Through with a Thru-hull

GilStump · #1 (**permalink**) 06-07-2009

First, thanks everyone for all the advice. Priceless. Hopefully one day, I too will have advice to offer.

Second, I just installed a Nature's head. I like it. But I am a bit of a hippie-tree-hugging eco-freak.

Anywho, I now have a thru hull that I have no use for. And I am on the hard. She's a fiberglass boat with a 3/4" chuck of plywood for a thru-hull backing plate already glassed in by the manufacturers. Should I fill it? If so, how? If not, is a dummy plug in a thru hull a really stupid idea?

MedSailor · #2 (**permalink**) 06-07-2009

I'm about to install a nature's head myself and have the same issue. Though for me it's 3 through hulls. One for water in, one for the macerator post holding tank and a larger one directly connected to the head that the sewer trout swim out through.

I'm going to use one which will be attached by hose to a waterless urinal. This will also provide a convenient dumping place for the urine bottle. This should have a valve that either has a removable handle or a lock in order to strictly be in compliance with coast guard regs.

If you have no possible use for them then yes, glass them up in the yard. Be sure that you can't use them though since they're already there. How about a salt water tap either with an electric or a foot pump? Could help with water conservation. You could also use one for a washdown pump for the anchor, or for a watermaker.

Not to hijack, but any advice for a soon-to-be nature's head owner?

MedSailor

GilStump · #3 (**permalink**) 06-07-2009

That's a great idea. A washdown pump.... hmmm. Honestly, I just installed it myself, so no advice as of yet. That sucker is tall ain't it?

KeelHaulin · #4 (**permalink**) 06-07-2009

Well I'd say use a thru-hull fitting that is blocked off such that it can't allow flooding. Maybe fill in the existing thru-hull with thickened epoxy and then cap it off on the interior (redundancy to simply capping it).

I don't like the idea of grinding back and then re-glassing; to me it does not seem like a stronger or safer way to plug a hole; but that's just me. The reason is that you have to taper the hole way out and then be sure you get a good bond on that bevel with the end-grain of existing laminate. I'd rather have a clamped bronze fastener on the hull than a piece of plastic glued on from the outside that has to stand up to hull flexing and shear stress transmitted across the glue joint.

sailingdog · #5 (**permalink**) 06-07-2009

If you have access to both sides, a double sided repair with a 12:1 bevel will make a repair that is as strong as the original laminate or very nearly so, especially if you're using EPOXY resin, which has a higher tensile strength than Polyester or vinylester resins, and a very high secondary (adhesive) bonding strength.

Merely plugging the through hull leaves you vulnerable if the brozne dezincifies, and requires you to do constant inspection of the through-hull to check for dezincification.

tager · #6 (**permalink**) 06-07-2009

Dog, have you ever done fiberglass work?

KeelHaulin · #7 (**permalink**) 06-08-2009

Quote:

Originally Posted by sailingdog

Merely plugging the through hull leaves you vulnerable if the brozne dezincifies, and requires you to do constant inspection of the through-hull to check for dezincification.

Generally bronze de-zincifies when it has a galvanic couple; which would require electric current passing through the water or the thru-hull fitting (if it is bonded to the ground wire). The presence of bronze in contact with seawater alone will not create a galvanic couple and therefore there would be no electric potential for de-zincification (the inherent corrosion resistance of the bronze is preserved). I have bronze thru-hulls that are original on my boat (30 years old) that look like they are new when polished to bright metal; no discoloration or red flecs in them. If your system plumbing keeps the bronze fittings isolated from DC electric current or galvanic coupling (dissimilar metals) you should have little trouble with de-zincification. The one thru hull on my boat that could be problematic with de-zincification is the engine raw water thru-hull since the engine is an electric ground. Generally this thru-hull is the one to worry about when it comes to galvanic corrosion.

A thru-hull that is blocked off and not plumbed to any water line or bonded to a ground wire should be the least worry in terms of galvanic corrosion.

sailingdog · #8 (**permalink**) 06-08-2009

Quote:

Originally Posted by tager

Dog, have you ever done fiberglass work?

Yes, quite a bit of it as a matter of fact. My last major project on my boat, in terms of fiberglass work, was installing a bridgedeck.