RE: Fins for a go 139
As long as you are a little patient, we will get all of your questions answered.
OK, let's talk about fins:
Yes, you could expect a small gain in lite wind early planing performance with a 54 cm fin, but what you gain on the bottom end of the spectrum, you lose on the high wind end of the spectrum.
If you use a 54 cm fin in your GO 139, you can expect perhaps a knot of gain over the 48 cm fin, but when the wind comes up, you will have more control issues earlier in the spectrum.
The "span" of the fin is the vertical length the blade of the fin extends below the bottom surface of your board and most fins are "classified" but the length or span.
But you can't just go and get a 70 or 75 cm fin and expect to plane in
even less wind. There is a point of diminishing returns here and if you try to use too large a fin, you will get so much drag that the fin will slow you down and be very hard to control once you get going.
The front to back "width" of the fin is termed the "chord' of the fin, and of course the wider the chord the more area you have, but again too much chord can induce a significant amount of additional drag and make you slower and introduce a different set of control problems.
The " thickness" of the foil or chord is another "parameter" here.
You can have a fairly thick "foil" that generates lots of power or lift at lower speeds, but as your speed increases, so does the drag, and you get yet another set of control issues with too thick a foil.
A thicker fin could get you planing sooner than a fin with less lift, but would probably cut your top speed as well.
So, selecting the right fin for your conditions, and yoiur sailing style is always going to be a process of finding the best compromise between
span, chord, and foil (chordal) thickness. Having a few fins in different sizes and different planforms (very vertical fins may be faster and normally go upwind better, but they are less "turny" and don't jibe as well as fins with a more raked back and curved outline.
Then you have specialized fins like weed fins that are raked back at 40-50 degrees to allow any weeds in the water to slide down the leading edge and off the fin to prevent the "build up" of weeds that can really hurt your performance.
Hope this helps,