What is tailwalking and what is wheelies?

[isobars]

[chophop]

I agree with James Douglas' analysis.

The fin is not always true vertical to the water surface. So it is a bit sideways lots of the time. I would also guess that the water passing rapidly under the tip of the fin causes some lift.
For me it is a given that fins cause lift and big "powerful" ones cause more lift than small thin flexible ones.

[B737]

[inarchetype]

This has been illuminating.... as a non-engineer, I have always wondered about the same thing (mystery of a symmetrical foil oriented parallel to direction of travel creating what people call "lift").

It dawns on me that the key is that it isn't perpendicular to the direction of travel, as lee way ensures that direction of travel is actually at an angle to the longitudinal axis of the board, creating the necessary angle of attack for lift to counteract the lee way.

Light bulb goes on... so thanks.

[boardsurfr]

Nice to see a pissing contest about text book formulas. Here's a much simpler way to look at this:
The sail creates "lift" that is more or less perpendicular to the sail (imagine a force arrow pointing straight away from the center of effort). Without a fin, a board would pretty much follow that arrow, going mostly downwind.

The fin keeps the board from slipping sideways. Therefore, it must generate a force that is (in first approximation) equal to the sideward force generated by the sail. Imagine a force arrow on the fin that is pointing towards the wind.

Most of the time while sailing, the fin is attached to the board (if this is not the case, tail walking will not happen, so we can safely ignore the rare exceptions). So take a fin, hold it at the top, and push the center of the fin sideways. Surprise: the fin wants wants to tilt! Attached to the board, it wants to tilt the board so that the windward edge comes out of the water. Longer fin = more leverage => more tilting power. Might have something to do why 70 cm long fins are found on formula gear, but not on wave boards.

So, based on this simplistic analysis, I get the suspicion that the "lift" of the fin (using the aerodynamic term) that is perpendicular to the fin surface creates noticeable "lift" on the windward side of the board simply because the fin is attached to the board on one end.

As to why a symmetric foil generates lift: look at the chicken strap on formula boards. If I understand correctly, it's in the center, and used when going straight downwind (a crazy thing that windsurfers on most other boards never do, except for a very short time when playing in waves). Going straight downwind is the only time when a windsurfer is actually traveling exactly where the nose of the board is pointing. At all other times, the board will also have a certain amount of sideway drift which creates a non-zero angle of attack on the fin, and therefore lift. Usually, we don't notice the sideways drift, but you can sometimes see it in other surfers.

Again, there's a simplistic explanation that may be easier to understand. Think about being on a board with a reasonable large fin and outside footstraps, like a freeride or slalom board, in marginally powered conditions. Often, the easiest way to get planing is to push onto the fin, for example with a foot wrapped around the windward rail. If you want to, think about the water pushing back, which generates the windwards-directed power on the fin. Or if you prefer, think of pushing the fin leewards creates a lower pressure area on the windward side that wants to "suck" the fin back. As before, since the top of the fin is attached to the board, this will torque the board so the windward edge wants to come up.

[jingebritsen]

BTW, flex fins, what ever their size make the sailor have to work harder for control in gusty choppy conditions. stiffer fins do not. for max planing power in glassier venues one goes for lotsa flex. for windier choppier conditions the opposite.

[bred2shred]

[isobars]

It seems pretty apparent that some posters still don't understand that aero/hydro-dynamic "lift" on a sail, a fin, an arm stuck out the window, or a paddle dragged in the water is a technical term unrelated to lifting an object vertically against the pull of gravity like we do in the gym's weight room. Others may not understand that lifting a fin vertically, through whatever real-world means -- is not going to make the board nose paw at the sky. To compound the confusion, ever-present hydrodynamic drag on a tracking fin always exerts a downward force on the board nose.

Mike \m/

[noshuzbluz]

[techno900]

jingebritsen said