some info found, from Sailnet resource "article"
I removed a few the way he explains(below). Also, have used a Sawsall (reciprocating saw) to cut off a gate valve handle, which was sideways. If the thru hull turns around in its spot, then it will need to be rebedded.
reference also: Don caseys book, " sailboat hull and deck repair" page 27 for illustrations.
I would use bronze ball valves for in-line feeds, where stresses are less, and seacocks ( with flange)if mounted to the hull bottom with support blocks and polysulfide.
Article....Removing Thru-Hull Fittings
...... What is the best way to remove or plug the thru-hull fittings that connect to the head?
Dan Dickison responds:
Once the boat is on terra firma, just dismantle the head and take it out.
Start by removing the hoses from the inlet and the outflow, and then unbolt the base of the commode from the cabin sole. After that you can break down the thru hulls and begin removing them from the hull. Once you have the seacocks or valves removed, the project may become a two-person job. If the nut that seats over the thru hull and fastens it to the hull is stubborn—and it's likely to be—you can always try positioning someone outside the hull with a flat file and a crescent wrench. Have them insert the flat file securely into the thru hull and then clamp the crescent wrench around the exposed portion of the file. Now you can begin to torque on the nut from the inside with a large wrench, probably a plumber's monkey wrench. If the nut or the threads are at all corroded or the threads on the shaft are galled, you may have to cut the thru hull out with a hacksaw.
Once you have all of the old parts removed and you're staring at two holes in the hull, you can begin to prep the holes for patching. Working on the inside of the hull, grind down the area around the holes so that you remove all the paint and expose the fiberglass. Don't grind too far, just enough so that your new glass and resin will adhere well to the existing material. Clean the area around the holes thoroughly with a clean rag and some acetone. You can then cut up pieces of chopped strand mat and fiberglass cloth in graduating sizes. Mix up resin and catalyst and wet out the fiberglass pieces that you have cut up and begin by applying them—the smallest pieces first—over each hole. You'll want to build up a pretty good thickness, roughly the same as the original thickness of the hull. Once the resin catalyzes, you'll have a fairly firm area to patch from the outside with resin mixed with a filler.
For additional security, you might want to grind a small area around the holes on the outside in a conical fashion and fill that with resinated mat as well. However, make sure that you leave a slight depression so that you can later fill that with resin that is mixed with an easily sanded filler like microballoons or cabosil. Afterward, you can sand the outside surface smooth and then coat it first with catalyzed resin or a primer for resistance to water penetration, and later with bottom paint.
All of this is well described in Don Casey's bible on maintenance entitled This Old Boat. Here's hoping that this information helps you in your project.
