I've been getting "up close and personal" with through hull issues over the last few days. I like the marelon, but I've already got bronze, so I'll stick with those. There is one issue I've noticed that I have not seen discussed anywhere. With the flanged bolt-on type of seacock, the backing block, it seems to me, must be made of fiberglass. Not wood. Any swelling or shrinkage of the hull or backing block could cause the fitting to leak. With a conventional through-hull, one can just tighten the backing nut. The flanged type can only be tightened from outside the hull, with a special tool, probably after applying a heat gun to the fitting (i.e. not something a diver could do.)