ELI5. How do some sails press down on the board

Lots of things can help Pin a board down. Seam shaping and batten alignment are the biggest factors.

Most Freestyle sails have a batten above the boom at an angle and not flat, this not only drives the board forward but also lifts the rider, helping with "POP". Straighter battens usually create more forward drive and less lift driving the board forward and not lifting the rider so much.

My understanding is that it's a leverage thing.
Mainly due to lift at the top of the sail.
An upright sail, will have better leverage over the board than a raked back sail.
More downhaul and more twist at the top of the sail will also lessen this leverage effect.

Mast position also alters the leverage over the front of the board, further forward will help to hold the nose down for chop, and further back will give a greater angle of attack for smooth water.

How that ties in with batten angle I don't know.

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Paducah said..
I've seen this mentioned many times especially in relation to slalom race sails but what are the actual forces that make this happen (apart from sailor pressing down on either harness or boom)?

This is a much misunderstood concept, so we get some very wrong answers to questions like these.
Sails don't actually 'press down on the board', but the rig exerts a leverage when sheeted in, and which is greater with tight leech sails.

Let's start with the basics.
When you stand on your board and the sail is not sheeted in, the sail material can flutter, because there's no forward drive. At that point the wind is flowing along the sail cloth with equal pressure on either side, like with a flag.

When you sheet in the sail takes on an aerofoil shape, and then it deflects the wind and creates forward drive, and your board if set across the wind direction starts to move forwards. If the board can not move forwards then you catapult, because the top of the sail moves forward when the board cannot. When you catapult your are ejected off the board in a forwards direction. If instead you let go of the rig, it moves forward alone, and often smashes the nose of your board. Damage is expensive, so it worth understanding the forces generated by your rig. The leverage forces need to be compensated by your body weight, so we lean back when sheeting in, direction the leverage forces of the rig through our feet and onto the board.

While most of the sail drive is then moving you forwards, other forces apply, such as the sideways force which the fin 'lift' or daggerboard must resist. But there is also a leverage force which acts downwards as you first sheet in, and we call that mast foot pressure.
That mast foot pressure can be directed by the sailor to help the board keep tracking without luffing into wind. The mast foot pressure comes from leverage at the top of the sail once that part is sheeted in.
The key is to lean back as you sheet in and, with the sail under control, you don't catapult but you move forwards and on to planing speed.

When you are planing along comfortably, there is less mast foot pressure but the luff length of the sail (or mast length) means there is still a leverage force acting downwards from the rig drive.
Modern slalom sails are built with leach twist to match the wind angle when sailing across the wind but that dynamic twist (along with a flexing mast) also allows the sail to dump power when you are over powered. If you control that sail twist with downhaul tension you can also create more downward force by using less downhaul tension.

What we find is the tighter the upper leach the more downward force there is. Another way to look at that is: the flatter the top battens are in the upper part of the sail the less power they generate, and the less downward force.
So the sails that apply the most down force will be tight leech sails and those with with fullness (curvature) cut in the upper seams.

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decrepit said..
My understanding is that it's a leverage thing.
Mainly due to lift at the top of the sail.
An upright sail, will have better leverage over the board than a raked back sail.
More downhaul and more twist at the top of the sail will also lessen this leverage effect.

Mast position also alters the leverage over the front of the board, further forward will help to hold the nose down for chop, and further back will give a greater angle of attack for smooth water.

How that ties in with batten angle I don't know.

The first part of your analysis is totally correct, but the second part is not.
If you put the mast foot forwards that does not increase leverage, but reduces it.
In windy weather we put the mast foot forwards to increase sailor weight on the tail of the board, and to take the mast foot pressure away from a railing fin, and that brings the board back into control. Nothing to do with 'holding the nose down', and more to do with using body weight to prevent tail walking.

Would it be wrong to think when going fast there is no sail pressure on the mast foot and actually there is an pulling up force ? Hence needing harness weight to hold it down ? So wouldn't it be more correct saying , "some sails have less lift than others " ? Not that i would want to try , if going fast with no harness weight and minimal arm weight on the boom and the mast base join suddenly and completely snapped , does the sail go up in the air or smash down on the board ? The faster i go there seems to be less wheight on the remaining part of the board that is touching the water. So the whole kit and caboodle is lifting off the water more than what the fin is lifting
Just seems less weight on the board. Also when moving the base forwards or back changes the centre of gravity combining rider and boom weighting with harness and arms , more so than changing the dynamics of the sail with more or less angle. ??? I believe in levering forces around the boom and how that changes with different leech tension and other aspects of the sail. I've got popcorn , this could get ugly.

