odd genset design

[team karst]

Our needs on the sailcat are driving some likely unique, and, knowing you people , comment worthy, features.

Going with a NextGen 5.5kW unit, mounted in the port bridge locker.
Will use a manual, rated switch to "convert" the genset back and forth from 240/120 to 120V. These switches force a OFF state, during the transfer. These gensets have two identical 120V isolated windings.
I have a portable 3HP 230V compressor that will need a softstart system to operate with this genset. Going to try the SR22-40 for this job. All other AC loads are 120V, but when the compressor is running, not much else will be. When the scuba tanks are full, the switch goes back to 120V, with over 40 Amps available. At that setting, I can run run both A/C units, plus a 1.5 HP watermaker.
I will use the LiFeP house battery bank for genset starting. I know that one will get comments. There is already a 100A breakered 12V pair running up to the adjacent locker containing a 1kW windlass. So, this scheme won't require a 4th battery system with its attendant charging system. This genset has no alternator. The starter motor is around 700W/12V rated. So, the starter is smaller than the windlass. The house bank is charged from two solar sources, and two engine alternators.
Pulling out a 300 liter water tank, leaving a 2nd tank in place.
Installing a 27 gallon diesel tank, in the vacated spot, to feed the genset. Don't let me forget to relabel the deck fill, from water, to diesel!
Installing a Rainman 30gph system.

There is, of course, another option to change out the 3hp 230V compressor motor for a 120V motor. That, unfortunately, takes the softstart cost into the stratosphere. You would then need a 80 Amp surge rated ST system, hard to find to say the least. Also, the compressor looses value at other locations as it would need around 30A @ 120 V run current, rather hard to come by.

I will keep my current 30A 120V shore power system, but adding the genset transfer switch before the panel. There will be no way to accidently provide 240V to the panel, with any switch position.

All the new stuff will still be lighter weight than the existing 300L tank, full of water.

[fxykty]

You already have a LFP house bank that is charged by engine alternators and solar, but by inference is too small to run A/C, the planned watermaker, or the dive compressor. Correct?

So you want to install a third diesel engine to run those big consumers. Why not instead upsize your LFP battery and increase the charging from your alternators? Why add a third motor that needs to be maintained and serviced?

A/C and electric dive compressors are both very big loads, so maybe a generator is a cheaper option? But unless you really want a high output watermaker that must have an engine to run it, consider instead a lower capacity DC-powered watermaker that runs off your battery.

BTW, starting a generator from the nearest existing battery is no problem. We did that with our former generator - started it from the port engine start battery.

[team karst]

Yeah, the kWh just dont add up for a battery/solar solution. Just the compressor is 2kW for 3 hours straight. And then i need to run the a/c for 6 more hours into the nite.

[Captain Bill]

I have a 5.5 nextgen and running 2 ac units and 1.5hp water maker is a bit optimistic. In my experience the voltage sag with something near those loads drops to 100v or less which is hard on motor windings. One can adjust the voltage up under high loads but then the voltage is too high under light loads. FYI, I have two AC units and a 1.5hp water maker.

[team karst]

Good input Bill! For the record, i have a 9k and a 12kbtu.