Re: Drains, valves, and seacocks: weighing the alternatives
If you do decide to add the ball valves, be advised that most valves will be NPT thread (ie tapered) and the through hull threads are not. What that means is that when you install the valves on the through hull threads you'll only get maybe 1 1/2 threads engaged before it tightens up.. not very secure. Check Mainesail's 'how-to' site for the RIGHT way to do this.
Replacing Thru-Hulls and Seacocks Photo Gallery by Compass Marine at pbase.com
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)