Gel Coat Rub Rail Cracking

What causes those consistent and persistent LNVT rub rail cracks and how can they be eliminated once and for all? The cracks are consistent in that they almost always appear on the upper (i.e. the skyward) section of the rub rail and are usually nearly vertical. They're persistent in that many owners have repaired the cracks only to have new ones appear. When Nellie D. was repainted last year her many rub rail cracks were ground out, filled and faired. These cracks were only in the gel coat and did not penetrate the underlying fiberglass. So, what's causing these cracks? I had an experience yesterday that got me thinking. But first I must digress.

An opening in the stern bulwark clearly shows the rubrail's curved indent

I've preformed some exceptionally ugly landings lately. Yea, it's been 10 months since I've had Nellie out—but still. Anyway, during the last 'crash' I managed to drive Nellie's hull, just below the stateroom's port light , rather abruptly into the corner of the floating dock. Fortunately the dock was well padded and left only a rubbery raspberry on the hull. After counting my blessings, and spending 30 minutes removing the offending blemish, I noticed a big, new rub rail crack (see photo) almost directly above the point of impact. Hard to believe my little bump didn't contribute to this new battle scar. On the other hand, some strong winds in the past few days had pushed us rather forcefully against the floating dock. Either event would cause the hull to flex and I've suspected that movement of the fiberglass substrate is the root cause. Flexing can be caused by many types of dynamic loading: a rough passage; bumping a dock, a piling, or even from a fender; and certainly from the straps during a haul-out. But why does this otherwise normal fiberglass flexing manifest itself as a crack only in the rub rail?

The new crack …

So here's the theory, the hull mold was upside down, i.e. keel facing up, when it was sprayed with gel coat. Remember, the hull mold includes not only the rub rail but the bulwark too. Imagine a guy standing inside the hull with a spray-gun trying to put an even, thin layer of gel coat on all the surfaces surrounding him. It's fairly easy to do on the large flat areas of the turn-of-the-bilge, hull sides, and bulwark, but the rub rail, which is the nexus between the hull sides and bulwark, is a narrow, 6", concave indent (see first photo). If the spray was too thick in the indented area, the gel coat would run and pool on the indent's lower back and bottom surfaces. The exact surfaces, in fact, where today's rub rail cracks appear. Thus it's this too thick gel coat that's the problem; unlike the thinner gel coat everywhere else, the too thick gel coat can't handle the nominal deformations of the fiberglass it's attached to. The only practical way to eliminate rub rail cracking then is to get out the sander and remove the too thick gel coat.

See: Getting a Handle on Gel Coat Cracking, by Bob Lacavara