I don't understand why a foil moves forward while in a wave

Prone Surf Foiling
21

So what makes sense to me is that while a foil is flying, the upward force is coming from pressure differences on the top and the bottom of the wing (I.e. lift). In the lift equation (L = Cl * A * .5 * r * V^2), the upward movement of the water molecules affects the coefficient of lift. Therefore, because lift is a function of the square of velocity, the differences in the coefficient of lift make a bigger difference to total lift at higher speeds than lower speeds. This allows the foil to generate upward movement as a function of speed in a way that is different than an object floating on the surface of the water (I.e. a normal surfboard) because the surfboard is not generating upward movement through lift. Is that correct?

22

IMG_1113

I think of the foil as an airplane and the waves are a moving scenario of this diagram where the mountain forces air up the slope. The faster the wind rushes up the mountain, the more lift and speed a glider plane is able to generate.

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23

The water movement comes into the lift equation in two places, the lift coefficient as you’ve identified and also the velocity term.

You have to take your own velocity vector and add the water particle velocity vector. Please look up some info on vector summing to understand this, lots of good visualizations out there.

From this vector sum, you get the actual flow velocity vector over the foil. Since a vector has magnitude and direction, it tells you two things:
-The magnitude is the V term in the lift equation
-The direction gives you the angle of attack. The angle of attack determines the lift coefficient.

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24

I wonder, as in the graphic, the angle of attack as foil moves through water also increases to increase lift (sometimes too much) and contributes to collapse of lift if you run into the back of a wave.

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25

Yes, absolutely. In my mind this is a big part of what makes going up and over such hard work when downwinding.

26

Thanks once again for all the detailed replies. I feel like I have got the information I need to understand the basics. It seems like there are still nuances and comparisons to traditional surfing that could be interesting.