If you are getting a really good price on the closeout F-Type 138, that is certainly
something to consider.
On the down side, at 90 Kg. I think you would be giving up alot of early planing,
and probably not getting the improved ride in choppy conditions that you feel
the smaller, narrower F-Type 138 would give.
There will be an improvement, no doubt, as downsizing from 100 cm to 88 cm will
get you a better ride in rougher conditions, but not as much as I think you are actually
If you wanted to spend a bit more money, perhaps one of the larger Futuras or Isonics
would give you more of what you seem to want.
I think you would be creating a significant gap between the Serenity and the
If you were looking at the F-Type 148, at your weight, that would be, in my opinion,
a better choice, but it still won't get you the smoother ride it seems you want in 15 knots and up.
For what your seem to be asking for, I'd suggest the Futura or Isonic 133 or 144.
They are narrower than the F-Type, so you would give up as much, if not a little more
in early planing (still a gap I'm afraid) but you would gain a much better ride in your 15-20 knots range.
In my opinion, the F-Types and '04 Free Formulas where "detuned" formula boards, and as the wind and chop gets up, they have the same issues with control, without the huge sails you find on formula boards in formula races, to hold them down.
Result...... not a real good compromise in 15-20 knots.
Narrower boards (<= 85 cm) are far easier to control in this wind range.
Hope this helps,