How a small faucet leak causes a non-stop water pump problem on a sailboat

Lauren Landers Sailing has a small water leak at one of the aft head faucets. Even though the drip itself is minor, it is enough to trigger the pressure-activated water pump to cycle on roughly every ten minutes, all day long. The root fix is straightforward: replace the faucet with a spare she already has on board. The complication is that she has never replaced one of these faucets before, and if the new fitting does not match she will be left with an open hole in the boat. Before starting the job she needs to locate a shut-off valve for the incoming water supply so she can isolate that line while still using water in the rest of the boat. She found a plumbing store nearby and went to source that valve before touching the faucet.