PLEASE, PLEASE replace the thru-hulls and valves on the older boats! If your boat is aging, the thru-hulls could have serious perforation from electrolysis or be starting to get brittle. I recall reading a message from one person on this list who went to close a valve and the whole thing came away in his hand with suitable ingress of water! Imagine the language! As suggested, the Marelon are great but I went with bronze (be aware that there are poor quality bronze thru-hulls available too) and with marine grade bronze ball valves (again, don't use brass). I got my valves from an industrial wholesaler for between a quarter and half the prices charged at chandleries – no exaggeration! The extra money paid for the better quality thru-hulls; never let The Admiral think that savings like that mean "We're going out for dinner"!
Although perforation was not so much of a problem for the Dragon as I tie up in fresh river water, I knew they should be changed when one of the nipples started unscrewing from the hull! No kidding. Bronze thru-hulls are either domed like a mushroom and protrude outside about 1/4", or they are flush mounted. The flush ones have to have the hull carefully shaped to take the tapered edge of the flange; much more difficult to install. Unless you are racing and need every 100th of a knot, get the mushroom type. Be careful to use the right sealant also; it must be for underwater use. Some people recommend 3M 5200, which means that it is there forever - it will rip the gelcoat off the laminate before letting go. I suggest 3M 4200 which means that it will seal really well, but can be removed if you work at it. Cannot remember which version of the Sikkaflex is best. In either case, make sure you have suitable added thickness of hull at the location to strengthen the hull. Do not use wood or plywood, (I removed the rotten plywood from mine) and make sure that the built up glass/resin inner hull surface is parallel to the outside surface.
How do you do that, I hear you asking?
Take your mushroom thru-hull and apply release agent or tape (I used masking tape) to the surface under the lip of the dome, over the dome itself and around the threads to stop resin sticking to it, then, pre-test that your thru-hull....with the tape on, will still go through the hole. Load your thru-hull up (under the mushroom) with as much caulking as it will hold, and gently stick it through the hole, FROM THE INSIDE, so that the dome pulls the caulking down onto the inside surface of the hull. And, the caulking should be resin thickened with agents and usually short chopped glass strands.
Hold it there while someone gently tightens the nut from the outside and jams a piece of wood against the thru-hull opening so that the nut is firmly wedged against the hull. Or apply a stretchy cord so that it pulls up evenly into the middle of the boat. Snug up a little more on the nut and let it kick off in a few minutes. On a big thru-hull like the head outlet, you want the bear-shit at least about 3/8" thick under the dome and the edges spread out away from the dome in order to distribute the loading of the thru-hull, especially if it receives the shock of being hit from outside later on. The flat pad left by the dome is larger than the nut surface anyway. When you remove the thru-hull you will find that except for a little trimming, the inside surface will be parallel to the outside of the hull, even if the old surface was all lumpy and not parallel. Voila!
Apply the sealant so that it gets squished out from the inner diameter of the dome toward the outer, and do not tighten it up hard all the way; just till it barely touches to get an even seal. Wait a couple of days for it to become set up semisolid rubbery and then snug it up the rest of the way. Try to have someone else insert something into the thru-hull to hold it stationary by the tangs or tabs inside while you exert torque on the nut inside; you don't want to tear the new sealant. Similarly, be careful about applying torque to the whole assembly when you tighten down hose barbs or whatever onto the valve body inside. By waiting until the sealant is set up, you will be doubly sure of a seal, as you will be applying pressure to a perfectly formed gasket. It will actually bulge a little around the edges from the pressure.
I used a toothbrush to paint the insides of the larger bronze thru-hulls with some spare epoxy to slow the galvanic action further.
Richard Britton, Friendly Dragon