Here is how I’ve gone about making my latest. It is pretty much standards procedure but may have some aspects of interest to some of you.
Fin box construction ideas.
Select the best quality ply for this if your sheet is not uniformly prime all over.
Cut the pair of sides exactly to the plan.
It is prudent to make a fiberboard template of the fin in the same thickness to build the fin box around and to make the locking holes. It will also make marking the aluminum precisely when making the keel itself. Strongly recommended to make this template. It will be also used to line up the fin box in place, vertically before fixing into place.
The inner surfaces need to be coated with a durable, 100% waterproof coating which is epoxied fiberglass matting.
Matting is available on hobby king in various grades, I use either
Super gives a very thin, dense finish.
The best epoxy for fiberglass is finish cure.
Don’t use penetrating epoxy.
You can use warmed up 5 minute epoxy but finish cure is made specifically for doing matting jobs and as it is so thin, I don’t warm it until after it is in place, and even then only mildly.. It will cure overnight and can be sanded (doesn’t need to be, for the fin box ,unless there are any lumps)
Wear disposable rubber gloves if you have them but epoxy cleans up readily, and cleanly, with Metholated spirits (Denatured alcohol ).
Cut an over size piece of cloth for each ply piece. And tap into firmly into place , fixing at the back of the ply this will be trimmed off after curing, so neatness is not an issue.
Mix up the final cure
Mix up the epoxy (finish cure has a lengthy pot life, warmed 5 minute has just minutes, at best) , and apply with a small flat brush, or even a gloved finger. The Hobby King product page gives an excellent “how-to” as follows (I’ve edited out bits that don’t apply for fin boxes)
“FINISH-CURE 20 min. epoxy is an excellent, low odor substitute for polyester resins. It can be used for applying fiberglass cloth to wood or by itself to give wood a surface ready for primer and paint. FINISH-CURE can be sanded the easiest of all our epoxies and is excellent for the sheeting of foam core wings. Allow 8 hours for full curing. For best results, FINISH-CURE should be heated to a temperature above 85 degrees F or applying light weight fiberglass, lay cloth on balsa first, then brush on FINISH-CURE. When fully saturated, go over the surface with a heat gun, and then squeegee off excess epoxy with a playing card from an old deck. Heat and remove excess several times for a light weight finish. If room temperature is below 70 degrees F use a heat gun on the surface several times for the next 2 hours. When dry, lightly sand., scrape off excess with a playing card. Sand to a smooth finish ready for primer. FINISH-CURE is best mixed in a disposable cup. “[Thanks to Hobby King for that advice]
That internal finish does not to be ultra-smooth, just abrasive resistant.
Use close grain soft timber for the top and sides, pine is very suitable.
I make the fit a bit tighter than Gary recommends, but as the keel will always be under pressure from some direction it will lock into place while in motion.
Only two holes for the locking pins are necessary and the holes in the keel head may be a little oversized to facilitate putting the bolts in place, particularly in cold weather. I use lightweight nylon bolts cut to length to suit, with a hole near the end to take a straight wire pin about 2 ½” long so they stay in place.
The cap on top of the fin box can be made to be removable, not likely to be needed but you can’t do it later if not set that way. If you do it, just place either thin layer of silicone under the cover strip (do it after you paint the timber, if silicone gets on to it i won’t take paint effectively. Then screw down AFTER silicone is cured.
IMPORTANT!! Gary highly recommends making the fin box first before fitting the bulkheads. Just follow his instructions.
Leave the section of the fin box unpainted where it goes through the hull to ensure a quality glue joint. But it’s a good idea to paint the rest of the box before emplacement as it is harder afterwards.