Journal of Applied Windsurfing Science

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... making a fin

by Richard R. Thomas

Forget about making a mould. Unless you are going into commercial manufactureing it is not worth the trouble.
The best way to make a fin for a standard finbox is to find some plate aluminium that will fits nuggly in the slot.
This (Al) plate is used as the frame for your fin. The plate is cut out like an inverted "T' shape. The top of the "T" being the base of the frame. This base can then be drilled and tapped to accept the lock tab and bolt. The back roll pin locator hole can be drilled as well.
The wider the "T" leg is the better so that a large surface area is available in the glass lay-up stage, but this is predetermined by the Cord length to a certain extent. The frame can be aggressively drilled to lighten it and act as keying holes for the lay-up.
The leg of the "T" should be raked aft between 20/25 degrees in relation to the base, ** more on that later.
Determine the side elevation profile of the fin and cut it out from a high density foam like Airex or Divinicell, (I prefer Airex). * Note, Airex requires a resin coat before lay-ups are done. Find the centre of the edge of the foam and mark a line around the outline so you can shape to those marks later.
Lay the frame on the foam and mark and cut out the inverted "T" leg so the foam and frame fit together. Epoxy glue the two parts together to form your fin.
You can then shape the Cambers and Taper of your fin by sanding them to the edge marks.
You can then lay-up the fin by using fibreglass and epoxy resin. You will need a minimun 2 or more layers of 200gm fibreglass. Each layer wraps around the fin. The free ends are extended past the trailing edge and can be cut off later. Don't lay the glass over the base part of the frame though, (it wouldn't fit in the finbox then would it). If you use the aluminium frame then you do not need to use carbon fibre or any other exotic materials at all.
You will wonder if you will ever get a proper shape because the fin looks very bad at this messy stage. It does get better.
After the epoxy resin has cured you can cut and roughly sand the fin to shape.
To obtain the final shape you use epoxy resin fillers like Phenolic Micro Balloons or "Q" Cells to mix a filler paste. This is applied all over the fin and sanded when cured to the final shape. This filling and shapeing may have to be done a few times.
A final resin coat can then be applied and rubbed back.
Easy, you have built a fin in less than a week and no more than one hour each day.

** Because the Bic has its footstraps near the Stern the planing line starts from the front footstrap to the stern. When planing the bow has a 20 degree +- bow up attitude. The fin rake therefore has to have a similar rake aft otherwise it becomes a forward swept foil, (big problems). Most fin manufacterers do not understand this still! Measure their base/rake angles and you will find very small rake angles, so guess what they become?
I build 350mm fins for my MPB's that weigh just over 200gm's installed. I don't usually use finboxes as the fins are designed to be part of the hull, (I don't need to use aluminium frames for these particular fins though). You should be able to build a lighter fin than any commercial moulded or cad-cam fin.
If you have different fin boxes you have to fabricate the frame base to suit.
You can shape the whole fin out of a single piece of material, G10 fibreglass, aluminium etc. But I think it's too hard to do.
Moulded fins would be much heavier than the framed ones I have described. The lock tab area is a weak point also with a moulded fin. (The metal frame fin eliminates this problem to a large degree).
If you reduce the Span, (lower the Aspect Ratio) you can still get the same surface area by increaseing the Cord. This will lower the loading on the span. It may be wise to go for a lower Span. There are some major disadvantages associated with high aspect ratio fins.

We have not discussed Cambers or Taper yet.


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