Doug Dotson wrote:

I don't trust plastic below the waterline. Marelon is
plenty strong but bronze is stronger. I have to install
a few more thru-hulls this spring. I guess I'll have to
do some thinking about it.

I had the same worry before I decided so I tried (as my compulsive
engineer personality requires) to analyze the threat. Corrosion is not
a problem. The main threats of marelon vs bronze are breakage because
of the lower ultimate strength and dimensional stability. Dimensional
stability results in sticking so routine exercising the valve and
lubrication are definitely in order. That is a good idea for bronze
also because chrome on bronze balls can flake and jam and stainless
balls can suffer crevasse corrosion between the seals. That leaves the
strength to be tested. I see 3 potential forces acting on a flange
mounted throughhull. The water pressure from outside for which the
flanges are far over designed, the bending force of the hose and
fittings but the fittings are much weaker than the throughuull and
potential impacts. Side impacts would be the big worry.

I tried to visualize what would cause a significant side impact on a
throughhull, probably in a knock down or roll over and came up with a
battery breaking loose, an anchor stowed loose in the bilge. So I found
an old used Forespar 1 1/2" seacock on ebay, mounted it on a piece of
1/4" steel plate and whacked it on the side with a 5 lb sledge. First
blow had no effect. Second blow produced a small crack about 1" long
leading from the valve stem. Third blow broke a bolt flange and
lengthened the crack some but it was still well attached to the
throughhull. I have beaten enough bronze castings to pieces to fit in
my crucible furnace that I don't believe a bronze throughhull would have
held up but a little better.

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
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