Can Sails Produce Downforce?

Mast foot pressure would seem to be the product of the sailor hanging weight on the boom. Some have said that sails can offer downforce too. Could that be true? I would have though they produce a little upwards lift and a lot of side force.

Push your mast base all the way forwards in its slot when powered. See what happens...

Whilst it would make the answer blindingly clear, I wouldn't recommend doing it in chop

I have experienced this phenomenon "the hand of god" like greg bob said sailing beside the third runway botany a few years ago. I was fully powered up in a black north easter .l got slammed .felt like the wind had hit be from above.the guys said l was sailing too close to the runway .

Maybe its a matter of symantecs. A race sail pins a board down more then a wave sail. Similarly less downhaul pins the nose down more and more downhaul makes it light. If its not downforce then its just adjusting the trim of the board between the front and back. Now a sail boffin can tell us the truth.

It's a rotational thing, forward thrust at the tip, pivoting at the boom, wants to rotate the whole rig into a forward loop, so this is applying down force at the nose, and up force at the tail. Obviously this is more pronounced the higher the lift in the sail is. Which is also affected by sail size/aspect ratio and our old friend downhaul.

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decrepit said..
It's a rotational thing, forward thrust at the tip, pivoting at the boom, wants to rotate the whole rig into a forward loop, so this is applying down force at the nose, and up force at the tail. Obviously this is more pronounced the higher the lift in the sail is. Which is also affected by sail size/aspect ratio and our old friend downhaul.

I know it is semantics but the sail is not producing a downward force.
The sideways and forwards power of the sail makes the mast want to move and that acts upon the board ..... but the sail is not producing a downwards lift force.

Mark is right, if mast is leant towards windward all it can produce is lift. This is 99% of normal efficient windsurfing. If you are grovelling in 5 knots and just balancing on your board, the mast may lean away from the wind and produce some downforce but who would care?

Interesting responses. I was/am suspicious of the idea that a sail would intrisically create downwards pressure as a result of its own aerodynamics. I'm more included to believe that the sailor's control of the power in the sail has the secondary effect of pressing the board down, the mast being one leg of a tripod formed with the sailor's legs. Essentially the sail and mast pulling forwards must be trying to rotate downwards around an axis formed where the harness lines attach to the boom... (Decrepits theory?)

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ikw777 said..
>>> the mast being one leg of a tripod formed with the sailor's legs. Essentially the sail and mast pulling forwards must be trying to rotate downwards around an axis formed where the harness lines attach to the boom... (Decrepits theory?)

Yep,

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ikw777 said..
Interesting responses. I was/am suspicious of the idea that a sail would intrisically create downwards pressure as a result of its own aerodynamics. I'm more included to believe that the sailor's control of the power in the sail has the secondary effect of pressing the board down, the mast being one leg of a tripod formed with the sailor's legs. Essentially the sail and mast pulling forwards must be trying to rotate downwards around an axis formed where the harness lines attach to the boom... (Decrepits theory?)

Yes that last line - but the to is not making the downards pressure, it is the sail's forward force making other tings wanna torque around a certain point.
I think I took your question too literally

The time when you find out about this is when you're sailing flat out a tad overpowered then get hit by a gust. If you sheet out, the power in the head decreases, the forward force at the head reduces, less down force on the nose and you're tail walking out of control.

Less upwards force is also kind of a downward force ;) I guess it depends on what you practically consider as zero. If sail 1 produces more upward force than sail 2, then in terms of practice, you could argue that sail 2 has more downforce.

That's a great reference Mikey. Finally someone with an estimate for the drag to lift ratio of a planing windsurfing hull. it's 0.15 !

So now we can answer that old question which comes up here time and time again. How much drag is contributed by the board vs. the fin? Well Jim has just ignored fin drag from my quick read. And you can see why.







The lift provided by the fin is 61.6 lbs compared to the lift of the hull of 216 lb.

216 * 0.15 = 32.5 lbs ,

the figure Jim gives for hull drag in figure 8.

Now the drag to lift ratio of a fin should be less than 0.15, it's a fully submerged foil, much better than a hull, as the moth foilers know only too well.

eprints.bournemouth.ac.uk/12235/1/Simon_Fagg.pdf

Simon Fagg gives a L/D ratio of a fin between 20 and 30. So the drag on the fin might be only 61.6/25 = 2.5 lbs. Less than 1/10th the hull drag. No wonder Jim has ignored it!


Can a sail produce downforce? You could use rocket science 101, equation 1.

F = d/dt(mv). (1)

If you can imagine the sail deflecting horizontally flowing air (mv) upwards then yes. My guess is that most of the air deflection is horizontal, or a little downwards. I'll go with Mark's explanation.