Forums > Windsurfing General

protecting the fin box

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Created by DUKE4 5 months ago, 16 May 2018
DUKE4
SA, 7 posts
16 May 2018 11:23PM
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has anyone thought of protecting fin box damage from hitting sandbars and reefs by semi-cutting the fin bolts at the right position ie between board and top of fin( thread insertion points) , therefore the bolts / screws are strong enough for threading but snap when hit an object saving the board.
cheers

boardsurfr
673 posts
16 May 2018 10:19PM
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I have not tried that, but it might be worth pursuing. I have had one quite dramatic encounter with some solid rocks in flat water where the fin came out, saving the board. It was a power box fin with the thread directly in the power box adapter (no brass nut). Not sure if a tuttle fin would also have come out, or if two screws would have held well enough to damage the board.

Another thing you could try is to use short screws so that you get just a few turns in the brass nut. I've had fins in rental gear come out when running aground (more gently) where that was the case.

LeeD
291 posts
17 May 2018 12:10AM
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Be aware, oftentimes the screw can fail with impact and the fin pivots back and slices into the bottom of the board behind the box. That can imbed the trailing edge of the fin as deep as 3" into the foam.

boardsurfr
673 posts
17 May 2018 12:39AM
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Select to expand quote
LeeD said..
Be aware, oftentimes the screw can fail with impact and the fin pivots back and slices into the bottom of the board behind the box. That can imbed the trailing edge of the fin as deep as 3" into the foam.


Pretty bad - but an easier repair than replacing a torn out fin box?

olskool
QLD, 846 posts
17 May 2018 3:39AM
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Heres how a pb 50cm supersport fin looks as a weedy!
Compliments of some murky water sandbank action. Screw did not let go. Fin pivoted n crunched up into deck. Only had finbox changed from Us to pb a month earlier.
So rebuild number 2 was done. Ya gotta do what ya gotta do to keep these ol raceboards alive.

LeeD
291 posts
17 May 2018 5:22AM
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Pretty simple fix to replace box.
Tuttle and pb box goes for under 30 bucks, and you just rout 3/16" around the broken box, add new box, and layer in several layers of glass then fill with microballoon mix, sand and glass over.

FormulaNova
NSW, 7431 posts
17 May 2018 7:49AM
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Select to expand quote
olskool said..
Heres how a pb 50cm supersport fin looks as a weedy!
Compliments of some murky water sandbank action. Screw did not let go. Fin pivoted n crunched up into deck. Only had finbox changed from Us to pb a month earlier.
So rebuild number 2 was done. Ya gotta do what ya gotta do to keep these ol raceboards alive.


Wow. That's bad luck. I've broken about three powerbox fins and each time the box breaks along the fin screw and the fin comes out. I think they designed the PB to do this so that the fins breaks and doesn't destroy the board.

MarkSSC
QLD, 327 posts
17 May 2018 9:05AM
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Select to expand quote
DUKE4 said..
has anyone thought of protecting fin box damage from hitting sandbars and reefs by semi-cutting the fin bolts at the right position ie between board and top of fin( thread insertion points) , therefore the bolts / screws are strong enough for threading but snap when hit an object saving the board.
cheers


Shear bolts/screws have been used for specific areas in aircraft design where the attachment is protected by the earlier failure of a lesser item. Your idea is a rough way of doing this, and may not be all that accurate as far as determining what degree of load force will snap the item. Personally, I would be looking for a better way to avoid the sandbanks.

Imax1
VIC, 1485 posts
17 May 2018 9:44AM
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On every fin i have i open up the thread hole in the brass nuts to 5.3 mm . ( standard is 5.0 mm ) . The stripping of the brass nut is the weak point you may be looking for . A well used fin hole over time will be close to that with sand grinding . Replacing a stripped nut is better than crunching a finbox.
Waaay too many fin crunches later , i have never broken a box . Never lost a fin either , because they kind of hang out bent back in a TB box with the front nut pulled through and the back nut hanging on the last couple of threads of a bent screw. . And with PB the fin comes totally out and are easy to find in shallow water.

Imax1
VIC, 1485 posts
17 May 2018 9:46AM
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Select to expand quote
LeeD said..
Pretty simple fix to replace box.
Tuttle and pb box goes for under 30 bucks, and you just rout 3/16" around the broken box, add new box, and layer in several layers of glass then fill with microballoon mix, sand and glass over.


Id prefer to loose a fin.

Mark _australia
WA, 18281 posts
17 May 2018 8:57AM
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The barrel nut in PB / Tuttle is actually machined with a loose thread so they will strip easily enough
IF they used the correct ones.

I had one let go when I was not planing (barely moving in fact) and touched reef. That was a bit of a premature letting go in my opinion and I lost the fin. Thus I really don't think we should be making shearbolts.

If u must, I like Imax's idea

John340
QLD, 1857 posts
17 May 2018 11:09AM
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If sandbanks, reefs, marine life and shallow water are an issue, invest in weedspeed fins. They draw less water than pointers for the same lift and the rake of the fin helps the board ride up over the obstacle, protecting the fin box.

The other alternative is to use carbon fins. They shear off when you hit an obstacle. But at $400+ a pop, it can get expensive.

Agrid
WA, 288 posts
17 May 2018 11:11AM
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There was a time when we used plastic screws.

Imax1
VIC, 1485 posts
17 May 2018 4:46PM
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Next board I make I'll wrap a couple layers of glass around the box and stuff a glass resin mix , half inch thick down the voids of the back of the box.
Indestructobox .
I glass over both sides of the box.
It will be the last part of the board intact before going to the tip , all for 100 gm.
Ill be able to smash into things with gay abandon.
One less thing to think about
Having said all that , I'm into overengineering , that's why my last board is 3 kg heavier than it could have safely been. At least , if I had to , I could sail over rocks

decrepit
WA, 8706 posts
17 May 2018 5:57PM
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I've seen a couple of boards snapped, from hitting something solid with the fin. I wouldn't get too gaily abandoned.

Imax1
VIC, 1485 posts
17 May 2018 8:17PM
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^^^
How about half assed gaily abandoned ?
Now that sounded bad as I was writing it



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"protecting the fin box" started by DUKE4