Nicely said Wal, you beat me to it.
The universal only stops the down force if you let go of the sail, (until the mast smashes the nose off anyway)
Select to expand quotedecrepit said..
Nicely said Wal, you beat me to it.
The universal only stops the down force if you let go of the sail, (until the mast smashes the nose off anyway)
I've done that experiment before.
Can confirm excessive damage to the nose of the board.
Select to expand quoteNotWal said..Nice sketch. The board, rig and sailor can be considered a fixed system. The flexible UJ doesn't matter. All that is necessary is that all the bits maintain the same general relationship to each other. So you have a system that is driven at the centre of lift of the sail ( somewhat above the boom) and resisted at the bottom of the hull. This generates a pitching moment.
This is equation of sail and board as rigid conection. Fs x 106=W x 29...
Imagine that sailor stand 100% upright,(his center of gravity is above his feet),he will exert no pull to the sail,so sail force must be zero.
Yet the diagram implies it makes no difference. think about it..
same applied on pitching moment as well
If sail, board and sailor stay constant, the following increases tail walking
Bigger fin
Lifts windward rail and allows wind under the board
Higher boom
The sail is raked further back and creates more vertical lift
Mast foot back
Shorter lever arm for mast foot pressure from rear of the board
Front foot strap back
Shorter lever arm for front foot pressure from rear of the board
Good sailors 'fly the fin'. They adjust the above so that the board is on the edge between fast and uncontrollable.
As far as I can see, lifting the boom higher will not rake the sail back further. The centre of effort and the center of resistance will remain the same so the rig rake will be the same.
The relationship postulated between the position of the front foot(strap) and weight on the board is also highly dubious if the body position remains in the same relationship to the sail CoE and the CoR. For instance, if I am reaching and there is virtually NO weight on my front foot (as I have often experienced), moving it forward or back won't change that (near zero) foot pressure. Moving the front foot back or forward does change the bodyweight centre of mass lower or higher, especially on a broad reach. So moving the foot forward will lower the sailors centre of mass and may increase mast foot pressure.
Or to think of it another way. If you are sailing along and you lift your front foot off the board, where does your weight (support) go? I suggest that any weight that was on your front foot, now is on the boom and through the mast foot. Try it. 
Select to expand quoteUSA46 said..NotWal said..Nice sketch. The board, rig and sailor can be considered a fixed system. The flexible UJ doesn't matter. All that is necessary is that all the bits maintain the same general relationship to each other. So you have a system that is driven at the centre of lift of the sail ( somewhat above the boom) and resisted at the bottom of the hull. This generates a pitching moment.
This is equation of sail and board as rigid conection. Fs x 106=W x 29...
Imagine that sailor stand 100% upright,(his center of gravity is above his feet),he will exert no pull to the sail,so sail force must be zero.
Yet the diagram implies it makes no difference. think about it..
same applied on pitching moment as well
The 29 is the distance the sailor's weight is offset from the centre. If he's standing upright on the centre line of the board that factor will be zero and that resolves to zero both sides of the equation.
Select to expand quoteNotWal said..USA46 said..NotWal said..Nice sketch. The board, rig and sailor can be considered a fixed system. The flexible UJ doesn't matter. All that is necessary is that all the bits maintain the same general relationship to each other. So you have a system that is driven at the centre of lift of the sail ( somewhat above the boom) and resisted at the bottom of the hull. This generates a pitching moment.
This is equation of sail and board as rigid conection. Fs x 106=W x 29...
Imagine that sailor stand 100% upright,(his center of gravity is above his feet),he will exert no pull to the sail,so sail force must be zero.
Yet the diagram implies it makes no difference. think about it..
same applied on pitching moment as well
The 29 is the distance the sailor's weight is offset from the centre. If he's standing upright on the centre line of the board that factor will be zero and that resolves to zero both sides of the equation.
You didnt understand me,I said if he stay 100% upright on rail(not on centerline).
If you stay 100% upright on the rail, sail force must be zero becuase your righting moment is zero.
This diagram implies it makes no difference.
Yes the diagram is impossible with a uni joint. if the sailor is exerting no force on the rig, then there is no force keeping the windward rail up, and the board would no longer be level. The sailor must be leaning back for that to work.
But nobody is saying that the sailor is upright in fig 3 the sailor is obviously leaning out
Select to expand quotesailquik said..
As far as I can see, lifting the boom higher will not rake the sail back further. The centre of effort and the center of resistance will remain the same so the rig rake will be the same.
The relationship postulated between the position of the front foot(strap) and weight on the board is also highly dubious if the body position remains in the same relationship to the sail CoE and the CoR. For instance, if I am reaching and there is virtually NO weight on my front foot (as I have often experienced), moving it forward or back won't change that (near zero) foot pressure. Moving the front foot back or forward does change the bodyweight centre of mass lower or higher, especially on a broad reach. So moving the foot forward will lower the sailors centre of mass and may increase mast foot pressure.
Or to think of it another way. If you are sailing along and you lift your front foot off the board, where does your weight (support) go? I suggest that any weight that was on your front foot, now is on the boom and through the mast foot. Try it.
Daffy, as the sail is raked back, the centre of effort moves back, which allows the sailor to straighten their front leg and lean back, the board rides on the last 400mm of length and hence the centre of lateral resistance to moves back to balance the movement in the centre of effort, as per the following
The higher the boom attachment to the mast, the greater the potential rake of the mast and hence the movement toward the the tail of the board.
Select to expand quoteJohn340 said..
Daffy, as the sail is raked back, the centre of effort moves back, which allows the sailor to straighten their front leg and lean back, the board rides on the last 400mm of length and hence the centre of lateral resistance to moves back to balance the movement in the centre of effort, as per the following
I completely agree with that,
but I dont think JUST raising the boom will do it. In any given wind strength and board speed I don't think raising the boom alone will rake the rig back further and make you go faster.
One way to test the theory would be to take it to the extreme and see what happens. 
BTW, you are looking mighty nice there John. 
Select to expand quotesailquik said..John340 said..
Daffy, as the sail is raked back, the centre of effort moves back, which allows the sailor to straighten their front leg and lean back, the board rides on the last 400mm of length and hence the centre of lateral resistance to moves back to balance the movement in the centre of effort, as per the following
I completely agree with that, but I dont think JUST raising the boom will do it. In any given wind strength and board speed I don't think raising the boom alone will rake the rig back further and make you go faster.
One way to test the theory would be to take it to the extreme and see what happens.
BTW, you are looking mighty nice there John.
Thanks Daffy, it sure felt good at the time
Select to expand quotesailquik said..John340 said..
Daffy, as the sail is raked back, the centre of effort moves back, which allows the sailor to straighten their front leg and lean back, the board rides on the last 400mm of length and hence the centre of lateral resistance to moves back to balance the movement in the centre of effort, as per the following
One way to test the theory would be to take it to the extreme and see what happens.
Daffy, I've successfully tested it the other way. I've lowered the boom by 25mm, when overpowered and flighty tail and the board had settled down
Select to expand quoteJohn340 said..
Daffy, I've successfully tested it the other way. I've lowered the boom by 25mm, when overpowered and flighty tail and the board had settled down
That is interesting.
Select to expand quotedecrepit said..
Yes the diagram is impossible with a uni joint. if the sailor is exerting no force on the rig, then there is no force keeping the windward rail up, and the board would no longer be level. The sailor must be leaning back for that to work.
But nobody is saying that the sailor is upright in fig 3 the sailor is obviously leaning out
look at my topic
www.seabreeze.com.au/forums/Windsurfing/General/Correct-way-to-calculate-forces-moments-in-windsurfing
