Engines Generati ng Underway

Let’s look at this first in the context of alternators attached to the main engine(s) and operating either while underway or battery charging at anchor. Almost all modern diesel engines have similar efficiency profiles, with peak efficiency for any given engine speed occurring at somewhere around 70 to 80 percent of the maximum power the engine can develop at this speed. If we attach a propeller to the engine, and size the propeller such that it absorbs the engine’s full power at full speed (this is the conventional mechanism for sizing a propeller), we find that at anything other than full speed (for example, at the normal cruising speed) the power absorbed by the propeller is substantially less than what is needed to operate the engine at peak efficiency. Although it sounds counterintuitive, anything we can do to add load to the engine will improve its efficiency. The immediate takeaway here is the more powerful the alternators we can fit to the engine, the more efficiently we will create electrical energy and also at the same time propel the boat—we gain on both sides of the equation, so to speak.

The limiting factor in this scenario is our ability to use the electrical energy being generated by the alternators. It may be possible to apply much of it to the boat’s ongoing DC loads, and/or channel it to AC loads via DC-to-AC inverters. Any excess energy the alternators are capable of pumping out will need to be absorbed by batteries. Once the batteries approach a full state of charge, and their charge-acceptance rate declines, alternator output will have to be ramped down and we will be back to the conventional situation. As such, the key to pushing the alternator side of things is generally a large-enough battery bank, with a high-enough chargeacceptance rate to high-enough states of charge, to fully exploit the alternator output (for more on this, see below).