I cant post on subject "lift your boom" because is too old...So i put here new topic...There was conversation about CLR,CE ...
Ian K said:
"Yes the centre of lift of the sail lies on that line. But the centre of lift of the sail also lies directly above the centre of hydrodynamic lift. (Turning moments about a vertical axis must be zero.) And on powered up slalomy boards, that are sailed flat, the centre of hydrodynamic lift is pretty well at the centre of the fin lift.
No matter where you put your boom or harness lines you will wiggle and distort your body so that the sail lift is over the fin lift and the board goes in a straight line.
Apart from the boom height altering sail shape by redistributing tension, boom height doesn't matter. If you need to "close the gap" just move the mast base forward."
It is not neccesary that CP lies directly above CLR if boards goes in straight line.
When sailor lean out ,mast is also slightly angled to windward and sail CP can be slightly behind fin when sail is rake(close the gap)
Then we have this situation on picture,CP behind CLR,but board goes straight because sail resultant force goes through CLR point..
(Terms center of lift,center of effort (CE) and center of pressure (CP) are the same but I like use CP because pressure differnece is what cause sail lift...
Jim Drake words:
"The interesting feature of this perspective is that all the horizontal forces interact at or behind the tail of the board. This feature is also unique to windsurfers as compared to all other sailing craft."
(one more thing about lift your boom,when lift boom you can transfer more weight to hold sail,pure geometry)
Yes the force can "act" " at or behind the tail of the board". But force is a vector . As well as the point of action you need magnitude and direction. The vector addition of "Drive" and "Heel" in the diagram is "F sail" which passes through the fin's centre of lift as depicted in the diagram.
The diagram assumes the drag runs through the centre of the board generating no moment about our imagined vertical line through the fin.
A board this wide is likely to be sailed on the leeward rail in which case the drag will act more on the leeward side and generate a moment about the fin. To compensate the "F sail" vector would have to be realigned to generate a counter moment about the fin if the board is to continue in a straight line.
You notice this on a wide board. As you ride more off the fin, getting the windward rail a little higher, you find yourself raking the sail back just a little.
