

Yep! Sky Ski!

Attached Files

•  sky ski relax.jpg   16.79K   8 downloads

[BoardSail]

Just a single simple foil.

[/media]

Well maybe I was wrong, looks like a canard but no adjustments.

[/media]

No, no canards on both videos.
On the second video on 6:23 kiting guy fells, this is what I plan to avoid with using canard.

May be next time I'll build canard-less foil.

[IC Nutter]

Just a single simple foil.



Well maybe I was wrong, looks like a canard but no adjustments.

Note in the first video that the non foiling board to windward is going faster!

[BoardSail]

Note in the first video that the non foiling board to windward is going faster!

yes, I saw it. May be he has larger sail, or wind is strong?
Also there may be impact of helicopter on the foiling board. Just guessing, I do not know how far it is from the sail.

Theory tells that foils should be faster in medium winds.
I calculated, drag of foil is about 8-10 times less than drag of planning board. And it was on flat water. Chops should make planning board less effective.
Just see how much of waves, splashes and white water the planning board leaves behind it. It is energy waist.
I know with foil there will be also friction and cavitation problems on fast speeds.
But in our conditions (Ontario) we have winds more than 10k not very often.

[narecet]

Note in the first video that the non foiling board to windward is going faster!

yes, I saw it. May be he has larger sail, or wind is strong?
Also there may be impact of helicopter on the foiling board. Just guessing, I do not know how far it is from the sail.

Theory tells that foils should be faster in medium winds.
I calculated, drag of foil is about 8-10 times less than drag of planning board.

How much certainty is there that this calculation was valid? Actual results do not seem to support it. 

[bruno]

those t foils don't have much lateral resistance

[dogwatch]

yes, I saw it. May be he has larger sail, or wind is strong?

Sail sizes look similar to me and not strong wind from the appearance of the water. Maybe 13-14 knots? To me it looks like the foiling sailor cannot take up an efficient stance to power up the rig.

I calculated, drag of foil is about 8-10 times less than drag of planning board.

I seriously doubt the difference will be that great, bearing in mind a board will plane with only a small section in contact with the water - and if it won't plane it's not going to foil either.

[Presuming Ed]

Adaption of the Airchair, AIUI
http://www.airchair....oducts/skis.htm

[BoardSail]

I calculated, drag of foil is about 8-10 times less than drag of planning board.

I seriously doubt the difference will be that great, bearing in mind a board will plane with only a small section in contact with the water

I doubt too. I've done it last year and lost Excel file. I've done it for quite slow board speed, 10-15K.
May be on higher speed foil drag increases (function is ~ square of speed).
May be at some speed planing mode becomes more efficient.

and if it won't plane it's not going to foil either.

Here I would disagree. There should be a range of speed (and wind-power), for a foil with enough lift, where board will foil borne but would not plane. Even if it is a wide formula board. How much wind it will be needed to make Formula planing with 8m^2 sail and 80kg guy? What will be speed of this planing?

The ideal should be a foil, that at some high speed starts to plane.

[IC Nutter]

May be at some speed planing mode becomes more efficient.

Planing mode becomes more efficient when the lift coefficient (CL) of the fully submerged foil drops below that of the planing surface. A fully submerged foil will generate a maximum CL of maybe 4 times that of a planing surface. But with a submerged foil you have to reduce the CL as speed increases to keep it submerged. With a planing surface you can maintain the same CL and the wetted surface area reduces as speed inceases. So at some speed the planing surface becomes more efficient because it has less wetted surface area than the submerged foil.

A way to get around this is to use a surface piercing hydrofoil with a constant CL. This is less efficient at lower speeds than a fully submerged foil, but more efficient at higher speeds.

[BoardSail]

May be at some speed planing mode becomes more efficient.

Planing mode becomes more efficient when the lift coefficient (CL) of the fully submerged foil drops below that of the planing surface. A fully submerged foil will generate a maximum CL of maybe 4 times that of a planing surface. But with a submerged foil you have to reduce the CL as speed increases to keep it submerged. With a planing surface you can maintain the same CL and the wetted surface area reduces as speed inceases. So at some speed the planing surface becomes more efficient because it has less wetted surface area than the submerged foil.

A way to get around this is to use a surface piercing hydrofoil with a constant CL. This is less efficient at lower speeds than a fully submerged foil, but more efficient at higher speeds.

I agree. Another way would be ladder foils,
or combined ladder T+V foils..
or even telescopic foils (I just "invented" yesterday ), that can increase/decrease span automatically, or at least, manually.
manual span control would not be very convenient on a board, but may be quite doable on Moth.

Just found another discussion on foilers. Very interesting... reading...
http://www.boatdesign.net/forums/sailboats/foiler-design-2447.html

[dogwatch]

There should be a range of speed (and wind-power), for a foil with enough lift, where board will foil borne but would not plane.

You've clearly done some analysis here while I'm a bit gut-reaction, but surely the point is that for planing you've got the whole surface of the board as a lifting area which in effect self-adjusts as the board rises from the water. Whereas the area of a foil is essentially fixed. If you've got a huge foil it might plane early but be a large drag at higher speed.

