Fenders, as most sailors know, act as bumpers to keep your boat off the dock and prevent its hull from being scratched or chafed by whatever protrudes from the dock, like cement or wood pilings.
The problem with all the battery-powered burglar alarms we have found at hardware stores is that the alarms were neither louder nor more frightening than a chirping canary.
The stuffing box is a critical part of the boat. It allows a driveshaft to spin through the hull without allowing water to pour into the vessel. These units are often deep down in the boat, behind the engine and with limited access.