The question is primarily, why run a generator when underway?
Isn't the output of the alternator(s) sufficient to provide enough power?
To elaborate ....
I understand if AC or multiple AC units are running.
Beyond that I am not imagining an electrical load necessary to run a generator when the main(s) are running.
However, I have zero experience with larger boats, with multiple electrical systems.
I have been looking at some and those had rather lower amp alternators. (100 amps or less) It is my inexperience about larger boats*. I would have thought much higher amp alternators would handle the load. And be more or less standard on diesels for this kind of boat. I am just surprised more don't have alternators in the 150+ amp range. Yeah they are more expensive and 250 amps or more are possible.
My experience stops at 30 feet with a single gas engine with a refer, lights and a sound system that has a good draw. I am looking at roughly 40 feet boats with twin diesels. And all of the power systems that kind of vessel has. I saw one with 55 amp alternators. It had a generator. Just seemed strange.
We have large alternators, yet still run the generator occasionally while underway. It's a bigger boat, with full household amenities.
Why big alternators? Mostly to minimize when we need to run a generator. As someone mentioned earlier, the fuel consumption for both is about the same, though I did an experiment a number of years ago that suggested for more moderate loads the alternators are more fuel efficient, yet for heavier loads the generator was more efficient. The big reason to avoid running the generator is to reduce maintenance.
Bigger loads that the alternators enable with are:
- recharging a large house battery bank
- operational loads while underway. Nav equipment is the obvious one, but the bigger power consumers are the ER vent fans
- Ability to run some large AC loads via inverters. These include a water maker, electric dryer, electric oven, and at least partial HVAC.
Why still run a generator? If the AC loads are consistently maxing out the alternators, I'll run a generator to keep ahead. There are also cases where the loads will consistently be above what the alternators can cover, and then I'll run a generator for the duration. Such loads would be:
- Continuous, full power HVAC
- 2 watermakers at the same time to make water really fast.
- Two or more heavy loads at the same time in any combination, like the dryer while also making water or running the oven. We usually try to run those things sequentially, but that doesn't always work out.