Select to expand quote
barbarian said..
I guess my question is about how much do the fins play a part in the shape?
A lot!
For widish-tailed thrusters, picture the board rolling in a turn: the tail rail sinks, but the center fins become lifted towards the surface, where it has less grip, because it is far from the rail. If you pay attention, you can feel the slight tail slide (before a full fin release) it generates in turns, as opposed to the quad where the inner rear fin bites deep into the water with the rail.
This phenomenon is less noticeable with narrow tails, where the center fin is not moved a lot upwards when rolling.
This slight tail release is what helps a wide-tailed thruster feel loose in weak surf, but can spinout in faster/hollower surf.
Quads can shine in weak surf too, but the rider will have to use the grip of the quad to generate more speed than with a thruster, getting looseness via speed instead.
The JP Slate has a very peculiar fin setup, with the rear quads very close to the rail and far back, this promotes a very drivey, "locked in" feel, where you have to engage aggressively for rail-to-rail turns. And as a thruster, you can see how much the center fin will get lifted as it is very far from the rails due to the very wide tail, providing smoothness, but loosing grip in bigger surf.

It would be interesting to try to have rear boxes placed in a McKee setup (rear boxes closer together), for instance along the channel ridges, it could provide a quad setup that could work in all conditions.
mckeesurf.com/index.php/fin-direction-graphic/?nbspExample of a McKee setup:

What you can try on your slate is to use a 5 fin setup: find the smallest fins you can fin (3.5" trailers?) and use them as rear quads + another one in the center both to keep a reasonable total amount of grip , and smooth the rail to rail by distributing more the fin areas.

.Here is one of my small wave SUP board with a 5 C-Drive setup:
The rears were their smallest size, the "Grommet".