OK one more image just in case you still think you can beat the odds and buy your fittings / valves at Home Depot.
This is a good example on two fronts:
#1 This should be evidence enough of why using a gate valve no longer satisfies marine safety standards. There was no way this valve was ever going to close fully even if the handle had not crumbled away. Seacocks should always be 1/4 turn ball or tapered cone type.
#2 This was a high zinc content valve most likely sold and intended for residential plumbing applications. When the owner tied to service it the valve literally fell apart in his hands. The pinkish/coppery color illustrates complete dezincification of the valve body which left it brittle. According to the owner who sent me this photo the bronze thru-hull fitting was in perfect serviceable condition but the brass valve just crumbled like tortilla chip.
This image is WORTH clicking on to get the full size picture! Look at it, let it absorb, then think about what would happen to your boat if this had happened to you while in the middle of the Atlantic... Bone chilling is what I see.
PLEASE DO NOT USE BRASS BELOW THE WATERLINE. MAKE SURE YOUR COMPONENTS ARE MARINE UL & MARINE BRONZE OR MARELON.