Enlarging a through hull fitting hole in your hull.
Marcus, There are two simple ways I know of to do this. If you want the larger 3/4" hole to be exactly slap-dab in the same place as the existing 1/2" one, buy a tepered wooden plug at West marine that you can pound into the 1/2" hole. Mark where it exits the hull with a pencil all the way around the plug, remove it and make a saw cut on or slightly outboard of that mark. Reinsert the shortened plug and mark the center of it with an awl or ice pick. Now use a 3/4" hole saw with pilot drill to exactly cut out a larger hole. The plug will keep everything centered and within seconds you'll have the 3/4" hole in your hull. If you can live with the hole being 1/8" off center then simply use that same 3/4" hole saw and cradle the bottom teeth in the bottom of the existing 1/2" hole. Using a variable speed drill (aren't they all like that today?) cut a new hole slightly (1/8") higher than the original. Tilt the drill down a bit so only the bottom teeth of the saw begin to cut the new hole. As it bites in, level the drill and finish the hole. That's the easiest and fastest way to cut a new hole, I've done it that way many, many times. Marcus, you really need to replace what's there with the proper plumbing, a 3/4" seacock. The problem with using a 1/2" through hull fitting (straight threads) and adding a simple ball valve (tapered pipe threads at both ends) is that the threads will only engage to a depth of about two threads rendering the assembly extremely vulnerable to any side blow which could result in the ball valve snapping right off the end of the through hull fitting, something none of us want to have happen in our waking hours, only in our nightmares. If you plan to have family or firends aboard, consider their safety and do it the porper way with a seacock. If tragedy were to occur, you'd never forgive yourself for not spending what would be chump change compared to injury or loss of life by reinstalling 1/2" mismatched plumbing. Do the right thing, Glyn Judson, E31 hull #55, Marina del Rey, CA