toe in - sensitivity - big fins

I recently made nyself a nice lookin 6 10 X 20 X 2 5/8 swallowtail big boy shortboard from a 7 A blank - nice low rocker. Being a larger gent (230 kg), I put some big fins on it - 4 5/8" long, 5" high and toed the side fins in just over 1/4".

How did it go? Well, the board went beyond belief hopelessly - it felt like the handbrake (parking brake US?) was on the whole time. I looked at my rocker, rails, foil - everything suggested it should’ve been a fast board. I decided it must be the toe in; ground off the fins and reset them to just under 1/4" toe in. The board now flies - the difference is truly amazing.

Is the general rule, the smaller your fins the more you can toe them?

Possibly BUT ive tried different size fins with more and less tow with similar results. A little too much tow and it slows you down alot and you cant carry speed through sections. On the other hand too straight and its really difficult to turn. Ive found that the difference between the fins working well and working bad was less than 1/16 of an inch tow.

I , on my 3rd. and to be best board toed in the left fin 1/2" by mistake and the left 1/4" as intended.

The board was a dog!!!

Slow, and spun out going right off the bottom but wouldn’t come off the top.

Going left (backside) it rode better.

It did make me think perhaps there is a case for asymetrical fins???

Still. I’ve since decided that 1/4" may be a bit less 5mm is good for me as it seems loose enough with plenty os direction

Hey Silverback

Good on ya for figuring out what works best for you!

I shaped over 40 twin fins, and currently still ride them.

For toe-in, my measuring method was to toe them in half the distance to the nose stringer, so it takes into account width of tail, length of board.

All the fins I used were pretty much standard short fins, no keels.

Longest was 8"ers on a 5’8" x 20 x 15" tailed squash, but that was specifically for maxed out Tarantulas. On the same board, I’ve used as small as 6"ers.

My G&S twin used 4.75 tall fins, and constantly spun out backside on any kind of bottom turn. I learned my lesson with that board.

I just figure a bigger fin can handle more surfer power, and I can always throw in more power, rather than try to hold a rail by feathering and feel.


Are you really 230kg?? 506pounds? Screw the toe in. Good on ya for getting out there. Hope you rip it man.

oops - 230 lbs - not really the worlds heaviest surfer, just quite heavy!

As you turn a fin from straight to more and more angled, its resistance to water flow increases slowly. Then, as some angle, small increases in angle result in large increases in resistance, or drag. This occurs where you start cause turbulence behind the angled fins.

The most common angle for rail fins is very close to the steep part of this function - the part where drag increases rapidly (and flow transitions from laminar to turbulent). So, if your rail fins are angled just a little too much, you turn a decent board into a dog.

Note - the critical angle changes a little bit with better foiling. But ideally you set your fins as angled as possible without running into the higher drag regime.

I’d recommend using a protractor for consistency, and you will easily verify these things for yourself. But a degree or two change, and you go from little drag going straight to tons of drag going straight.

Excellent response B, but does this critical angle change according to the size of the fin - ie can you toe a bigger fin in less than a smaller one before encountering too large drag?

Yes, the angle for a “wider” fin needs to be a little less than for a narrower fin. Wider refers to the fore-aft direction, or base width. This is especially problematic at the base. Cutaway shapes are a response to this problem, where the fin is narrower at the base, and flow separation occurs a few degrees greater angle of attack than a wide base fin.

But fin height does not impact this. As side fins get longer and longer, they cannot and should not have bases much longer than normal side fins (anything much over 5 inches is going to be a bust). But the height can grow and grow.