Groco SBV Safety Seacock
A proper seacock is more than just a mounting flange with holes for bolting it down. It contains specially arranged internal threads, top and bottom which is why an ordinary ball valve or gate valve should not be used in a hole in the bottom of your boat.
The "mushroom" that is the actual thru-hull has straight threads (like a bolt), so that a locking nut, or the base of the seacock itself, can screw down on it for its entire length and make a firm seal against the inside of the hull.
The pipes that make up the plumbing in your house have tapered threads on both mating pieces, intended to screw together only a few turns before they jam and make a watertight seal. A household-type tapered valve screwed onto the straight threads of the mushroom would also achieve only a few turns, but it would be weak and leak-prone.
At the top of the seacock, the story can be different and more complicated. Look at the descriptions of seacocks and you will see that some have straight threads top and bottom (NPS, for National Pipe Straight), and some have NPS bottom and National Pipe Tapered (NPT) at the top. The tapered thread at the top is probably more common. It receives the tapered threads of tailpieces for the connection of hoses. Straight threads at the top are used when some other piece of hardware, like a strainer, is to be installed. The important thing is to know the difference, and to select the correct mating pieces for the application.
Safety Seacocks include a side port that allow you to use your engine's raw water pump in an emergency to pump high bilge water overboard through the exhaust system. The SBV Bronze Full-Flow Safety Seacock models, shown in the diagrams, include a Service Adapter fitting that lets you flush your engine with fresh water, antifreeze or a salt removing treatment such as Salt-Away.