Most boats have a gasoline or diesel-fueled internal combustion engine. Many also include gas-burning appliances for cooking or heating. Having one or more sources of fuel on board presents a potential danger, especially if you are unaware of leaks or vapor accumulation.
Many boaters have some serious misconceptions about fuel fumes in the bilge or engine compartment. One is that the engine blower fan will eliminate dangerous fuel vapors prior to starting a gasoline engine. In fact, any sort of fuel leak can create new fumes as quickly as the blower fan can clear them out, so the risk of explosion is always present, whether the blower is running or not. And although diesel vapors are not explosive, high concentrations can make you sick. One approach is to recognize that any fuel vapor in any part of your boat presents a risk serious enough to warrant the investment in a fume detector.
Fuel vapor detectors will detect more than just gas fuel vapors - they are sensitive to any combustible vaporcooking fuels, hydrogen, solvents, and certain cleaning compounds. Here's how they work: a special wire in the sensor has a small electrical current passing through it whenever the unit is turned on. The presence of combustible hydrocarbon vapors causes a change in resistance in the wire. This change in resistance in the wire changes the amount of current passing through the wire, causing the alarm to sound.
The Effects of Carbon Monoxide
Carbon monoxide (CO) can overcome a person very quickly, and in high concentrations, it can be fatal in just minutes. The most prevalent source of CO is exhaust from engines and generators. These fumes reach boaters either from a leak in the exhaust system, fumes that are sucked back into the boat through intake vents, from exhaust that lingers outside the boat while the boat is moored in a boat house or marina with a motor left running, or from other boats. Because carbon monoxide is not humanly detectable, the installation of CO detectors aboard all recreational boats is recommended. Remember, make sure that your boat is well ventilated.
(From Boat/U.S. Tech Notes)