Heres a SUP on a foil, the video shows the instability of a flat foil.
I dont know how to put a video here but heres the link.
I looked at this pic and its a great idea in action…
As the speed increases the foil lifts up reducing the foil area in the water. Its a great idea for foiling at a constant speed.
But a " perfect" surfing foil should lift and reduce its area automatically to match any speed.
To make a foil automatically adjust to any increase in power, the foil could be tapered like a standard fin to reduce the area or changing the angle of the foil to reduce the lift performance.
As the speed increases and the foil area reduces it also needs to remain stable.
With this setup…
the area that contacts the water is a fine point under the rider which is OK for skiiing at a stable speed but for surfing the foil needs to make a wider ‘footprint’ to remain stable especially as a surfcraft can go from 0 to top speed in a few seconds.
The best plan I have is to design a 3 dimensional foil that changes its area and aspect and remains stable.
Something like this is a starting point , the curved foil is a good idea but here its planing area remains in contact with the water at all speeds.
So I looked at reversing the rounded foil so the flat is at the top and the arms sit out low and wide.
From the front the foil is flat at the top and rolls out on both sides with the tips at ~ 45 deg. Width from tip to tip about 18 inches.
This shape will mean that as the speed increases, the foil lifts and the amount of the foil area in the water is reduced,
the angle of the foil changes to reduce efficiency
and the ‘footprint’ of the foil widens for stability.
The foil arms are each shaped like an elongated fin. So that as the foil lifts the planing area is slightly less.
Ply prototypes…
Now as a 3 dimensional foil it covers a depth of about 8 inches so that it has a defined dimension for lift rather than having a 3 foot strut between board and foil like this…
The foil has a width of 18 inches
and is 24 inches long with a 45 degree twist in each arm…
The flat front is on the left of the pic below.
Side view.
I started glassing over the ply foil, I’ll do about 20 or 30 layers of whatever Ive got on hand.
Thinking about what to attach the foil to, Ive put them on bodyboards but I think I need to made a standup drone type of board. Something that provides a platform to stand on and is quick to paddle but the hydrodynamic design is unimportant.It could be any shape but a thicker shorter drone board is probably easier to balance and control.
I found a template at Blending Curves and turned it around so it looks like a pig shortboard with the fuller nose at the tail.
I’ll keep it about 3 in thick with some nose rocker and then flat all the way thru to the tail for a fast paddling speed. It might only be 5’10" long but it should have the same volume as an 8 foot + LB.
Theres other things to work out too, like where to put the foil in relation to where the surfer will stand.
centreing it between the riders feet will make it easier to regulate pitch.
And the height of the strut is important, currently the sky-chair ski foils have a strut about 3 foot long and thats fine for steady speed cruising but because a surfboard can accelerate and stop and then take off again in a few seconds a much shorter strut makes sense. As the board accelerates it rises up and down in response to the speed and pitch and leaping up and down makes it hard to ride smoothly or even enjoy the ride. If you checked out the SUP video earlier you would see that he is having trouble controlling the height and pitch. Currently the struts Im using are 11 inches long and they feel too long when doing a cutback. So I’ll get some struts made that are about 6 inches and start from there…
Will 6" leave enough clearance between board bottom and water? A telescoping leg from an heavy duty aluminum tripod would give you adjustability.
I like that idea Dlock !
Not sure if the 6 inches will be enough but I 'll give it a try and see what happens. I’ve got the 11 inch struts and a pair of 3 inch struts as well to swap around and see what’s best.
Lots to consider…
I’ve used plenty of tripod legs for various “tests” and homemade tools. Would love to see this one pan out for ya foils. Nice looking design by the way and thanks for the sway’s stoke, testing and sharing.
Even the Moth sailboats use a longer strut and flat foils and they suffer from the same instability and pitch problems.
Moth stacks here…
http://www.youtube.com/watch?v=kURlmsC5Ifc
It’s amazing that no ones developed a more stable foil considering how expensive sailing is…
Hey surfoils, long time, no post.
glad to see you are still trying new things.
It is not necessarily the flatness of the foil or the length of the strut. Many folks who try to do hydrofoil tech have a 'planing hull' mindset, which is incorrect. Hydrofoils are essentially underwater aircraft, and require either a positively stable planform (like a hang glider) to which only weight shifts will be the only controlling mechanism, or they need control surfaces, just like aircraft. Unfortunately, positive stability (think of aerodynamic/hydrodynamic pitch stability like a steering wheel that wants to 'self center', when suspension caster angle is correct) is a draggy proposition. Neutral or even negative stability is much more sleek and with less drag, but you pay a price in a distinct lack of control, unless control surfaces are computer-controlled, like in modern fighter aircraft.
The problem with a shallow foil is that it becomes a hybrid. Not part planing craft, not pure hydrofoil, as at times I bet it is a little of both. Do you have vids of you surfing the foil a page back? I'd be interested to see....
JSS
Hi max, yes it’s been a while, I remember you being in on heaps of threads a few years ago.
im getting a GoPro for Xmas, gotta buy it for myself and then hide it from my sons…the last video I bought my eldest mounted it on his bed head…
Why weren’t videos around when I was single ?
A dozen layers of 6 oz and innegra over the top of the ply.
Popped it off the mold and ready to do a dozen layers on the underside.
Ive already worked out a better design for the foil but I’ll go thru with this one and see how it rides.
Im plannig to mount the GoPro on the nose but aimed beneath the board to see the foil in action. Any ideas of what hardware to do that ? This is where dlocks aluminium poles might help…?
Fantastic I cant wait for the videos. I mount my gopro with an fcs plug I would think that it would work
ok on the underside of the nose, they take quite a beating without blinking.