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Imax1 said..
Would it be wrong to think when going fast there is no sail pressure on the mast foot and actually there is an pulling up force ? Hence needing harness weight to hold it down ? So wouldn't it be more correct saying , "some sails have less lift than others " ? Not that i would want to try , if going fast with no harness weight and minimal arm weight on the boom and the mast base join suddenly and completely snapped , does the sail go up in the air or smash down on the board ? The faster i go there seems to be less wheight on the remaining part of the board that is touching the water. So the whole kit and caboodle is lifting off the water more than what the fin is lifting
I've got popcorn , this could get ugly.

I'd agree with that. I've never felt that once over 18-20 knots board speed the mast foot was pushing down on the board much if at all. And I've had a Boge UJ pin break and I fell backwards and the rig went more upwards than downwards, although that would have been to the new forces at work at the time.
This topic has been discussed on the old Boards forum for decades. That forum wound up 2017ish, but the topic was recurrent. Some insisted that MFP, pinning the nose was always there, usually the really old school type or wave sailors who have never gone that fast. That always got ugly and no agreement between some.
One poster (JKinhismassivevan) who used the North shock absorber extension said he stopped using them because at speed they were extending. And he did have a massive van and was a fast sailor. Back then, I found a video on UT from Nik Baker promoting the extension, which had a cam on the front of the board looking back, and this was showing the extension extending as the board started planing and going faster. It wasnt clear that there was a net lift at speed, but it certainly didnt show the extension compressing at speed. I cant find the video now, it must have been deleted.

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Subsonic said..

PhilUK said..

Imax1 said..
Would it be wrong to think when going fast there is no sail pressure on the mast foot and actually there is an pulling up force ? Hence needing harness weight to hold it down ? So wouldn't it be more correct saying , "some sails have less lift than others " ? Not that i would want to try , if going fast with no harness weight and minimal arm weight on the boom and the mast base join suddenly and completely snapped , does the sail go up in the air or smash down on the board ? The faster i go there seems to be less wheight on the remaining part of the board that is touching the water. So the whole kit and caboodle is lifting off the water more than what the fin is lifting
I've got popcorn , this could get ugly.

I'd agree with that. I've never felt that once over 18-20 knots board speed the mast foot was pushing down on the board much if at all. And I've had a Boge UJ pin break and I fell backwards and the rig went more upwards than downwards, although that would have been to the new forces at work at the time.
This topic has been discussed on the old Boards forum for decades. That forum wound up 2017ish, but the topic was recurrent. Some insisted that MFP, pinning the nose was always there, usually the really old school type or wave sailors who have never gone that fast. That always got ugly and no agreement between some.
One poster (JKinhismassivevan) who used the North shock absorber extension said he stopped using them because at speed they were extending. And he did have a massive van and was a fast sailor. Back then, I found a video on UT from Nik Baker promoting the extension, which had a cam on the front of the board looking back, and this was showing the extension extending as the board started planing and going faster. It wasnt clear that there was a net lift at speed, but it certainly didnt show the extension compressing at speed. I cant find the video now, it must have been deleted.

I get where the thinking comes from, but sheet out whilst at speed and see what happens.

If you sheet out, you've unhooked and there arent the same forces through the sailor as before. The only way to measure the forces through the mastfoot are by measuring it via a loadmeter. The only thing in existence we could current use is that old North Shox extension and video it. I see Duotone dont make that any longer.

Found that video with the North Shox. And its less clear than I remember, but I still get the impression that as he sails faster the extension is extending. I dont think its compressing. But it is with 2013 sails, not todays.

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Matt UK said..

Basher said..

Matt UK said..
Lots of things can help Pin a board down. Seam shaping and batten alignment are the biggest factors.

Most Freestyle sails have a batten above the boom at an angle and not flat, this not only drives the board forward but also lifts the rider, helping with "POP". Straighter battens usually create more forward drive and less lift driving the board forward and not lifting the rider so much.

