I am puzzled by some of the comments regarding foil shimming to change the angle of the foil to the board. I recently shimmed my foil - the Smik sup I have has a lot of tail rocker and the foil box is quite far back. My logic was that when I was paddling into a wave - the foil was definitely nose down causing drag not lift. Now my logic is that although this will affect paddling in general - on a steep enough wave - it will be fine as you will be standing well back anyway. But I want to do downwinders so I think that it should be at least parallel on the normal paddle position - if not slightly lifting. I have heard people say that it make the foil unstable AFTER take off. For the life of me I cannot understand that? Unless you are lucky enough to live at a class wave spot - take off is everything, a few degrees will make lot of difference. But once you are flying surely the board is just as happy - a few degrees up or down from optimal . I have a few kite foil boards - they all fly at different angles to the foil and I dont care. Crucially with a kite you have so much power and something to push against - that take off is easy.

Why do some think angle matters after take off? am I missing something ?

It matters heaps if you Foil is facing down it drags like an anchor and brings your wave count way down , plus when you are up and foiling the nose of the board has to point up to get the foil level making hard to ride.
Just shim it so it level to the deck and all is good.

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Piros said..
It matters heaps if you Foil is facing down it drags like an anchor and brings your wave count way down , plus when you are up and foiling the nose of the board has to point up to get the foil level making hard to ride.
Just shim it so it level to the deck and all is good.

That's my thinking as well Piros.
However my Naish 120 Crossover designed for SUP foiling without shims the Naish XL foil points down.
I am told this is a deliberate design feature.
The Naish 122 SUP foil board that has no tail rocker has a channel to obtain the same down foil angle.
I use the same Naish foils for kite foiling on flat skim boards so foil is same angle as the deck and all good.
What do you think?

It affects some foils worse than others , on some you notice it on the first stroke. Not sure if Naish is doing that deliberately or that's the way they came as you cannot offset track mounts but you can offset Tuttles to suit the rocker line. Some boards float different so another way to check is to put it in the water and let it float and have a look under water so maybe that's were Naish is coming from. It may look wrong on land but sit straight in the water , regardless of that If your foil is on an angle with the board floating on the water your board will need to be on a angle in flight. Foils do not foil on an angle they foil flat. It's not rocket science , tilt that front wing up or down in flight you get launched or face planted.

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Piros said..
Foils do not foil on an angle they foil flat. It's not rocket science

Well, they definitively foil at an angle in order to provide enough lift to support the rider. And it is not rocket science, it is hydrodynamics.. rockets do not travel in water :-)

The angle depend on the foil profile and the speed, but for a thin foil taken as example in this MIT course, the optimal angle maximum lift/drag ratio() is 3 to 4 degrees: web.mit.edu/2.972/www/reports/hydrofoil/hydrofoil.html

But I guess you meant that foils do not foil at a large angle of attack, and you would be right: "A steep angle of attack is not needed in the design of the hydrofoil. On the contrary, small angles of attack are used on hydrofoils to optimize the lift to drag ratio as explained before". But there must be a positive angle, even if it is a small one.

Usually we are talking small angle - do you really care if your board is up or down 3 degrees when up and foiling ? I agree level makes sense>

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colas said..

But there must be a positive angle, even if it is a small one.

This is NOT CORRECT.

Our surf foils are not thin they are thick, heavily cambered foils. Otherwise the kiters would have been surfing on their foils a decade ago. What Alex Aguerra did to break open this space was to built a big fat Cessna style foil.

Any introductory course in aeronautics will teach a concept called "circulation." It is the rotational flow created by either the camber (arc'd shape of the wing), or angle of attack of a symmetric foil. Heavily cambered foils generate significant lift at ZERO angle of attack. These foils also generate THE LOWEST DRAG at a NEGATIVE angle of attack.

And yes it is related to Rocket Science because both Airfoil design and Rocket science start with Bernoulli's principle. This is a law that is so NON-Intuitive they put a guy's name on it.

