# space between fins and its effect?

I have two boards and decided to measure the fins placement. Both thrusters one inch different in length same width standard bigger guy. The 6’6" one has front fins further apart from the other and also has a wider tail by one inch, it is squash. The 6’7" the fins are obviously closer togethr but the other difference is the distance from the tail i greater than the first, #2 is diamond tail which may have somthing to do with that?

did you take any science courses in highschool?(not trying to insult your intelligence here just trying to jog your memory.)first thing in every lab only one variable should be changed. everything in a surfboard affects everything else

Ok. How would the further apart fins affect performance assuming all else was the same?

Now measure and compare distance from side fins to adjacent rails on both boards.

Report back your observations and any conjecture you may have formed based on these observations.

Make a table. For each board, measure

1. the distance from the tip of the tail to the rear of each fin

2. the distance from the rear base of each rail fin to the rail

3. the tail width

Let us know the results, and we will be happy to guess how this makes the boards feel different under your feet.

ok here goes: squash 6’6" side fins 2 1/8 from front middle of side fin box to rail. tail is 15.25". Back of trailing fin box 3 1/4 from tail. 1’ between front of side fins.

6’7" Diamond tail. 2 1/4" from front middle of side fin box to rail. tail is 14.25". Back of trailing fin box 3 1/2" from tail. 11 3/4 between front of side fins.

oh and the rear base is about 1 inch for both. aloha

From RedX Website. Generally speaking.

“Putting the cluster of fins closer together loosens up the board while positioning them further apart makes the board stiffer.”

fins closer to the tail = tighter

further from the tail = looser

fins closer together in a thruster = looser more pivoty feeling with shorter more square turns

example: generally speaking

shortboard 6’3" fins are set around

3 1/2in from the tail trailer

11in front

1 1/8 off the rail

fins towed or pointed 2in off the nose

now a 7’0"

3 7/8 trailer

11 7/8 front

1 1/16 off the rail

same tow

use that as a starting point ride the board and make adjustments

…if you ride a small board in small to medium surf (average conditions), and you put your back foot really near the very tail, you should go with :-the fins near the tail…-the front fins near the rear …

Quote:
ok here goes: squash 6'6" side fins 2 1/8 from front middle of side fin box to rail. tail is 15.25". Back of trailing fin box 3 1/4 from tail. 1' between front of side fins.

6’7" Diamond tail. 2 1/4" from front middle of side fin box to rail. tail is 14.25". Back of trailing fin box 3 1/2" from tail. 11 3/4 between front of side fins.

Measurements are generally made from the bottom, rear, of the fins. Not from the center of the box. Classic competition measurements are 10.5 inches for the rail fins, 3.5 inches for the rear fin. But today a lot of variance is found, and the averages on consumer boards are closer to 11.25 inches and 3.5 inches, and quite a few in the NorCal area are 12 and 4.

The Diamond tail board has the fin cluster 1 inch closer together in the fore-aft direction, it should be noticeably looser/twitchier/faster to change directions.

look at it this way…

if you have a honda civic - small wheelbase(closer fins) - the car turns very easily, but can be a bit scary at high speeds.

if you have a cadillac - long wheelbase(more space between trailer and side fins) the turns are more drawn out, but smoother at speed.

i have a 6´0" egg with the rear of the sides at 14 3/4 and 1 1/8 off the rails that is a rocket ship, but doesn´t really snap super hard unless you really push it.

hope that makes sense…

Boards that are shorter naturally will turn sharper but I believe what we are addressing is how the fin spacing on the same size board will affect performance. If we move the rail fins on a 6’6" in from the standard position on the rail toward the board center (with the chance of sounding redundant) the boar will be more centrophic. By this I mean the fins will not make the board climb the wave face or turn with as much amplitude or in as sharp a arc as if they were place in there normal positition as long as we keeping tow in, cant and fin shape the same. At least that’s the way I see it.

Mahalo, Rich

halcyon,

yeah, i guess i didn´t get my point across that well…

what i really want to say is that with the side fins further up, trailing edge at 14 3/4" instead of 11 1/2" makes the board faster - more space for the water to exit - but at the sacrifice of smaller, more pivot style turns. i wanted a short,wide board to ride in big waves, so i put the fins on based on longboard set ups.

anyway, if the boards are the same - in everyway - then closer = loose.

if the fin is closer to the rail, it becomes more drivey feeling, also.

OK, so what if I haven’t decided on what fins to use, what would be the rear plug placement on a 7’6" ?

Quote:
OK, so what if I haven't decided on what fins to use, what would be the rear plug placement on a 7'6" ?

A bit of a loaded question, not knowing anything about the board except the length, or even what kind of plug we’re talking about. Roughly an inch of fin advancement for a 7’6", the rear at 4.5", the fronts at 12.5"

But there’s lots of ways people vary placement with length, these will work as “middle-of-the-road” positions.

Also, Halcyon is quite right about fins too far inboard from the rail. As the fin moves outward, it gets more and more drive. If it goes too far out, it gets real pivot-ty and loses hold and drive. But generally, small movements of the fin inboard from the optimal position make relatively small changes in drive and hold, but small movements of the fin too far outboard rapidly trash the fin action. Fins do not work on water alone, they need to direct the water against the hull. The fin too far outboard loses this.