Not sure you need to replace through-hulls
I did the same thing you're contemplating; replacing gate valve with ball valve. My through-hull fittings were in fine shape, solid, no leaks, etc. As such, I essentially just unscrewed the old gate valve and screwed on the new ball valve - while the boat was floating in my slip. Have a wood bung at the ready. In sum, about 1 quart of water made it inside the boat. Below, I've copied in the steps I followed and previously posted to the archives.Good luck!The new valve is in! Submitted by Paul on 06/09/2004 at 06:18AM regarding General_interest It's done! With the tips provided, I replaced the seacock without hauling the boat. It was a relatively simple process, once I screwed up my courage (and double checked the bildge pump was working). Here are the steps I took:I spayed the through-hull threads and old valve with Liquid Wrench then let it sit for about 10 minutes. With a wooden plug at the ready, using a small pipe wrench and minimum torque, the old gate valve came off rather easily. (To anyone who tries this method, go slowly to ensure you don't torque loose the through-hull fitting. Also, Chris was right; first "tighten" the valve by a hair, then unscrew it.)Once the valve was off, I jammed a wooden plug into the through-hull fitting. The water that did come through was minimal and at low pressure. The plug worked like a champ.With the plug in place, I used a wire brush on the through-hull fitting's threads, dried everything, then wrapped a few layers of Teflon tape around the threads. With the new BALL valve in hand (and in the "closed" position), I yanked out the plug and screwed on the new valve. Less than a quart of water made it into the boat during the whole process.The new valve is in place, new hose is double-clamped to the valve, and the trusty wooden peg is tied near the valve.Thanks for all the helpful advice from everyone who responded to my question. Together you saved me about $100 in haul out fees. If you lived in my area, I'd buy you a beer.Paul