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Old 5th December 2010, 04:47 AM   #1
Join Date: Aug 2008
Posts: 29
Question Deep Tuttle fin box question

Hi Roger,

I have Deep Tuttle fins for my board and noticed when I tighten the bolts that the root sinks down below the surface (approx. 1/8"). The fin feels securely in place and the bolts get tight however, the root is not perfectly flush with the bottom of the board. I have observed this with two different fins on this board.

Does this have any impact on the board's performance and should I try to modify the fit? If so, is it better to modify the fins or the fin box and do you have suggestions for this?

Thanks in advance,
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Old 5th December 2010, 09:34 AM   #2
Dream Team - School Guru
Join Date: Aug 2006
Posts: 2,177

Hi John,
There might be a small difference in performance.
I've had this with some of the demo boards I've had over the years.
It's an easy fix really.
If you have a fin that fits in another board flush with the bottom of the
board, try that fin in the fin box you are having trouble with.
You have 2 choices as to how to fix this problem.
1/ If the board is fairly unique to one or 2 fins (i.e. you don't use the fins
that fit this board in other boards and vice versa) then the easiest way
to fix this is to take the fin heads that go in too far, and simply wipe a
thin layer of thickened epoxy filler (Marine Tex works.... so does JB
Weld or JB Weld Quick... or you can mix up some epoxy resin with
microballons or foam dust as a thickener.
Try to work just the back taper first.
Put a dab of the filler on the radius on the back of the fin root.
Spread it with a gloved finger or with a plastic spreader/spatula.
Take some masking tape and carefull tape over the filler, being
careful to keep the filler on the radius.... do not alow the filler to
flow around to the sides of the fin root.
It only takes a little filler as you are only wanting the "build up"
to be < 1/16" per layer.
When the epoxy has set, remove the tape and smooth out
the layer you have added.
Try the fin in the fin box.
If it's now above the bottom surface of the board more that 1/32"
to 1/16", do a bit more filing to drop it into the fin box.
Ideally your fin should slide into the fin box quite snug (a push fit)
on the sides and the front and rear tapers regulate how far it goes in.
Pushing in by hand should leave the fin root between 1/32" and 1/16" proud
(above the bottom surface) of the bootom of your board.
The last little 1/32'-1/16" is the "draw" on the front/rear tapers and you pull
the fin root in that last little bit with the fin screws.
The idea is that you get a very tight and solid fit so the fin screws are not
supporting the fin in the box, but simply holding it in.
To remove a perfecty fitting fin, you must take out the fin screws, then use
a rubber hammer or your hand to "pop" the fin head taper loose from the
fin box.
2/ If this board sees alot of different fins, then it may be better to build up the
back taper in the fin box.
This is tricky, but uses the same materials.
Mix up your epoxy filler, get some waxed paper (borrow some from your
Smear the epoxy in the rear radius of your fin box, but not all the way down to
the bottom (say about 1/2 the distance to the bottom).
Now cover the epoxy with the waxed paper, making sure the wax paper covers the
filler completely.
Now install your fin and tighten the screws very lightly so the bottom of the fin head
(what you see when it's installed correctly) is parallel with the bottom fof your board.
Let the epoxy set up hard.
Remove the fin and the waxed paper.
Try your fin.
If not enough... repeat the process.
Ultimately you need to "fit your fin" using a bar of soap as your "contact marking
Rub the soap all over the fin head.
Slide it into the fin box.
Pull it in lightly with the fin screws.
Remove the fin and look for the "shiny/dark spots" along the front and rear tapers.
Use a single cut file to remove just the shiny spots.
Rub with soap..... repeat process... until you have at least 75% contact (top to bottom)
on both the front and rear tapered radii.
Now your fit fits the way Larry Tuttle intended it to fit.
Hope this helps,
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Old 5th December 2010, 11:21 PM   #3
Join Date: Aug 2008
Posts: 29

Thanks Roger for your very thorough answer as per usual. I'm going to borrow and insert another fin as you suggested before I make any modifications to see if my fin box or the roots are the culprit.

Thanks again,
johnk is offline   Reply With Quote
Old 6th December 2010, 05:34 AM   #4
Dream Team - School Guru
Join Date: Aug 2006
Posts: 2,177

Hi Joyn.
Might want to take a flashlight and look down into the fin box on the rear taper.
Sometimes the factory fin box tolerance gets a bit large and they have been
know to stick a blob of black epoxy in the rear taper to shim up the original fin
to the correct level before they ship the board.
Pretty sure they do it very much like the 2nd option I suggested.
Take a look and if you see a lump of epoxy about 1" down on the rear taper
radius in the fin box, that was the factory's way to fix an oversize fin box.
I've seen a few where the lump of epoxy broke off and the fin now drops too
far into the fin box.
Before you apply the epoxy to the fin root.... or to the taper in the back of the
fin box, try to wipe it a bit with a paper towel with a little acetone in it.
Just be sure not to spill any acetone on the board or in the fin fox as it will
really make a mess..... an clean and prepped for exopy application mess.
The acetone will prep the fin root or fin box so the new epoxy you apply will
stick solidly.
Also, remember, the fin bolts are only there to pull the tapers into engagement
and ensure that they stay engaged while you sail the board.
They add nothing to the structure of the fin or the fin box, so cranking the screws
down real tight normally causes more problems that it fixes
That is usually what dislodges the Cobra applied lump of epoxy shim..
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Old 21st December 2010, 10:35 PM   #5
Join Date: Aug 2008
Posts: 29

Hi Roger,

Happy Holidays!

So, I have tried out another deep tuttle fin in my board and concluded that it probably makes sense to build up the fin box seeing that all of my fins consistently sit low in the front... slightly raked forward if that makes sense. Am I correct in assuming to build up the front taper in the fin box since the fins sit low only in the front? Also, is Marine Tex ok to use for building up the fin box or would a thickened epoxy adhere better?

Thanks again,
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Old 22nd December 2010, 05:57 AM   #6
Dream Team - School Guru
Join Date: Aug 2006
Posts: 2,177

Hi John K.
Hmmmm.... I've never had to build one up on the front taper.
Very interesting.
Did you look in the fin box with a flashlight to ensure there isn't already a lump of epoxy just below the surface on the back taper of the fin box?
I would rub some soap on the fin root and pull it down into the box and see where it's touching.
Should be a real tight "push" fit on the sides, and then the longer front taper should pretty much set
the angle of the fin.
I'm puzzled as to why the front would drop in lower than the rear.
Also have a look all the way down in the fin box and check for manufacturing debris that would prevent the back of the fin root from seating deeper in the box.
You should only be able to get a full contact on the front taper when the fin is at the correct angle and the root will be flush with the bottom of your board.
Can you send me some photos of your fin roots?
I may be able to check the angles to see that they are correct (with Larry Tuttle's original concept.
Hope this helps,
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