Sorry mate, but that's so wrong.
It is however true that freestyle sails can be relatively powerful for their size. That is where they are cut with a tight leach - or set will little downhaul, to retain more shape in the upper sections of the sail. A freestyle sail is set for acceleration, and not for top end speed.

'Pop' is something totally different.
Straighter battens would in fact provide less lift, or less drive.
The rig does not lift the rider - although that is a complicated discussion about equal and opposite forces.

What do I know.

I have worked for Pryde, Naish and North sails as a sail maker.
Seams are usually shaped under the batten pockets as the batten supports the seam shaping. As the angles increase at these points then so does the lift. A straighter better will produce more forward drive and an angled batten more lift.

I also said it helped the rider with POP, not produces it. The Dacron luff panel also helps with a quicker inflation of the luff area.

Well thanks for your explanation but I still disagree.
It is true that broad-seaming shapes the front of the sail and can determine if a sail is more 'forward-pulling' or not, but the batten angle is irrelevant. With no broad-seaming, you tend to get a sail that is more sideways pulling - and that's often the better choice for wave sailing.
What's important is the profile at the front of the sail and what overall foil shape it creates. In addition to broad-seaming, we also have luff round and vertical seaming for twin luff sails as contributing factors to overall sail shape. But changing the angle of the batten will not create upwards or downwards lift.
From my own sailmaking days we'd tend to align the battens so that they were parallel with the airflow when the board was in a locked down planing position - and that was to minimise windage/drag.


There have been several other posts here about whether or not mast foot pressure (MFP) exists, and that discussion is of course a bit of a rabbit hole.
My view is that when we first sheet in and accelerate there IS mast foot pressure, but once planing the load on the mast foot becomes more neutral. Board trim is actually determined by the sailor's weight over the board, and how the rig is set in terms of mast rake matched to sailing stance. I feel it's meaningless to say the sail 'holds the nose down' .

For people using slalom gear, or freeride kit with a long single fin, there's another discussion to be had about how the fin rake controls the boards planing trim. But that's another rabbit hole.

PS. I think I mostly agree with what imax1 is saying, above, but which way the mast foot goes when you break a U/J varys, mostly because of the different boom heights we use. The discussion is a useful reminder to check U/Js and deckplates before the start of a new season.

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Matt UK said..
Lots of things can help Pin a board down. Seam shaping and batten alignment are the biggest factors.

Most Freestyle sails have a batten above the boom at an angle and not flat, this not only drives the board forward but also lifts the rider, helping with "POP". Straighter battens usually create more forward drive and less lift driving the board forward and not lifting the rider so much.

Batten alignment makes no difference, overall sail shape does and that is down to seam shaping as well as other things.

probably get different results for every rider and every session

It would also be good to be able to measure the aerodynamic lift from the front of the board.

yes armchair mode can be wrong.
But saying that, I still go with upper drive, transferring through boom and sailor, to foot srtaps. imparting a rotation moment to the board, pulling up on the rider and pushing down on the nose. Yes this will be a stronger force at lower board speed, when there is more force in the sail. But as long as there's any force in the sail, this effect is still present.

Vertical lift from the fin, will push the back of the board up. Depending on where the riders weight is will affect if that pushes the nose up or down. Certainly the rotational force will lift the windward rail, increasing aerodynamic lift. I suspect it's this effect rathe than fin vertical lift, that gets the nose up. But I have no experience of this as all my finds are stiff short weedies with no vertical lift element.

My uneducated take is that you need the mast raked at least a little to windward to get downward pressure on the mast base. Two reasons why:
- The net lift force on an airfoil is usually perpendicular to the wing surface. If the mast is dead vertical then the force is going to be horizontal, but when the mast is raked to windward then a component of the lift force is in the downward direction.
- Try sailing a foil downwind on a broad reach with the mast upright (or even raked forward) versus raked to windward a bit. Maybe it's just me but it feels like a huge difference in downforce.

I'm not even going to bother on this site any more, theres far more people on here that have probably never built a sail that know far more than a sail maker does.......

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Ben1973 said..
It's wouldn't be that hard to fit some strain gauges under the deck plate, on the boom and under the foot straps then it would be easy to see what's going on

I was thinking that too. Nothing beats measurement. I've got a couple of wheatstone bridge strain gauges in my gash box. I suppose they would have to go on the extension and modified harness lines so they can be calibrated easily ?. I'm busy at the moment. But might investigate when I become bored.