RE: Lessacher Duo 40 for F-Type 148 - Sail Sizes?
First, If you have a little gap under the "overhang" at the juncture of the
top of the LE and the bottom of your F-148, why not fix that first.
Rub some soap (like "a bar of soap") on the front and rear of your Deep Tuttle fin head, install the fin in the board as you would normally, then remove it and see where the soap residue has gone "shiny". That's were you need a few strokes of a single cut file. Just take off the shiny soap, and a little more.
If the fin root is hitting hard in the box down low on the root then that is
not allowing the front of your fin to drop into the fin box. Repeat the process a few times, and I think your "gap" will disappear.
If the overhang has a definite "incorrect" angle to it, you can easily put wax paper on your board, a little thickened epoxy, Marine Tes, or JB Weld Quik on top of the wax paper, some wax paper on the front of the fin root (so the epoxy doesn't get into the fin cavity and stick your fin permanently) and you can fill that gap. Not to worry if the epoxy sets up in a sort of a "puddle or glob, as you can easily file it back to the shape of the front overhang of your fin. Better still get some sheet or round soft lead material, and shape it so you get a nice "fllet" between the fin overhang and the bottom of your board.File/sand the filler material to give a fairly solid but radiused edge and you won't even need the foam "gasket" any more.
Back to your question though, I find the Lessacher Formula Duo does really well and I can't tell you exactly why Wolfgangs 40 cm Duo seems to have as much or more spin out resistance when compared to larger symmetrical weed fins. Perhaps it's the concave/convex blending into convex/concave that's the "magic" with Wolfgang's designs.
I think the True Ames and the Tangent Reapers go upwind a little better, but it does seem to take more fin area to achieve this.
Wolfgang's fins are as fast if not faster, they just change the "feel" of the board a little as they are so far back behind the fin box.
I'm still trying to get Wolfgang to make some progressive rake "DUO " fins that are set forward. I think that will change the "feel" for the better and may result in the "ultimate weed fin".
He's sent me some photo's of prototypes but I haven't been able to get any fins to test yet.
Since you are not racing, and you do need the shallow water and weed shedding features, the Lessacher Formula Duo 40 cm could be just what you need. You can decide if the "different feel" is OK or not.
If you come to Hatteras, or if I come up your way, let's get in touch and you can try out the Formula Duo I have in my demo fin quiver.
Glad the "comparisons" on the FT-158/FT-148 seem to be working for you
I hope my suggestions were pretty much on the mark, performance wise.
Hope this helps,