Long story short this stuff if largely imperical and therefore 'What feels good' is what works (Unfortunately). I brought the negative angle of attack foil mount theory to Stretch (board designer) in Santa Cruz (take a look at your thruster fins if you want to see negative angle of attack employed with cambered wings) he summed it up by saying

"There is so much going on down there we just make the foil parallel to "flat" bottom of the board."

I found there is definitely a downside of having the mast angled forward too much. You can create a bigger angle of incidence that will have more lift on takeoff but this makes it harder to control the foil at higher speeds. Most foils already have an angle of incidence of about 2 degrees relative to the fuselage (and to mast at 90 degrees to fuselage) and like Beasho said, at high speed you need to fly the wing at zero degrees or even a negative angle to keep it from creating too much lift. So if the mast is angled forward you need to fly with the nose of the board tilted slightly downward to reduce the angle of attack at higher speeds.
You want to fly the foil low to the water surface for more control if you are going fast and any touching of the water surface with the nose angled downward is a show stopper. Having the right wing angle is especially critical for downwind foiling where you want enough lift for low speed takeoffs but you also need to be able to control the big wing when you are flying low to the water surface on fast moving bumps with cross chop. This is much easier to pull off if you can trim the foil angle of attack close to zero with the bottom of the board close to parallel to the water if that makes any sense. If you even just slightly touch the water surface with the nose angled downward, you are going over the handlebars.

Beasho, Cessnas definitively fly with a positive angle of attack.

This is because they need lift to support the plane, just like our foils need lift to support us, unlike surfboard fins.
The lift value at the optimal lift/drag ratio is not enough at normal speed.

But we are speaking of very small angle values anyways.

After initially mucking around putting foil track mounts into an old SUP that needed lots of shimming to set the angle. I got a Sunova 2 in 1 Foil board & the first couple of sessions on that with the Naish Large wing felt like I was dragging a boat anchor.

With a little trial & error I found a perfect shim - trim for this board. Paddles well, catch waves & easy to control even at speed.
You would think that a purpose build board wouldn't need shimming, but they do.
It could even be down to little things like how heavy you are, where you stand, etc that change the way the board sits in the water. Shimming is a quick fix. Just adjust with small increments - it won't take much. Not enough it sucks to paddle. Too much & it's unsteady at speed.

I get blue planets explanation perfectly except having a convertible Smik Hipster, I have a lot of rocker at either end. If you are riding a fugly Dave Kalama style board ( to be clear Dave is a handsome guy - the board is fugly - but I still would have one in heartbeat!) I am sure its a big deal tho. No I really do not get the description unsteady at speed from Spartacus ? I have been cruising at 30 knots on a kiteboard and the angle of the board to the foil really doesn't matter at all . I have had an early sroka board that did seem to ride nose down - I wouldn't say it was unstable - just that it was kinda weird and perhaps a little unsettling as like blueplanet says you will feel like your going over the front if you make too much of a mistake. Is that what you mean?

I'd like to ask about how the angle is measured for a foil wing.

If it's say, 2? positive, is that measured from the chord line or the flat bottom of the foil (assuming it's flat.)

I've wondered about this and am about to mount up my homemade foil.

Slight update - I was surfing some fairly big sets at the weekend and definitely felt the takeoff would be better if I lost some angle ! I think there may be a market for an adjustable plate.

Hi Icharus,

Your right it's not unstable to ride along the wave, but it does make it harder when you try to pump back out. I too found cruising along with a kite with the foil not "tuned: was not an issue. You have plenty of time foiling with the kite & you learn to adjust to that balance - even if it's with more pressure on one foot then the other.

Surf foiling is a lot harder then foiling with a kite. The instant acceleration on taking off on a wave, the speed/dynamics of the wave & then when I'm pumping (or is that trying to pump) back out - that's really when I notice how much the board balance plays a part. The better tuned & balanced helps speed up the progression.