How important is it to have the base of the fin flush with the bottom surface of the board?
I find it is really difficult to get it flush at both front and back, especially if the same fin is used in more than one board. Are there any tricks for doing this?
If you can't get it flush, is it better to have the base slightly proud of the bottom of the board, or slightly recessed?

If you only have one board where the fin sinks further in then the fix is fairly simple.

On the board where it sinks in further

Lightly sand the inside front edge of the offending fin box (just to rough it up a bit)
File a small chamfer (3mm) on the bottom front edge of your fin base (the edge which goes in the board first)
Apply some vasoline to your fin base especialliy the front edge and bottom.
Mix up some Kneadit and make a push it into the front of the fin box
Insert fin and use screws to pull the fin down to the desired level
The Kneadit should then be moulded to the fin box making the fin box the correct size.

The same method can be used on fins just applying kneadit to fin rather than finbox

If fin is to large then file back more than necessary and use Kneadit to get it back to the correct size

This question was asked on the Fanatic Forum some time in the last 12-18 months, and Caspar answered and said that it is best for the fin to be slightly recessed (recess fills with water when sailing) than slightly proud (which creates resistance and turbulence). I had a quick look to find it for you but ran out of time due to a couple of work commitments.

Are there any tricks for finding the high spots which prevent the fin from seating properly?
I have one which is sitting a little proud of the box but I can't work out why, as it doesn't seem tight at all.

Try marking the box with chalk and see where iy marks the fin Anita!

Select to expand quote

Windxtasy said..
Are there any tricks for finding the high spots which prevent the fin from seating properly?
I have one which is sitting a little proud of the box but I can't work out why, as it doesn't seem tight at all.

If you rock the fin back and forward, it pivots on the tight spot. If it seems loose, and rocks on the leading and trailing edges, it means the fin is "bottoming out". In other words the box isn't deep enough for the fin. If you measure the depth of each at the front and back, that should tell you how much to remove. If the fin is too long for the box, that will also stop it seating, but it will gradually get tighter as it wedges into the base. In this case I sand the rear face of the fin head, as it's smaller than the front end. I try to keep front and rear ends concave, to help stop the fin pivoting back and forwards. As stroppo says, chalk can help work out where it's too tight.

I find the best way, if needed, to have the front of the fin slightly recessed and the end a bit proud. Being completely recessed creates a lipping affect at the end, if one observes water streamlines (if this is not important, why then bothering with sanding the tail end of the board as pwa guy do it to remove any uneven surface from the bottom) this would create a breaking effect. If the end part is proud than it is more of a stepping waterfall than a breaking efect, the water release is free. ... IMHO the change to the rake is minimal

I have similar problem as I use a 20cm delta in 2 boards.
In the big one I think it doesn't seat all the way ( raised ridge at base of fin above the box.It can also be moved a bit side to side but it fits ok with the smaller board. I have to use foam to pack up the rear in both boards to prevent it moving back and exposing a gap ( they are powerbox).
In my case I just figure I have to make do as if I modify it for one if won't fit in the other.

Assuming the fins are Tuttle base, very carefully sand the front or rear edge bearing surface of the base until it is a perfect fit in the tighter box. Then in the larger box insert a thin plastic 'packer' strip at the front or back when fitting the fin.

I cut these from the lids if butter or margarine tubs and have a stack of them in my spares box. Make them a bit narrower than the box width and slightly shorter than the end of the box you are fitting them in so they sit on the bottom of the box but don't protrude out the top when fitted. Also, fit you fins in a sheltered areas when using these 'packers' as it's annoying when they blow away as you try to fit them!

I have seen people use Mylar strips and strips of Coke can aluminium as well.

Oh, one more important thing. I have found that some Tuttle boxes are made a tiny bit too shallow (or the fin bases are made too deep). Check that is not the problem before doing the above operation!