Unless you plan to propel your boat with a steam engine or turbine,
your engine will probably be some type of internal combustion device.
Either gasoline or diesel fuel power these engines, and the engines
themselves are of either a two- or four-cycle design.
Gasoline Engines - 4-Cycle: These engines power everything from ski
boats to mid-sized family cruisers through drive systems such as
inboard/outboards, Vee drives, straight drives, and so on (see Drive
Systems).

In the gasoline engine, a mixture of gasoline and air is introduced
into the cylinder head (or heads). The mixture is then compressed to
about 200 psi (pounds per square inch) and ignited by a sparkplug. The
resulting heat expansion drives the piston downward, supplying power to
a rotating crankshaft. Most gasoline inboard engines are a version of a
truck or heavy duty industrial engine. Remember, your car gets to coast
going downhill, but your boat engine is pushing uphill all the time.
Also keep in mind that a boat engine is driving the hull through a
heavy fluid, not just air.

Some advantages of 4-cycle gasoline engines a 1) They produce more
horsepower per pound than their diesel counterparts, 2) with the new
ignition systems, they are very reliable, and 3) they are much less
expensive in initial cost.

Disadvantages are that they burn more fuel than a diesel engine and
will not last as long. And let's not forget the inherently dangerous
nature of gasoline! Although gasoline has a lower heat producing
ability than diesel fuel (124,000 BTUs per gallon for gasoline, versus
138,000 BTUs for diesel fuel), it has a lower flashpoint. In other
words, gasoline will explode when exposed to an igniter. Because
gasoline fumes are heavier than air, they can sink to the lowest part
of your boat (the bilge) and stay there until something sets them off.
Forget to ventilate and Ka-Boom! Voyage terminated!

Best regards
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