All instructors and profi windsurfers tell if you want put nose down,you have too high boom so you must lower boom...

Every time I thinking about this I have confusion.

When we put boom up,harness lines stay more vertical so we can transfer more weight on sail ,therefore mast foot pressure(MFP) also increase.taking some of your weight off your feet and pushing it down on the mast foot instead.(in theory)
Bigger MFP must hold nose down more..(in theory ,but MFP is just internal force)

1) Can someone explain physics explanation why higher boom lift nose up more ?
2) Does MFP exist,is mast base in tension or compression,and can MFP put nose down,because MFP is internal force ?

I have my opinion,but I want hear your too...

(We have burnning discusion about this topic on boards.uk forum, there some members say that MFP does not exist at all,that universal joint is in tension during sailing not in compression etc etc..)

http://forums.boards.co.uk/showthread.php/78577-Boom-up-nose-up#XVkQRZ3LCGjfeKCo.97

Don't worry about it just sail and enjoy

I dont know the physics of it but raising the boom definately makes the nose of board flighty when powered up.

The only explanation I can think of is if you take the point where the boom meets the mast as the apex of a triangle formed by that point, the UJ and somewhere on the board between the two footstraps. If you raise the apex of that triangle, then you end up with a taller, narrower triangle, because the distance between the UJ and the point on the board hasn't changed.

Now we would probably compensate a little by perhaps leaning the mast further back or standing more upright so we don't have to stretch so high, but most of the effect would be less weight on your front foot as you need to "reach" to that higher point, and placing more weight on the foot over the fin.

i would suggest it's less about the mast foot pressure and more about human foot pressure.

Try using a chest harness and it's the equivalent of lowering your boom about 6 inches or 15 cms.

A seat harness will lift the nose of the board higher than a waist harness for sure.

With a low boom you are not pulling down much thru your lines,more a lateral pull so the nose of the board will not lift much.

Same with using a harness with a very high hook height - the pull is more lateral so you'll get more control but less lift.

Higher boom rakes the mast back further when you are in the straps. Therefore the centre of effort of the sail is also further back.

Here's my simplistic explanation: think where the boom connects to the mast as the pivot point.. When it's lower, the distance between the centre of effort of the sail and the pivot point increases while the distance between the pivot point and the mast foot decreases, resulting in a higher MFP. Overly simplified and ignores angles etc but I think the force on the sail not the weight on the harness lines is the most significant component of the MFP most of the time. This also explains loss of MFP when sheeting out.