He said at 3:28 "basicaly you press board up or down with boom height"
Can you explain why boom up or down change board trim,what is physics behind this,what push nose up/down??
I see only two forces that can change board trim,sailor weight moving for and aft and sail drive force moving up and down at sail,but what boom height has with them?
Boom connected to mast, mast to board up at mast track.
Down pressure on boom weights mast track.
Side pressure to boom less.
Sheeting IN adds mast pressure.
Sheeting out doesn't.
Boom higher your natural stance will be more back foot weighted as front hand/shoulder/hip will be higher taking weight from the front foot.
Boom lower will drop your hand/shoulder/hip and weight the front foot more...
so higher boom makes the board fly more on the fin because back foot is weighted more. Lower boom makes the board contact the water more because front foot is weighted more...
so higher boom = faster while you have control, lower boom= more control but slower if you don't need it.
of course you can physically fight against this, but in a natural stance that's how it is.
I'm with you on this one Hotlap. I dont think these explanations really explain the physics. 
But many good experienced sailors do feel it makes a difference so something is going on.
The mast base is the fulcrum, top of the twisted sail is the weight, and the boom (with rider mass attached) is the "lifting" force point of attachment.
Bump the boom down and the lever ends up with more load at the fulcrum. Bump the boom up and the load on the fulcrum lightens=less load holding the nose down.
Select to expand quotesailquik said..
I'm with you on this one Hotlap. I dont think these explanations really explain the physics.
But many good experienced sailors do feel it makes a difference so something is going on.
Answer from Cald seems physicaly correct ,he just need include pitching moment from sail drive force...
Select to expand quotesailquik said..
I'm with you on this one Hotlap. I dont think these explanations really explain the physics.
But many good experienced sailors do feel it makes a difference so something is going on.
I take picture from Jim Drake..
in steady state net force=0 ,net moment=0
if drive force increase from 32,5 to 50(for example sheet in), lets find where would board lift move..
moment aorund point A...Ma=0
-216.5x X +16.5x13 +32,5x80=0
X=19.4 inches
we can see that board lift is moved toward nose (from 13 inches to 19.4 inches) that mean nose is press down when sheet in..
(I use board lift position as indicator what nose is doing,because board lift position depend on wetted surface,so if board lift position is moved towards nose,that mean nose is press down...)
Also we can see with this easy example if we increase center of effort height(less downahul),board lift will again move towards nose,that explain why less downhaul more press nose down .
But I still dont have idea what happend with forces,body position and sail angle when you move boom up/down..
Select to expand quotecald said..
Boom higher your natural stance will be more back foot weighted as front hand/shoulder/hip will be higher taking weight from the front foot.
Just moving the front arm up or down vertically does not change pressure on the feet.
One explanation I've heard is that a lower boom leads to the mast being tilted less towards the back and more towards the nose. That would put more of the rig weight onto the mast base. I can also see how it would tilt your body the same way, resulting in more pressure on the front foot. Makes sense to me, but I'm not sure it's really the correct explanation of what's going on.
Select to expand quotehotlap said..sailquik said..
I'm with you on this one Hotlap. I dont think these explanations really explain the physics.
But many good experienced sailors do feel it makes a difference so something is going on.
I take picture from Jim Drake..
in steady state net force=0 ,net moment=0
if drive force increase from 32,5 to 50(for example sheet in), lets find where would board lift move..
moment aorund point A...Ma=0
-216.5x X +16.5x13 +32,5x80=0
X=19.4 inches
we can see that board lift is moved toward nose (from 13 inches to 19.4 inches) that mean nose is press down when sheet in..
(I use board lift position as indicator what nose is doing,because board lift position depend on wetted surface,so if board lift position is moved towards nose,that mean nose is press down...)
Also we can see with this easy example if we increase center of effort height(less downahul),board lift will again move towards nose,that explain why less downhaul more press nose down .
But I still dont have idea what happend with forces,body position and sail angle when you move boom up/down..
Added confusion.
Low harness hook pulls down on boom.
Sheeting the sail in pulls down on mastbase.
LESS downhaul really powered adds weight to mastbase.
Select to expand quoteboardsurfr said..cald said..
Boom higher your natural stance will be more back foot weighted as front hand/shoulder/hip will be higher taking weight from the front foot.
Just moving the front arm up or down vertically does not change pressure on the feet.
One explanation I've heard is that a lower boom leads to the mast being tilted less towards the back and more towards the nose. That would put more of the rig weight onto the mast base. I can also see how it would tilt your body the same way, resulting in more pressure on the front foot. Makes sense to me, but I'm not sure it's really the correct explanation of what's going on.
Did you just disagree with me then explain why you do agree? basically, as you said it's not "just moving the front arm" is it. but it does result in either more or less front foot pressure. Potentially more or less mast base pressure as well but its hard to feel that, not so hard to feel ones feet...
In the 80's we had super high booms. I felt like I was sitting in the harness and need the straps to hold my feet down.
The days I feel like I have heaps more pressure on my feet, to the point where I have to be real quick getting into the back strap.
I don't know about others, but i don't really see boom height as a setting that should be toyed with to suit conditions. There's THE spot for it because having it in the wrong spot can have effects on rig handling that other rigging alterations cant fix.
If its too low in relation to your height/hook height then you're going to really struggle to stay on a crosswind/upwind track, and your fore/aft board trim will be like that vegemite sandwich you left in your school bag for a week with the books on top of it. Too high and the rig will be really forehand heavy, and you wont be able to tune it out, no matter what you do.
