as long as you maintain the thinning toward the tip - so after you cut it you'll need to take quite a lot of thickness out of the last 5-6cm.
that will mean re-foiling that last bit too, so take note where the max thickness is (as it is a percentage of chord so is not say 20mm from the leading edge all the way down.

I've had it done with several fins, but do not touch the tip, just remove the old box and make a new box! That way you can even re-rake a fin if you need or want to!
Marko

I've done quite a number of fins, exactly the way Mark describes and for the life of me can't understand why the red thumbs?!
If you follow the chord ratio, why not??

Mark _australia said...
I want to know how re-boxing makes it shorter?

Sure you can cut the base off and re-box .....but then you just have a shorter fin - it will be stiffer and thus not twist as much.
If that effect is what you want to achieve then fine..... but if he cuts 5cm off the base of a 44cm goldwing and reboxes it he will not have the same thing as a 39cm goldwing.

I assumed it had tip dmage anyway...?

I think it was lotofarts that red thumbed you, but I helped fix it

DunkO said...

Mark _australia said...
I want to know how re-boxing makes it shorter?

Sure you can cut the base off and re-box .....but then you just have a shorter fin - it will be stiffer and thus not twist as much.
If that effect is what you want to achieve then fine..... but if he cuts 5cm off the base of a 44cm goldwing and reboxes it he will not have the same thing as a 39cm goldwing.

I assumed it had tip dmage anyway...?

The top is has more flex than than the bottom so if you take from the bottom you end with a flexier fin than if you took from the top.

Save you a great deal of sanding wouldn't it?

Believe it or not, it will actually be stiffer. Get an old ruler, flex it, cut it in half and try again.

a ruler is not tapered in thickness and width.

before you start sanding, which new fin will be stiffer??

the best way to refoil it is too sand both sides along the thickest part of the foil, flat, when you have both side exactly the same roll the corners off to the leading edge, then repeat the other side , then sand the to the trailing edge, if you try too foil one side at a time its hard to keep it symmetrical

DunkO said...



before you start sanding, which new fin will be stiffer??

Irrelevant - as you said "before you start sanding" and nobody would use number 3 with just a blunt cut and thus a 10mm thick tip.

My point is
he has number 1
He wants a shorter fin.

If you do number 2, which is just cutting the base off, yes you have a shorter fin but it is now so much stiffer with more draft at the base. It will not feel the same as the next size down in that fin design.

If you do number 3 (cut the tip) and shape it correctly it will be more like the next fin size down.

Mark _australia said...

DunkO said...



before you start sanding, which new fin will be stiffer??

Irrelevant - as you said "before you start sanding" and nobody would use number 3 with just a blunt cut and thus a 10mm thick tip.

My point is
he has number 1
He wants a shorter fin.

If you do number 2, which is just cutting the base off, yes you have a shorter fin but it is now so much stiffer with more draft at the base. It will not feel the same as the next size down in that fin design.

If you do number 3 (cut the tip) and shape it correctly it will be more like the next fin size down.

I respectfully disagree with this. The chord thickness percentage won't change as you cut off the base. The thickness of these fins is around 9% of the chord/width, which means the max thickness of the fin at any point along the fin is 9% of the chord at that point of the fin.

Cutting the tip off will leave you with longer chords/width relative to the next size down, which will mean the entire fin will be thicker then the next size down, compared to shortening the fin by cutting off the base.

You'd have to refoil the entire fin to then get the stiffness down and get the taper ratio correct to be even close to the next size down fin, and you'd end up with a handfoiled fin, which would be inferior to the original CNC'ed fin.

I think l will keep the 44 cm fin as it is& buy a talon tectonics 39 cm these fins are made by professional people as I dont want to stuff it up

Haha great idea. Then you have 2 fins to choose from. I'm sure the debate will continue regardless [}:)]

Mark _australia said...
^^ but Chris how you refute my argument also refutes yours.... cutting the base off has the same effect - as you still have a fin that is thicker in draft compared to it's length either way you do it.

My issue was with Dunk0's question "which will be stiffer?" and I propose that is irrelevant to this discussion as nobody would cut the fin like in #3 and then go use it.

I maintain the best way to shorten is to cut the tip off and reshape as you can take thickness out as far up as you desire..... whereas just cutting at the base and reboxing makes it stiffer and introduces errors (is it now perpendicular? is it in line with the board? etc etc)

Cutting the base gives you a thinner fin. ie If you cut fin 5cm off from the base, the fin has a narrower chord/width at the new base. So 9% of the narrower chord at 5cm up the fin, is thinner then 9% of the wider chord at the original base of the fin.

chris, thankyou for shedding a bit of logic.
mark, the tip is the thin flexible bit why would you chop it off? sure you could sand a new fin out of the chunk of fin you have left but why would you not just thin down the top if you desired more flex.

personally both ways sounds like waytoo much work

DunkO said...

personally both ways sounds like waytoo much work

Agreed on that one.

Re-boxing and getting it right is not as easy as it sounds.

Re-foiling takes a few fkups to get right, and makes you itch for about a week.

OTOH there is the happiness in making something....

OTOH #2 I can't believe the ground-off buggered-up fins that people discard, yet 10ml of JB Weld and 20mins work makes it good as new. Hmm... a $30 old beater to save buying a $180 new one works for me.