How Does an Outboard Motor Alternator Work?

The alternator on your boat is only slightly different from the one on your car’s engine. Both create electricity by induction, and both involve relative motion between copper windings and a set of fixed, permanent magnets. The automobile alternator's rotor is turned by the pulley on its face to generate its electromagnetic field. The approach used by an outboard's alternator is slightly different.

  1. Driving the Outboard Alternator

    • The charging system on some larger inboard and inboard/outboard engines is much the same as that of an automobile, with a separate, pulley-driven alternator. An outboard engine, though, has an “internal” alternator. The system of permanent magnets is mounted in the interior of the engine, on the flywheel. The flywheel is surrounded by a spoked metal frame. The spokes are wrapped with copper wire.

    Electromagnetic Induction

    • As the crankshaft runs the flywheel, the permanent magnets on the flywheel move past the copper windings on the spoked cage. The magnetic force of the permanent magnets whirling past the copper windings excites the electrons in the windings' copper wire. This electricity is called an “induced voltage” or “electromagnetic induction.” More windings of copper wire increase the amount of available power the alternator produces.

    Raw and Uninhibited Voltage

    • As the permanent magnets spin inside the copper windings at the speed of the engine, the electricity’s polarity changes, or alternates, as the south and north magnetic poles of the permanent magnets pass the wires of the windings. This is somewhat similar to the household alternating current, but to be useful for charging the boat’s battery or for operating AC appliances aboard the boat, it must be changed to the one-way flow of direct current -- DC -- to charge the battery. Its voltage also must be contained at a level and pressure that household appliances can accept.

    Tamed but Still Lively Voltage

    • A series of diodes -- called rectifiers -- and voltage regulators are in place in the electrical system. Diodes operate as electrical check-valves, allowing a one-way flow of direct current to charge the boat's battery. Voltage regulators keep the voltage output of the alternator in check so it doesn’t overwhelm and damage appliances connected to the boat’s AC power source. The voltage regulator ensures the voltage is limited to about 110VAC and the pressure, or number of shifts in polarity per second, remains at about 60 cycles -- the same as household appliances.