I am fitting a new finbox to a JP supersport. The original boxed rolled out so I lost any reference I could use to align the new box. I need to find the centreline, and had planned taking some measurements to find the middle of the tail and using a trammel to find equidistant points on either rail further up the board line across and then divide in two etc. This all feels very imprecise, however. Is there a better way?
Here's an idea I've never tried, make a paper or plastic template of the board and fold it in half.
Almost impossible.
Every box install I've done, I put a straight edge from nose to tail, then check it with two T-squares, rail to straight edge. The more you measure the more it gets confusing, I don't think I've ever found a board with a true centreline and boxes on it being straight. Factory boards are all over the place.
Given you only do the important bit, ie: planing - on the back 1/3 of the board (at most) I suggest its best to mark from (1) centre at tail, to (2) centre just back from the widest point, say 1000 - 1100 ish, and mark that line. Use that.
I mark centre at tail, nose and middle then use a builder chalk line to mark centreline.
agree with mark. Factory boards are all over the place. Different rail shapes either side, fins not straight etc etc.
I used a string line.
Q: Does it really matter to have Planck length accuracy ?. It's just a Supersport.
Select to expand quotejn1 said..
I used a string line.
Q: Does it really matter to have Planck length accuracy ?. It's just a Supersport.
Just a Supersport ??? 
Don't measure just at the tail - that'll be very imprecise. Instead, find the tuck-line of the board (the place where the rails transition into the undersurface) and run a piece of masking tape up along both sides for 1M or so such that the inner edge of the masking tape is right on that line. It'll be pretty easy on a board like the Supersport because that rail is pretty hard. Now take ten pieces of string and tape them down diagonally from one side of the taped tuck-lines to the other, starting on each side at equal distances from the tail (important) and criss-crossed in the middle. You'll now have a series of co-linear "X"s down the middle of the board. Trace that line back to the tail (another piece of tape) and that's a good approximation of center.
For a non-planing board like an old Superlight you'd use the outer edge of the board's curvature on each side rather than the tuck-line. Back in the '80s I used to do that pretty often for guys who wanted a more powerful fin than the old Mistral kick-back piece of plastic.
A laser is handy for that task. I place the board upright at the wall and align the laser with the nose and the tail. Some pencil marks along the laser line and then I use a straight edge. As most of the boards are not symmetrical, a slight offset will probably not matter.
David
Production factory boards can be wonky , especially the tuck lines . It's easy to see . Put your board on stands , get down low , close one eye and look from the front or back . Move side to side and you will see any differences . I wonder if the blanks are wonky or if things twist in the vac bag ? Just the other day I was at a shop looking at a new 140 litre ish JP that was on stands . I flipped it upside down and looked down the board from the back to see what V /, cocaves it had . I wouldn't want to buy it . It was so twisted it would have to go one way different to the other . The rail tucks were also different . It was such a pretty purple board .
I do what Mark does , using a long straight edge . On single fins , I think it's more important to have the fin parallel to the board than where it is .
^^^^
Don't do it . It may make you sad . What ever you do , don't get a straight edge and compare one concave to the other 
Everyone can go upwind one way better than the other , it's probably not the board .
Select to expand quoteImax1 said..
^^^^
Don't do it . It may make you sad . What ever you do , don't get a straight edge and compare one concave to the other
Everyone can go upwind one way better than the other , it's probably not the board .
In my case anyway it's probably 99% technique and the rest the equipment 
Yep i kinda stop listening when people start talking about 1mm here, 1 degree there. Ever tried to shape then build a board that accurately.
If you find a board that makes you smile its a keeper. Wonky or not.
Select to expand quoteAwalkspoiled said..
Don't measure just at the tail - that'll be very imprecise. Instead, find the tuck-line of the board (the place where the rails transition into the undersurface) and run a piece of masking tape up along both sides for 1M or so such that the inner edge of the masking tape is right on that line. It'll be pretty easy on a board like the Supersport because that rail is pretty hard. Now take ten pieces of string and tape them down diagonally from one side of the taped tuck-lines to the other, starting on each side at equal distances from the tail (important) and criss-crossed in the middle. You'll now have a series of co-linear "X"s down the middle of the board. Trace that line back to the tail (another piece of tape) and that's a good approximation of center.
For a non-planing board like an old Superlight you'd use the outer edge of the board's curvature on each side rather than the tuck-line. Back in the '80s I used to do that pretty often for guys who wanted a more powerful fin than the old Mistral kick-back piece of plastic.
I think my t-square does the same thing a damn sight faster lol
It also shows that boards have too much variation to find a true centreline in most cases.
If it's straight relative to the rails in the back 1/3 of the board is about as good as u will get. If that means its pointing an inch off the nose, so be it.
And Gestalt I suggest it can be done with care. My board I'm building now has zero rocker variation after 3 vac bag steps and using a a rocker spine/jig of course. (some have around 2mm extra rocker at tail which is easily rectified after laminating). Never had a board twist.
It should be 59.4 wide, and I leave a little bit of fat on the rail apex to allow for vac bag squishing. Its unfortunatley 59.6 wide, a grave injustice.
Now lets talk toe-in
Mark if you are within 2mm then my hat is off to you.
Can we talk me buying fin boxes from you at a besty price??
Some great advice here and some useful reality checks. Thanks for all the input! I will do my best to find the centreline, glue the box in and think about it no more.
Don't use fast epoxy for the install. Foam melty, melty not good after taking forever to set up...
Use a couple of builders squares, easy to work off these and find the centre, do it at 300, 1200 and 1900 and then a straight edge to line them up.
As everybody says, it's interesting seeing how bad the Cobra stuff is, particularly since the Covid evacuation.
I guess all of the gun staff had to leave the country?
Everybody goes on about symmetry but how symmetrical are any of us?
Don't agonise too much over it.
Select to expand quoteTe Hau said..
As everybody says, it's interesting seeing how bad the Cobra stuff is, particularly since the Covid evacuation.
I guess all of the gun staff had to leave the country ?
Did he ?
Select to expand quoteBrent in Qld said..
Don't use fast epoxy for the install. Foam melty, melty not good after taking forever to set up...
For those thick resin applications, I use Surfset Flex . Half a day to gel and 2 days to go hard . Very little heat involved . Then it's only hard plasticy , not brittle. Good for box and plug install and those dodgy inject de lam fixes .
Select to expand quoteManuel7 said..
Is the mast track centered and aligned with the center line?
This.
Yeah, it might be set wonky too, but its really the only useful straight reference point on the whole board.
Select to expand quoteHmm dunno
If U use a long straight edge on the deck using the track as a guide then mark nose and tail, then flip it and use those marks us will have a quick n easy centre for sure. Problem is the rail and tuck variations.
I'd rather go off the rail in the back half as that's what will be fighting the fin if it's crooked
Look at the toe in thread to see just how significant a degree is 
I had a board last week from a very very good shaper and a centreline marked from finboxes was 30mm off at nose but correct with respect to most of the rail measures I took
If I used nose to tail when replacing it would be off
Select to expand quoteMark _australia said..Hmm dunno
If U use a long straight edge on the deck using the track as a guide then mark nose and tail, then flip it and use those marks us will have a quick n easy centre for sure. Problem is the rail and tuck variations.
I'd rather go off the rail in the back half as that's what will be fighting the fin if it's crooked
Look at the toe in thread to see just how significant a degree is
I had a board last week from a very very good shaper and a centreline marked from finboxes was 30mm off at nose but correct with respect to most of the rail measures I took
If I used nose to tail when replacing it would be off
Yeah, makes sense. I hadn't thought of that.
