Our new boat came with plastic seacocks, and when I closed one of the small ones, the handle broke off along with part of the tapered plug, leaving an open hole in the hull. I immediately pushed the broken piece back in place to reduce water flow, but it was still coming in faster than the bilgepump could handle. I needed a quick remedy.
I keep a can of Plumber's Putty in my toolbox. I jammed a wad of this into the hole, put a small piece of wood over the opening, taped the whole thing in place with electrical tape, then stood up and admired my handiwork. The leak was stopped.
Plumber's Putty resembles modeling clay and is used to bed fixtures like the drain in the middle of your sink. It can be molded into any shape, is always semi-flexible, and leaves no residue when removed. It's perfect for plugging sudden leaks and should be in every sailor's toolbox.
I also carry a can of Marine-Tex and a small piece of fiberglass cloth I can shred into fibers to mix with the Marine-Tex. This makes a composite putty-and-glass lump that can be molded in place. For example, when a friend found a hole in his exhaust system, I put some Marine-Tex mixed with shredded glass over the hole and let it cure. The next day we motored home, and everything was fine. My friend, who is a thrifty sort, never did get around to replacing the damaged exhaust system. It was still there when he sold the boat. Rod Glover