Smaller rear fin size

General Discussion
1

Hi gang,

 

I’ve been bitten by the fin building bug.  Way too much fun.  I’ve got enough G10 left over from my last fins (for my fish) to make one more fin.  I’ve done some research on running a smaller rear fin with my regular thruster setup on my regular shortboard and it sounds intriguiging.  The only problem is that I have no idea how small the rear fin should be to still get some drive but loosen everything up. 

What do you guys think…how small is too small?  My fins right now are plain old G-5 FCS fins (I think).  They’re just a standard cheapie set.  I’d imagine the depth is somewhere around 4.5", so maybe a 3" or a 3.5" fin would be good?

 

Thanks!

2

“how small is too small?”

A good place to start is the same size and shape as the side fins.  

I have played with decreasing the size of the rear fin until the board spun out… like a lot of things, you don’t really know for sure until you’re past the limit.  

3

A fast way to find out is an algorithm called “binary search” aka. “bisection method”.

Simply explained:

  1. try a half size fin
  2. if too small, try 3/4. if too big try 1/4
  3. ...
  4. keep refining until you reach your stop condition.

In numerical mathematics, this is known to be a fast and stable method.

Probably the fastest for this case.

You might also want to experiment with moving your fin more forward on the tabs.

4

If you only have enough material to make one fin, shoot for about 80 percent as large as your side fins.  No point making your last fin so small you will spin out. 

Then, get some more material and make a set identical but for size (like Hans says, or just make a 2", 3", 4"fin) and ride them all the same day (assuming not glass on fins…) You will quickly find out how the size affects handling.

Then pick the fin you liked best, and start changing the template (more rake?  Straighter back?) Then change the material it’s made of…report your findings when done!

 

5

Excellent ideas.  I’ll start with roughly 80% of the side fins to start with as suggested.  I should also mention, this is more for chest high days, maybe waist high if the waves have great shape.  Anything bigger than chest high and I won’t want to mess with the rear fin, I like how the board rides as the waves get bigger than chest high.

 

I’ll let you all know how it goes once we get a decent day to try it out!

6

Ride report required.

all the best

7

Tried a GX rear with G5 sides many years ago. The GX was loose but lost drive. I’d say it’s verging on too small. Here are the specs: Base 9.80 cm 3.86 In;** Depth 10.00 cm 3.93 In; **Area 71.40 sq. cm11.07 sq. In; Sweep 33.20 Degrees

8

Interesting.  I’ll have to check my fins again and verify that they are G5’s, but regardless, thanks for the fin dimensions.  I may shoot for a 4" depth, assuming I have G5 side fins. 

 

I’ll certainly report back with results once I give it a go…probably Monday or Tuesday!