Have you considered using an x-rental foam board as the standup mule- they catch waves easily and you could mount the foil in the same way as you have on the bodyboard.
How much drag do the foils add do they slow you down when paddeling out & does the foil make it easyer or harder to catch waves?
RyanUK, I was thinking of using a deckplate and then an Al pole sticking out the front, with the GoPro turned back and pointed down, as the board rises you’ll see the foil come up to the surface.I’ve got a ton of bodyboards so I’ll find one that’s best for the job.
The amount of drag the foil creates was a primary concern, if it was slower than a standard surfboard then no one would want to ride it so I keep the foils thin and angled for lift so you can’t feel them when paddling. Keeping the strut short helps too, the closer the foil is to the board the better the feel when paddling and riding.
When you paddle into a wave the foils added planing area gets you up faster, with the bouyancy and planing area of the board, plus the planing ability of the foil, it works at a low speed , even on unbroken waves.
A longboard has a planing area of about 2000 square inches and a foil has 250 sq in… There’s a lot more power within the wave than on the surface.
Those flexible microphone stands are cool tools too.
Dlock, Do you think I could use poly pipe…? Maybe 40 mm…?
Thanks for coming back to me, The idea of flying across a wave has an irresIstable allure, so if its alright id like to have a go at making one of these myself.
Any tips would be apprecated 
RyanUK, I’d be stoked to help you out !
Here’s someone in your neck of the woods doing kitesurfing foils…
http://www.lsbu.ac.uk/esbe/research/humaninteract.shtml
Sure Ryan, always happy to help, what type of foil are you interested in?
The flat ones are easy to make and give you an instant understanding of the feel and how to ride. Doing it on a bodyboard might be easier than on a surfboard.
Which way do you want to go.?
Thanks!
that kite surf looks quite highly developed.
I’ve never riden a bodyboard so want to go standup and i think a flat makes a lot of sense. I am not looking to throw fancy moves but experence a new way of surfing.
How did you calculate the surface area required and how did you sort the placement under the board?
Looking at the pics the lateral load could be an issue, have you had any problems with load from front to rear? are you using the vertical supports to generate drive?
Lots of arms and angles on that kite foil ! I think it’s a hexagonal frame with the board on top. He’s working with twin lateral foils (front and back) using the ‘boat’ style of foil design.
I’ve already tried it on a longboard and a short board but you’ll need to strengthen where it attaches to the board. With the new drone / mule I’m planning to use FCS plugs and a few closely spaced stringers to ensure the plugs are well secured in place.
Working out the area and placement was trial and error, I made about 60 different foil shapes and just kept moving them around. I’ve got heaps of pics of the early versions if you want… The foils is 246 square inches and I’m 80 kgs and it lifts me easily, so vary the size to suit…
The struts are 4mm aluminium, 11 inches high X 3 inches wide, they’re set 10 inches apart. No issues with lateral loading or front to back, it all works smoothly from takeoff to finish but it gets a bit squirrelly in the tube…that’s why I’m trying to develop the automatic foil that regulates lift based on speed.
i didn’t plan to create forward drive with the vertical struts and it doesn’t ned any more than it naturally has, just shows that you can create drive without any fins or rail length.
After one wave you’ll know a lot more than you did before.
Before my first wave I thought I could get nailed severely here, absolutely shredded, so many bad outcomes were possible…
After the first wave I thought How fucking cool was that !! And easy !
heres a pic of the flat foil over an inch grid if you want to start from there… It’s garden variety 3 ply with 1X carbon on one side and 1 X 6 oz on the other, didn’t filler or gloss coat, very basic.
A more tapered nose outline softens the entry into a turn and pitch control.
Surfoils,
You may already have visited this site, but it is very interesting.
http://www.hydrofoil.org/history.html
The fact that the original foil for a high center of gravity was 2 separate lifting surfaces, separated by a significant distance and the front lifting surface(s) being the bigger load bearing ones is very important. If you look closely, there is some decalage.
JSS
Hi Max, thanks for the link and I think I picked up some invaluable information from there too. Thankfully there is a world of free information on the net for amateur builders.
Re: 2 foils Vs 1 foil....
All of the science would suggest that twin foils are the way to go and there's a lot of practical applications for them ...
however in the surf it's a different system of water forces, changing densities between deepwater inside the wave, surface froth and foam and air. Twin foils always encounter different physical situations due to their distance apart. When taking off, the front foil is skimming on the face while the rear foil is still dragging afoot and a half underwater. Even out on the face the front foil can be planing on a smooth surface while the rear foil is in a much steeper part of the wave and battling thru the turbulence from the front foil. It's a recipe for greater confusion and trying to balance the different environments for each foil to get a single cohesive performance. Each foil would be working at a different level of efficiency so whichever foil is less efficient is holding back the overall performance. A bit like a learner on his first set of snow skis.
With a single foil it's relating to a more consistently even area of the wave, controlling it gives a more uniform outcome because the effective area is as concentrated as possible, there are variables from the wave interacting over the area of a single foil but they are more similar than the forces on 2 foils that are maybe 3 feet apart.
Apart from the performance benefits of a single foil, it's also a much simpler object to build and attach to a board. I made dozens and dozens of twin foil combinations in aluminium, plastic, fibreglass/ carbon and wood and only 3 single foil set ups and I wouldn't dream of going back to twins, the difference in speed and control is black and white.
The flat foils are great, you can bottom turn, cutback , stall, even bunny hop at top speed so more aerial stuff is likely.
http://www.youtube.com/watch?v=kdzrqDb4tDg
I'm hoping with the latest surface-piercing foil that I can make a more efficient single foil to tap into more of the energy within the wave.
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edited (by Huck) to add...
[video:http://www.youtube.com/watch?v=kdzrqDb4tDg]