If you have a gate valve threaded onto a through hull and the through hull does not have a flange you need to replace the whole thing. This is a commom "lazy way" of installing through hulls and is not supported by most insurance surveys and is also not ABYC or Coast Guard recommended.
The problem with buying a through hull, sticking it in the hole with 5200 then threading the nut onto it and then threading the valve, gate or ball, is this. Through hulls are what they refer to as straight thread meaning the threads are NOT tapered like the NPT threads of a gate or ball valve. In most cases you are lucky to get two or three whole turns down the threads of the through hull before it starts moving thus possibly breaking the seal. These threads are NOT meant to be threaded together. The straight threads of a through hull are meant to be threaded onto a flanged sea ****!
A seacock, the proper way of keeping water out, is a tapered valve or a ball vavle as made by either Apollo, Groco or some others who have them made by Apollo or Groco and it has a built in flange. These valves are made from silicon bronze and NOT brass! When installing these you must bolt them directly through the hull with a sufficient backing block. This does two things it gurantees a solid mount that can't be "snapped off" by a heavy item in rough weather, or by lightning and it guarantees that the through hull won't spin, breaking the seal, when you are changing the 1.5 inch odor safe hose attached to it. You are alo NOT dependant on two or three threads when trying to micky mouse a combination of straight threads and NPT threads. If you must use a threaded on ball valve Groco makes a bronze flange with male NPT threads! No one makes a through hull with NPT for a reason!!!