When looking at a new Yanmar engine, alternator options begin at the 3GM30F. Yanmar offers a 80 amp and 115 amp alternator. Stepping up to the 3JH3E series/40 Hp they offer the 80 and 115 options + the possibility of a 2nd 80 amp alternator. Standard unit is 55 amps.

Universal/Westerbeke begins offering optional alternators at 32 HP. You can add a 72 amp alternator, upgrading from the standard 55 amp unit.

Once you reach the Westerbeke 63C Four they offer a single output 90 amp alternator and dual output 135 amp, 165, and 190 amp options.

This list of options can provide guidance when you are considering upgrading your existing engine's alternator. In general if it's not at least a 3 cylinder engine, be wary of side affects.

Another aspect of alternators is mounting. Yanmar engines use what is referred to as a dual foot mounting with 3" separation. I.e., 2 bolts separated by 3 inches. Universal/Westerbeke uses a single 1" mount making the alternator from one line incompatible with the other.

An every once in awhile a question is in regards to N and P type alternators. This has to due with the location of regulation within the circuitry.

Yanmar alternators (AKA Hitachi alternators) are N type and are internally regulated. Many after market regulators are P type. For a N type alternator to work it must be modified.

Finally the idea of taking, for instance, an alternator from a Yanmar tractor and putting in on a Yanmar marine engine is often brought up. Essentially marine alternators are designed to produce energy quickly for fast charging. Non marine alternators aren't designed this way and so won't perform as well.