Seacocks are required to shut off the flow of seawater supplied via a through-hull fitting to onboard equipment, including engines, gensets, livewells, air-conditioners and the head. A seacock is not simply a ball valve. Its threads mate with those of the through-hull fitting, which has NPS (National Pipe Straight) threads. In-line ball valves have NPT (National Pipe Taper) threads. If you screw a valve onto a through-hull, you won’t get a good seal and won’t be able to screw the valve down flush with the hull, and since valves aren’t fitted with grease ports, bonding wire tabs or drain valves, winterization, service and maintenance are problematic. A “proper” seacock is fitted with a wide flange, is capable of being fastened to the boat so that the assembly can’t spin, and features a hole in the handle into which you can insert a socket wrench to extend your reach and operate seacocks deep in the bilge. Seacocks may be made of bronze or Marelon. Here are the steps for a solid fiberglass hull.