no belly, its a slight single to double with a bit of vee right at the back.
...I think that you are talking about other problem; I mean, fins are one thing but sink a rail is another and have nothing to do with fins.
First, you say 5 9 but I use a 5 2 fish; because I am not big or fat, a 6 fish is for super big guys; a 5 10 with more meat than that is for big guys. Simon Anderson, M Richards, Malloy, etc are big guys for reference. You need to have good "leverage" to make a wider design works right.
So are you sure and this is the only important point, that the length and overall area is ok for your size?
Then, what type of waves? A fish, a modern fish or a rocket fish are not rookie boards or small fat surf condition design.
The thickness in the last 1/3 of your board is too much.
Regarding fins: with that toe in, you cannot describe a down the line ride fast BUT you can turn on a dime (but you need to have the right tilt and the right rocker to compensate; depends on for what the board is intended; pocket warrior, down the line, slalom, cutback surfing etc)
You need to put the rear foot back, similar to a thruster.
Regarding that straight edge from the fins to the tip of the nose: for a 5 9 and for most, you do not point to the tip; the compromise is always off the tip.
THEN, the rear fins have HALF of the toe in.
Im tall and thin, 6'2 and about 170lbs (close to 80kg), been surfing for 17 years now. this is a copy of a board I made that I liked, although it is wider. I've tried in a few different types of waves and had similar problems. thanks for the tip on the last 1/3, I agree.
A few have mentioned getting the back foot back further, so I've ripped off the tail pad that was on there and I'll do some more tests.
I can confirm the fins do not point to the nose but ahead of it, there is less toe on the rears but after carefully measuring again I do think it is worth re setting at least 2 of them. i will measure off the stringer this time, 1/4" in the front and 1/8" in the rears.
It could be fins, my back foot, or like you mention, just a bad balance between design features, but I'm not giving up just yet.
1. Surf it more. You would be surprised how you adapt to what's under your feet.
2. It looks like you have too much volume farther towards the tail. While it;s not a massive amount, for my eyes and my experience, the bulk of your foam is too far back. This might be causing you to sink too far inot the wave when surfing. Not enough foam to keep you planing under your feet in the center and front of the board... my guiess is that youa re burying rails and plowing as opposed to planing. So if you are going to keep her, I would move the stance back several inches.
Did you put a straight edge on the fins? Inside face to the nose? If that’s all good; you are then most likely off on the distribution and foil of the foam. I shaped a longboard once that had the overall rocker too far forward. The board pushed water on paddle and was a bit squirrelly under foot.
Did the previous board have V in the tail?
I tried some V off the tail of an iteration of a board design that I really like.
I hated the V.
I've made a few boards that I couldn't dial in. I think it's in the fins you are using or the location/angle of tow. The post about where you are standing is another thing to think about.
Have you tried using just 2 fins in the front boxes? If that still doesn't work, I don't know what to tell you. Maybe try going down in size on all the fins.
It does look like your front fins have a lot of tow. For my boards, I've found that I prefer less tow and cant. I don't have any tow on my rear fins in quads and I also tend to keep the rear fins straight up, no cant. Most of my boards have between 1/8" and 3/16" in the front fins, but everyone is different, so don't take that as the way to go.
Here's a 5-9 x 22" I made it as a quad, but I also ride it with keel fins in front and tiny trailers I made because I don't like twins will sometimes wash out. These trailers are not the ones I use now, but they are the same size. I have a another set of keels that are a little bigger than these, and single sided foils, these are symmetrically foiled. A different set of fins changes everything.