Two generators?

[Palarran]

I'm looking at switching from my 56' sailing catamaran to a motor yacht in a few years. After looking at a lot of spec's online, I'm wondering if very many below 60' motorboats have twin generators. It seems overkill to have a 15 to 20kva generator running all or most of the time. I currently have a 12 and 8 kva but don't need the 12 at all and will be removing it this fall. We an run all systems, A/C, watermaker, dive compressor, refrigeration, etc with the 8kva (though not all at the same time). For a motorboat I'd prefer to have something like a 12kva and a 4 kva autostart where the 4kva is running a majority of the time to keep the batteries charged for inverting.

 

[Eren]

I think that 2 generators are only needed if AC is run 24 hours or if the boat is chartered and loss of generator support is unacceptible. Other than that, single generator is more than enough for private use.

 

[jfm]

No or virtually no non-custom 60 footers have twin gens. Too many space and cost issues. Frankly there is no problem running a 18kva (with a/c; 12kva without) under light load- it will do 10k hours then just replace it- and so this is what most 60 foot builders do. Quite sensible imho for most of the market.

Twin gens tend to happen at 70 feet optional and 80 feet standard equipment, as a generalisation. But even then I would spec 2 big ones not a small and big- I have 2x23kva on 80 footer and would do the same again. Reason being that I want each one to be able run the whole boat, and I don’t care if they sometimes run at 25% load.

Remember twin gensets needs different architecture for the boat’s wiring so is far from simple- if you want to run the 2 together. It’s easy if you’re content to run one at once. Reason is that you won’t be able to synch them easily so you need auto relays to split the boat automatically into two separate 230v systems when the two gen sets run together or if you want a gen to supplement (rather than replace) shore power. None of this is small in terms of equipment and installation though is entirely doable in new build if the owner wants to pay. Remember builders like princess don’t do this- buy their 88 footer with twin gens and you’ll find when you get home that you can only run one at once because they cut out the cost of a proper twin installation.

 

[luptonj]

For a motorboat I'd prefer to have something like a 12kva and a 4 kva autostart where the 4kva is running a majority of the time to keep the batteries charged for inverting.

If it's just to save the bigger generator running all the time, why not have a bigger battery bank, some solar panels and set your 12kva to autostart whenever the batteries run low, that way if your inverter is providing the power, your generator will only run to charge the batteries and then only if solar can't keep up.
When you're running heavy loads you may as well have the big gen running and charging batteries etc at the same time.