Inverter vs Genset

Fix or replace the generator. If you decide to do more later, fine. But you need the generator and you may find it easier to just run it, rather than trying to find reasons not to.

We have generators to use.

I think we all said that 60 posts ago but it is like the Energizer Bunny.

Are you planning on going camping????? (No A/C, No hot water, no stove?) Or owning a yacht? You NEED a new generator. An inverter is NOT a replacement for a generator. How do you plan on recharging the inverter bank of that size with just the engines if you're out on the boat for more than the day? What about all of the heat that radiates through the boat from the split engine room design and exhausts that go through the sides of the master stateroom?

Real swamp folk ett Opossum-pheffer (stew)….

You will de-value the boat with out the proper generator on board. No one really wants all that inverter stuff to rely on completely. Hatteras buyers want a generator. A new one will add value to the boat for the time you go to sell it.

Do you plan to do the Delta at all? Get's warmish and buggy over that way.Don't forget that the AC also provides reverse-cycle heat for those cool and foggy mornings. A space heater will take the bloom off the inverter batteries pretty quick, too.
Count me in the "fix or replace generator" camp. Ultimately, the best way to replace the energy used in an inverter system is with a generator. A really big problem with high output alternators for main engine mount is the drive system. It takes more than 1 V-belt to drive a 150 amp alternator at full output. One belt WILL slip, and it WILL have a short life span. I would be surprised if your Detroits could accommodate a double belt or better yet, a serpentine type wide belt.
I've run boats with huge battery banks, and in the final analysis, for anchoring overnight, energy management just isn't fun. One boat had 12 L-16 batteries at 400 ah +/- each, and the big home type fridge, additional freezer, icemakers, and flat screens in the cabins didn't last the night in many situations. I've been in generator curfew zones with a Nordhavn 55 that sucked down the batteries overnight with guests aboard, except the generator was broken at the time. Trying to charge the batteries from the main with 150 amp alternator was no quick fix. It took all day cruising and the next night on shore power to get the batteries topped up. With a generator, you can charge the inverter batteries after a quiet night on inverter AND have enough power to make coffee and breakfast. Maybe take the foggy chill off.

We have a similar situation to what you just described. We have twin 20kw gens, a Balmar 130 amp alt, 3kw inverter with a large house bank.( Think the previous owner did not want gen noise at anchor) The Balmar has to run at 1500 rpm for high amp output, we always have the port gen running that carries the boat with AC and provides hydraulics for the stabilizers while cruising. Rarely ever run the inverter at anchor because of running the AC. or other high amp needs. In the near future plan to downsize the house bank to minimum needs for inverter. Probably keep the Balmar alternator if case the house charger /inverter fails. Just keep on running 0ne or both gens depending on needs.

The boats I ran could cruise in temperate conditions without generators, since hydraulics came from mains. There was no appreciable benefit to that, though, other than keeping the gen hours down. You get used to avoiding cooking underway and not having to fire up the gen. Silly, really, that it becomes a "thing". It's not like we had to hand crank the genny .
On the yachts I've run with "Night" inverter systems, I found that they were often troublesome to manage, since I couldn't manage the energy consumers, meaning the owners and guests. Lights, TV's, the stereos for the TV sound left on all night, ice makers working flat-out to restore cocktail ice, and innumerable little things. Nothing like waking up to a "Low Voltage" alarm 3 hours before the boss wants to wake up in the morning to peaceful silence and you're sweating the minutes before it's permissible to fire up the genny.
Inverter systems are great technology, but they're no kind of magic bullet. They're perfect for owner/operators that get used to operating within the system design limitations. Building in excess capacity equals a lot of space and weight. The 12 battery L16 banks on one yacht I ran was the size of a regular side-by-side refrigerator lying on it's back. For those who haven't had the pleasure of shifting the 114 lb. beauties, they're 16.5" high, 7" thick, and almost a foot wide. About $3,000 to replace them all, Every 3 to 4 years.