I always presumed that the most optimum angle of the sail to the wind is to be perpendicular 90 degrees to the wind- and with apparent wind shifting the wind to come more from the direction of movement perhaps even a bit smaller than that, especially when going upwind.

So on a perfect beamreach this would mean to keep the sail directly in line with the board or slightly closer to account for apparent wind.

When trimming the sail I find this is not the case at all and the fastest angle seems to be perhaps 110 degrees to the wind or so - so maybe 20-30 degrees to the board. And while it is definetly true that the more windward I turn the sail needs to be held closer to the body the effect isn't as strong or linear as predicted. However, when going perfectly downwind the fastest speed does seem to be achieved by holding the sail 90 degrees to the wind and 90 degrees to the board.

Can someone explain this?

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Kit3kat said..
I always presumed that the most optimum angle of the sail to the wind is to be perpendicular 90 degrees to the wind- and with apparent wind shifting the wind to come more from the direction of movement perhaps even a bit smaller than that, especially when going upwind.

So on a perfect beamreach this would mean to keep the sail directly in line with the board or slightly closer to account for apparent wind.

When trimming the sail I find this is not the case at all and the fastest angle seems to be perhaps 110 degrees to the wind or so - so maybe 20-30 degrees to the board. And while it is definetly true that the more windward I turn the sail needs to be held closer to the body the effect isn't as strong or linear as predicted. However, when going perfectly downwind the fastest speed does seem to be achieved by holding the sail 90 degrees to the wind and 90 degrees to the board.

Can someone explain this?

It's not just about sail angle to wind angle.


when you're properly powered it lets you shift your weight back by straightening your front leg, which in turn changes the sheeting position of the sail out (because it's tethered to you by the harness lines). the whole weight shift and more open sail reduces the mast base pressure on the board, and allows it to trim on its tail. Less board in the water=more board speed, with everything else kept in trim of course.

The apparent wind angle change isn't as drastic as you might think sailing on a beam reach. It's when you dive off wind that it really starts shifting.

Apparent wind changes everything.

Are you planing?

If you use the parallelogram rule to work out the apparent wind, you will find that the stronger the wind the more you can bear away.
A good explanation:
seatern.uk/wp-content/uploads/2017/08/The_Physics_of_Apparent_Wind_version2.pdf

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Grantmac said..
Are you planing?

Yeah, reading OP again, its a fair question.

yes sorry I forgot to mention this. I am a kitesurfer so I am well aware of weight reduction i.e. in lightwind you can go upwind by keeping the kite very high which reduces your weight from i.e. 75kg to 20kg which now means you need hardly any "power" to push your 20kg effective bodyweight upwind even though the kite technically generates less momentum/power for you by being so high- the same obviously goes for a windsurfboard where more "lift" makes it easier to go upwind and faster crosswind/broad reach.

However, this is on a longboard while it's not planing. I checked some formulas and the theoretical 100% displacement mode formula is basically just waterline lenght x a constant which on a longboard is only around 4 knots of boardspeed. We all know that you don't feel like the board wants to plane at around twice that + a bit extra (i.e. my board feels like it's lifting properly out of the water at 8 knots or so of speed and at 9-10 knots it definetly seems to be mostly planing). So possibly going 4-8 knots there may be a lot of lift contributing to partial planing and speed increase even though the board doesn't really feel or look like it's planing.

That said I have been out in 3 knots of wind and I find even in full on displacement mode the angle of the sail isn't really much different and if anything slightly more open - presumably due to lack of apparent wind. I.e. when going crosswind in 3 knots of wind the sail seems to still work best around 30-35 degrees to the board or so. And that such low speeds you can't hang off the sail at all, you basically hold it upright and stand superclose to it. But i don't know whether the daggerboard will change things and as such low speeds its all rather unstable anyway and the wind isn't that "feelable".

The effect of fully sheeting in on board speed going crosswind is profound. If I go i.e. 5 knots perfectly crosswind and sheet in so the sail is at 0 degrees to the board it essentially will lose most of its speed. If I go i.e. 10 knots and the board is planing I find it harder to tell anyway as there are too many other factors at play and the board will still go reasonably fast when the sail is held at 0 degrees to the board though definetly slower than i.e. 20 degrees to the board.

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remery said..
Apparent wind changes everything.

Apparent wind means the wind will come more from the direction you travel at which theoretically means you want to hold the sail closer to your body? I do think this kinda does happen as I find the faster I go the closer to the body I want to hold the sail - which would be well explained by apparent wind shifting the wind more to come towards the direction of travel?

edit: just reading the paper you posted. They also seems to suggest that the wind comes more leeward the faster you travel if going upwind. So the sail should be held closer to the body? Unless you are going downwind where the wind will shift more windward?

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Kit3kat said..
yes sorry I forgot to mention this. I am a kitesurfer so I am well aware of weight reduction i.e. in lightwind you can go upwind by keeping the kite very high which reduces your weight from i.e. 75kg to 20kg which now means you need hardly any "power" to push your 20kg effective bodyweight upwind even though the kite technically generates less momentum/power for you by being so high- the same obviously goes for a windsurfboard where more "lift" makes it easier to go upwind and faster crosswind/broad reach.

However, this is on a longboard while it's not planing. I checked some formulas and the theoretical 100% displacement mode formula is basically just waterline lenght x a constant which on a longboard is only around 4 knots of boardspeed. We all know that you don't feel like the board wants to plane at around twice that + a bit extra (i.e. my board feels like it's lifting properly out of the water at 8 knots or so of speed and at 9-10 knots it definetly seems to be mostly planing). So possibly going 4-8 knots there may be a lot of lift contributing to partial planing and speed increase even though the board doesn't really feel or look like it's planing.

That said I have been out in 3 knots of wind and I find even in full on displacement mode the angle of the sail isn't really much different and if anything slightly more open - presumably due to lack of apparent wind. I.e. when going crosswind in 3 knots of wind the sail seems to still work best around 30-35 degrees to the board or so. And that such low speeds you can't hang off the sail at all, you basically hold it upright and stand superclose to it. But i don't know whether the daggerboard will change things and as such low speeds its all rather unstable anyway and the wind isn't that "feelable".

The effect of fully sheeting in on board speed going crosswind is profound. If I go i.e. 5 knots perfectly crosswind and sheet in so the sail is at 0 degrees to the board it essentially will lose most of its speed. If I go i.e. 10 knots and the board is planing I find it harder to tell anyway as there are too many other factors at play and the board will still go reasonably fast when the sail is held at 0 degrees to the board though definetly slower than i.e. 20 degrees to the board.

Makes more sense now.


if you're on a beam reach and sheeting the sail 90 degrees to wind direction, you're effectively giving the wind a "wall" to hit straight on. The sail will work much more effectively if its a glancing blow on the sail. Less speed, less apparent wind angle change too.

it would be interesting to attach some tell tales to the sail and see exactly what the optimum sheeting angle is.

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Subsonic said..
it would be interesting to attach some tell tales to the sail and see exactly what the optimum sheeting angle is.

I think that will depend on sail shape, lots of draft forward should allow tighter sheeting