RE: How to know when you are overpowered?
It almost sounds like you are so concerned with staying upwind, that you aren't heading off enough to get your board fully planing.
Is this sort of the case?
In 18 knots, on the GO 155, unless you are dealing with some pretty major currents that want to take you downwind, you should have no problems at all staying upwind.
My guess is that you have some board tuning issues.
Since you GO 155 is not a formula board, the formula racer's axiom that all the way forward gets you upwind does not really apply.
Which rail are you using to go upwind? Do you "tip" your board so that the upwind rail is down, or do you tip the board a slight bit lee rail (the downwind rail) down to get the fin to give you the most upwind lift.
You may also have some "stance" issues here that make the back foot pressure somewhat unbearable over a long session.
TOW will strenghten you back leg so you can sail upwind with 90%+ of the pressure on your back leg for several hours.
Less wetted surface probably will not reduce the back leg pressure, but it will give you much better speed, and allow your board to skim over the water much more freely than what you are experiencing now by having the mast foot pressure push the entire front of the board down toward the water. The sort of makes your board "plow" and often this results in a very hard bound/rebound cycle that makes control a rather big issue. Get the mast foot back, and the nose up a little, so your board planes more freely and you should be able to "rail" your board slightly to leeward bu pullling up slightly with your front foot while pushing across the top of the fin with your rear foot.
Once you get this mastered, you will never go upwind on the upwind rail unless you aren't planing.
Much better speed and a much better angle to windward with fin lift working for you.
If you are "compensating" for something you experieinced when you had less skills (the nose riding too high when you weren't planing fast)
you may have developed some bad habits.
The nose attitude is controlled in 2 ways.
When you are sub planing, where you put your weight controls the fore and aft (pitch) attitude of your board.
As you get closer to fully planing, you need to move your weight back on the board, at a rate that promotes constant acceleration. Move back too slowly and the board never gets up and over it's bow wave and the rocker transition.
Move back too quickly and the nose pops up too high and your acceleration dies off.
Get it right and your GO 155 will simply "slide" up onto a plane if you have enough power in your sail.
As far as getting the mast foot too far back, yes, you can do that, and your board will begin to tailwalk (dance up out of the water on the fin).
So, move the mast foot back until your board tends to "tailwalk" and then move the mast foot back forward a couple centimeters at a time until you just get things under control.
This is where the board will go the fastest and speed= better/easier sailing upwind.
looked at the North 2006 Crossride, and while it's a bit more free race oriented than I originally thought it's still more a free ride, supercorss sail and this may not give it the best upwind characteristics.
Perhaps you will need (due to your wieght) to get a sligthly larger fin on your GO 155. I'd think something in the 56-58 cm range would make quite a difference in upwind sailing.
We've all done "the walk of shame" at one time or another, so don't feel bad. TOW and better tuning and stance will soon have you zipping back upwind without even thinking about not getting back to where you started. Also, try to at all times determine what the "favored tack" is (to get you back to where you launched) and concentrate on spending more time on that tack.
Do you have any photos of you sailing your GO?
Send me an email (firstname.lastname@example.org) with a couple of photos of you sailing (upwind if possible) and we may be able to see some issues you have and help you to get beyond them.
Hope this helps,