Carbon / Foam keel fin

[Herman]

You could opt for 2 versions:

-1. A centre spar, or to make it easier, 2 centre spars, wrapped around glass or wood (do not expect for the wood to do much in terms of strength. Laminate thickness of the spar webs are in the region of 8-12mm for your size of project. (please check yourself). These consist mostly of UD and +/-45. Using 90 is not a brilliant idea in compression, it can cause microcracking of the 0 direction fiber, and finally complete failure of the structure. (same counts for the skins)

Then do the foam fairings, and wrap them in UD and +/-45, which give you extra bending stiffness, and torsional stiffness. Use a progressive layup, beef things up considerably at the hull exit, then go thinner towards the end.

or 2. Make an upscaled dinghy board, so foam covered with laminate. adding fiber between the foam sheets does not bring anything substantial. just use bog, hotcoat the foam before, and preferably use vacuum for consolidation.

Use a good quality epoxy, and make sure you slow-cook the thing to reach the specs of the epoxy.

When using UDS, make sure they are placed absolutely parallel to each other on both sides (use the trailing edge or leading edge as a guidance) or the thing will twist.

It is a very nice but labourious job, but not too complicated. Make sure you have your calculations right, though.

[dougfrolich]

A ditto to Herman---
In my design above; I use a the D.Fir core because it will contribute to stiffness somewhat--and it provides a substrate to laminate on. The section properties of the spar were chosen to resist bending primarily, but with an 80-20 mix of uni to off axis fibers it will also begin to resist torsion. As the fin is finished with forward and aft shaped sections of foam covered in off axis carbon fibres the torsion consideration is compleated. I am quite happy with this arrangement in terms of strength and damage tolerance, and of course weight---it definatly delivers the lowest center of gravity, at the cost of very precise labour, over that of a fabricated steel strut/fin. The design time is also much greater with the CFRP strut and fin.