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.

if you hold it right

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

One of the local spots where I sail sometimes is a smallish coastal harbour that has two islands, north and south, linked by break walls to mainland. Theres a marina and jetty etc, all these create wind shadows in different wind directions.

One of the phenomena we experience is a thing we call the "The Hand of God". If you see it happen it looks just like someone blasting along fully powered up and then, like a massive invisible hand just slams the person straight down. It looks bloody hilarious.

When it happens to YOU it feels like the wind changes direction suddenly 180 degrees and you get slam dunked straight down. Very disconcerting!

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 .

I say no.

Ignoring the hand of god scenario..... the sail produces forward and sideways power as we know. As a wing producing lift and canted to windward the lift has to be a tiny bit upwards surely?

The more the sail is canted to windward, the more the lift would be in upwards direction. If the sail is perfectly upright, the lift is all sideways / forward drive.
You could not make downforce with a sail unless the foil was inverted, ie canted to leeward to quite some degree.

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.

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swoosh said..
if you hold it right

Upon reviewing everyones answers offered here, I think swooshes is probably the most accurate

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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Mark _australia said..
I say no.

Ignoring the hand of god scenario..... the sail produces forward and sideways power as we know. As a wing producing lift and canted to windward the lift has to be a tiny bit upwards surely?

The more the sail is canted to windward, the more the lift would be in upwards direction. If the sail is perfectly upright, the lift is all sideways / forward drive.
You could not make downforce with a sail unless the foil was inverted, ie canted to leeward to quite some degree.

I believe you are right about that. Further more, I believe that instead of a downward thrust, a uplift is possible because as you rake the sail towards the back of our board, your sail is actually facing the wind at an angle relative to horizontal axis. That will generate a vertical lift, potentially helping you to plane.

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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.

I say no, this is exactly why we have to hang from the boom to provide mast foot pressure. And like Mark said, lean the sail to windward and lots of uplift.

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?

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Mark _australia said..

>>>

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.

Yes agreed, but in the context of trimming the board, where you want it to stay flatter, (going over chop for instance), this rotational force is what helps in pushing the nose down. And is maybe what the op is talking about.

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?)

I thought newton said, it's the board that produces the down force on the sail...

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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,

I agree with Decrepit. IMHO only..... This is why moving the mast foot forward helps in light wind. We're attached to the board with our feet and the sail with our hands on the boom. The third point of the "triangle" is the mast foot, where the sails power drives the board forward. There will be other vectors too, but sail downforce has to be one of them.

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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

I went out last week and produced nothing but downforce in 5 knots of wind.

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.

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decrepit said..
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.

Yep! Been there, proved that!

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.

I would defer to an aeronautical engineer, and the father of windsurfing, for an answer to this question. RIP Jim.

joewindsurfer.blogspot.com/2008/04/jim-drakes-windsurf-physics.html

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MikeyS said..
I would defer to an aeronautical engineer, and the father of windsurfing, for an answer to this question. RIP Jim.

joewindsurfer.blogspot.com/2008/04/jim-drakes-windsurf-physics.html

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.

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Ian K said..

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.

Designer may already thought about different sail configurations







to prevent this to happen


BTW great link Jim Drake bookmarked it