RE: Go 155
Yes, use 3/8- 1/2" increments, and then when you really like the way your board handles, try even smaller increments, but try to only change the mast foot position. That's the key to finding the "balances"..... only change one thing at a time so you begin to understand what each type of change can do for you.
OK, you have a bit larger chop to deal with than I was thinking.
So, you are actually kinda making little "chop hops" when your board takes off the top of a swell or wave.
This may be a huge part of your problems with the fin letting go.
When we "chop hop" and get the board up off the water and the fin mostly out of the water we need to instantly change stance a little or try to tuck the back foot under our butt. You cannot keep "pressuring" the fin when most of it is out of the water or the fin will go back into the water pointing alot higher upwind than your board is going.
If you "land the fin" it needs to be pointing a little downwind of your course so it can reenter the water without any sideways pressure.
Even if you aren't getting the fin clear out of the water when you go over the larger chop, if the board comes off the water, the amount of fin span (length) in the water will be reduced as well. If you continue to push on the fin, the board will move away from you and the fin will settle back to its full depth pointing much higher than the course your board is sailing.
This "overloads" the fin and leads to spin out (the fin lets loose) virtually all the time.
The roll attitude of your board essentially controls direction due to the way that the water flows along the bottom.
If you have the lee rail lower, the board will tend to turn off the wind (like setting up for a jibe).
If you have the upwind rail lower, the board will tend to head upwind.
There is another dynamic here that it sounds like you may be ready for.
That's the concept of sailing upwind on the lift from your fin.
If you tip your board very slightly lee rail down, and push across the top of the fin with your back foot, the fin will become super efficient and will tend to take you upwind very fast and at a better angle than you can achieve with the upwind rail down.
If you aren't fully planing, then you simply have to use upwind rail down roll trim to get upwind.
If you are fully planing, try lifting slightly with your front foot in the front footstrap and pushing horizontally across the top of the fin with your back foot.
This will roll your board slightly lee rail down and increase the fin lift (horizontal lift here) to the max. and allow you to simply rip upwind, "on the fin".
Pitch attitude is the fore and aft angle at which your board travels through/over the water.
Pitch attitude can be changed to make the board faster and more lively in marginally to normally powered up conditions and also changed to push the nose down more to help your board "stick to the water" in really overpowered conditions.
The things that have the most significant "effect" on pitch trim are the mast base positions (fore and aft along the centerline of the board in the mast slot or track); footstrap positions (fore and aft), and boom height harness line length can also have a somewhat mininal effect.
What's the "best" pitch trim angle...and how would we measure it?
Depends alot on the design of the board, the discipline we are sailing,
the amount of power we have to work with and to some degree the fin we are using.
My version of a "perfect" fore/aft trim is when the board is very fast and lively and virtually on the verge of tailwalking.
If I have enough wind, I will move the mast foot all the way back in the track, and hope the board actually does tailwalk or becomes hard to control. This means I have too much fin, or I've moved the wetted surfase/waterline length too far back for me to control the board.
Using this as a starting point I move the mast foot forward until I'm just slightly uncomfortable and the board feels lively and almost wants to tailwalk. This usually proves to be the fastest setup for me personally.
Formula racers (I did that a few times) like the mast foot much further forward (at or near the front of the mast slot) to give the best upwind performance and to hold the nose of the board down somewhat going off the wind.
So, it's a very personal "what you like and what works for you" kinda thing.
I do know that most boards can be slowed down and will stick to the water better with the mast foot a little further forward, but you need to be careful that you don't get the mast foot forward to a point that the hull of your board gets into a bound and rebound cycle. Then it becomes very hard to control as it seems like you are constantly "slamming' into the chop. Move the mast foot back slightly and you can normally find a "sweet spot" where you have the best mix of control and speed.
But, it's always good to try slightly different mast foot positions, until you really learn your board and what small differences in MFP will do for you.
Same idea applies to all of your rigging and tuning. Keep trying something slightly different, and see what happens. When it all feels perfect, mark it so you can get as near perfect as possible the next time your sail.
Hope this helps,