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A large percentage of recreational boats over 28 feet (8.53 meters) in length, as well as work craft of all different sizes, carry marine generators to provide electric power to various boat systems and accessories. A marine generator, while not an absolute necessity, is handy to have in circumstances where the main engines are shut down or shore power is unavailable, i.e. anchored at sea. This alternate power source saves battery power and uses considerably less fuel than the main engines.
The typical marine generator, like all mobile generators, burns either diesel fuel or gasoline. A diesel powered boat will, therefore, ordinarily carry a diesel generator, a gasoline powered boat, a gasoline generator. The power generator on a boat will typically be tied into the vessels engine fuel line, thus doesn’t need a separate, space-consuming fuel tank. There is some excitement in marine circles these days regarding the emergence of efficient and clean-burning hydrogen-powered boat generators. The only disadvantage to a hydrogen-powered marine generator at present is locating a fueling station to provide the hydrogen fuel.
Marine generators come in all sizes and are usually rated by the amount of kilowatts of electrical current that each produces, a kilowatt being a thousand watts. Smaller recreational boats may require a generator capable of no more than three kw while larger, recreational or working boats, such as luxury yachts, fishing vessels, ferryboats, and harbor tour boats often have generators of 25-plus kw capacity on board. A boat owner considering an electric generator would be well advised to total up the wattage usage for all appliances, instruments, and systems anticipated to be powered by the generator, then consult to ensure the right marine generator is chosen for the particular boat.
A marine generator differs from the ordinary residential or industrial generator in that it is designed and constructed to withstand the moisture and corrosion problems associated with the marine environment. Additionally, the vast majority of marine generators don’t need a self-contained cooling system, being cooled instead by hydraulic pumps constantly circulating seawater through the generator core. Thus, a marine generator will take less space in the usually cramped engine spaces of the typical recreational or work boat.
Beware of confusing an inverter with a marine generator; each is a separate electrical power producer. An inverter will convert a vessels battery power, which is direct-current (DC) into a weak alternating-current (AC) to provide power. A generator, on the other hand provides constant AC electricity independent of the batteries. This can be a serious consideration when attempting to start an inverter-equipped boat with drained batteries.
This article is for the most part correct, but I would add a point or two. Unlike generators built for stationary use on land, Marine Generators are typically fitted with special oil pans. These special pans are often deeper than their land based counterparts, and are sometimes fitted with baffles inside the pan. The extra deep pan and the baffles help minimize sloshing of the oil as the boat rocks and rolls in the waves. Without these added features the oil pump may loose suction as the oil sloshes around the pan. (For those who may not know, oil is the life blood of any engine. Without oil an engine will come to a speedy, and often dramatic halt)
The cooling system of a Marine Generator is another area of interest. As stated in the article above some small Marine Generators use the water the boat is floating in for engine cooling water. That is to say that the water from the lake, river or sea is pumped directly through the engine block to provide cooling. If at all possible you should avoid buying this type of engine. Nothing good can come of this type of cooling system, especially if the boat is going to be operating in salt water. Engines with this type of cooling system are particularly susceptible to damage by corrosion from the inside out. Damage to the engine from corrosion will be invisible from outside the engine, but is absolutely a given with this type of cooling system.
Instead you should look for an engine that utilizes clean fresh engine coolant, and uses a heat exchanger (like a radiator) which uses the cool water that the boat is floating in to cool the engine coolant without mixing the two. There are several different styles of marine heat exchangers available and your engines manufacturer should be able to recommend a model or style that will work for your engine.
You can check out this link for examples of marine heat exchangers.