Operationally, foiling boards sound like a huge PITA. You are taking something light you can beach start and putting a large, complicated, fragile and expensive structure underwater. So much for beach-starts.

[BalticBandit]

There should be a range of speed (and wind-power), for a foil with enough lift, where board will foil borne but would not plane.

You've clearly done some analysis here while I'm a bit gut-reaction, but surely the point is that for planing you've got the whole surface of the board as a lifting area which in effect self-adjusts as the board rises from the water. Whereas the area of a foil is essentially fixed. If you've got a huge foil it might plane early but be a large drag at higher speed.

Operationally, foiling boards sound like a huge PITA. You are taking something light you can beach start and putting a large, complicated, fragile and expensive structure underwater. So much for beach-starts.

Well remember that you can change the amount of lift a foil can generate somewhat by changing angle of attack so you could have a foil that lifts fairly quickly but only has moderate drag

The thing that most likely makes a foiling windsurfer less meaningful is that with just a planing board, you can use about 40% of the lift generated by the sail to lift the board thereby reducing drag quite dramatically. With a foil that sort of adjustment is much harder since with the foil you've added 3 more degrees of dynamic freedom that have to be kept in balance and offered the rider a dramatically less stable platform

So the sail lift reduces drag quite substantially

[BoardSail]

....
So the sail lift reduces drag quite substantially

Yes it does. However nothing is for free. Lift takes energy from thrust. I mean the vector of thrust has more pitch in this case. At some point it creates optimum equilibrium between thrust, drag and lift.
With a foil, where Ligt/Drag coefficient is higher that a planning board Lift/Drag C, the vector of thrust is more horizontal, because you do not need to use sail lift, lift is created, may be more efficiently by foil.

I totally agree about difficulties with balance control.
But boards are already more difficult to control balance, comparing to other vessels, and this challenge just adds fun.

[Amati]

I calculated, drag of foil is about 8-10 times less than drag of planning board.

I seriously doubt the difference will be that great, bearing in mind a board will plane with only a small section in contact with the water

I doubt too. I've done it last year and lost Excel file. I've done it for quite slow board speed, 10-15K.
May be on higher speed foil drag increases (function is ~ square of speed).
May be at some speed planing mode becomes more efficient.

and if it won't plane it's not going to foil either.

Here I would disagree. There should be a range of speed (and wind-power), for a foil with enough lift, where board will foil borne but would not plane. Even if it is a wide formula board. How much wind it will be needed to make Formula planing with 8m^2 sail and 80kg guy? What will be speed of this planing?

The ideal should be a foil, that at some high speed starts to plane.

But a section that rises to a plane would ideally have a flat bottom. Flat bottomed sections have limitations when immersed compared to more cambered sections. What do you believe the trade offs are in this ( implied ) design balance?

[Amati]

There should be a range of speed (and wind-power), for a foil with enough lift, where board will foil borne but would not plane.

You've clearly done some analysis here while I'm a bit gut-reaction, but surely the point is that for planing you've got the whole surface of the board as a lifting area which in effect self-adjusts as the board rises from the water. Whereas the area of a foil is essentially fixed. If you've got a huge foil it might plane early but be a large drag at higher speed.

Operationally, foiling boards sound like a huge PITA. You are taking something light you can beach start and putting a large, complicated, fragile and expensive structure underwater. So much for beach-starts.

Well remember that you can change the amount of lift a foil can generate somewhat by changing angle of attack so you could have a foil that lifts fairly quickly but only has moderate drag

The thing that most likely makes a foiling windsurfer less meaningful is that with just a planing board, you can use about 40% of the lift generated by the sail to lift the board thereby reducing drag quite dramatically. With a foil that sort of adjustment is much harder since with the foil you've added 3 more degrees of dynamic freedom that have to be kept in balance and offered the rider a dramatically less stable platform

So the sail lift reduces drag quite substantially

But how much? Most of your weight is at the least below the harness riding on the board. I've mentioned it before, but as a quick and dirty experiment, I used a lat bar on my weight machine to model a windsurfer boom, held on to it, had my feet on my weight scale, and all I could get rid of was 10% or so of my weight. Try it! What was interesting too was how little weight in the stack took to balance my weight leaning out on the bar. But that's why everyone was trying to pull down on the wish boom a few years back? But then that conversation moved to how it was better to put all of the sailors weight that you could on the mast foot, and that went around and around.

Granted, weight loss bearing down on an object moving through the water is pretty dramatic, but like boardsail points out, you lose thrust forward.

[BoardSail]

The ideal should be a foil, that at some high speed starts to plane.

But a section that rises to a plane would ideally have a flat bottom. Flat bottomed sections have limitations when immersed compared to more cambered sections. What do you believe the trade offs are in this ( implied ) design balance?

It was just a wild idea.
Also planning board or section usually touches water by its rear part. And that part in many cambered sections is flat, or even hollow, like with a flap down.