Mechanics/statics facts:
Fact 1:
Universal joint cant transfer moments,only forces.
What does it mean in windsurfing?
That mean that sail cant transfer moment to the board and vice versa.It can only transfer forces,for example "mast foot pressure" is vertical force.
Fact 2:
Steady state:
-Sum of all horizontal forces must be zero
-Sum of all vertical forces must be zero
-Sum of all moments must be zero.
What does it mean in windsurfing?
That mean that board is sailing with constant speed,not accelerate or decelerate.Also that all parts in system are not rotate in any direction. (A moment causes a rotation about a point or axis.)
What is rotation in our case?
Rotation are all situation where moments are not zero,catapult is rotation where moment from sail is greater than moment from sailor,lifting windward rail up is also rotation where moment from fin is greater than moment from sailor feet weight over rail etc etc...

Assumption 1
I will consider that sailor use arms only for control sail,so 100% his pull is made through harness line.That will simplify calculation.
Assumption 2
Sailor ankle when stay in footstrap can make additional moment(lift toes/push heel) against fin moment.I will not take this in calculation in board balance(step 5 and 6) because of two reasons:
1)It will simplify calculation.
2)It is very hard to determine how much moment ankle can produce ,depend on muscles/tendons..







Task




Calculation:

















So if you want ride board flat you can put smaller fin(solution above-step 5 ) or you can ride board with windward rail up(solution below-step 6)
Lets see if fin will be too big and rollover board or board will be in balance even in railing mode...Note when windward rail lift up,force Fhydrolift will shift to the right because left part of board is not in contact with water any more...

For this example board is in balance in railing mode too ,super we dont need change fin if we want railing!


You can see from calculations that force Fsail depend on ;sailor weight,sailor lean out angle,distance (L),harness line T1 angle,COE height etc..
You can play with geometric(angles and distances) or sailor weight and see how Fsail ,feet vertical force,MFP and other forces changing.