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I'm having to dig way back into the recesses of my memory right now, but IIRC a 12-wire generator can be configured for 3-phase power and if you can get what's called "low wye" configuration you can get 120vac from line-neutral, 240vac from line-line, or 208vac 3-phase with the 3 lines. If you configure like that then the compressor can run direct off of the generator *a 2hp motor won't need a soft-start on a 8kw generator unless all of the AC's and a microwave are running* and it means you can get a 2hp vfd instead of a 5hp vfd *when used for phase conversion the HP rating of the VFD must be doubled to handle the 1.7x input current since it only has 2 lines coming in vs. 3*.
You would have to reconfigure your electrical panel for 3-phase power and you would want to do that somewhat strategically for when you are connected to shore power since at least one of the legs wouldn't get power, but that could be dealt with.
If you rewire for 240vac split-phase *if you had a big pump I would recommend doing 3-phase on the boat, but for a 2hp motor I would use a VFD* which it sounds like it is you just have to make sure that you flip the breaker closed for the compressor based on your transfer switch setup when connected to shore power. Boats are rather complicated beasts because they have to contend with US based shore power at 240vac on 2x legs vs Euro spec which is 230vac on 1x leg so you have to figure all of that out. The way you say it's wired now is similar to RV's where you have 2 sides of a panel but when connected to a single leg of power either a dogbone jumps L1 and L2 together or a transfer switch closes and they're all on the same leg. Can't do that with 240vac split-phase appliances so have to make sure you have the right kind of transformers in place. Victron makes something called an Autotransformer that is designed specifically for boating applications to deal with this stuff though it doesn't help with 3-phase only the Euro 3-wire to American 4-wire, but different discussion.
Nothing terribly special about my Rix but I wanted it to be able to run on a normal extension cord so I put a smaller motor on it and also went with a 6-pole 2hp/3phase so instead of turning at 3600rpm like the stock motor it turns at 1200rpm which allowed me to use the same pulleys and it turns at a third of the speed, essentially it's now a 2cfm pump but per the engineers at Rix, so long as I keep the pressure down to around 3500psi or so it is considered continuous duty. I'd rather have a 2cfm pump run all day long than have to duty cycle a 5cfm pump, it's also considerably quieter. I did remove the stock cooling fan and put an electric fan on it from AC infinity that turns whenever the pump is plugged in and switched on. When it hits the pressure cutoff, the fan will still turn to push off the latent heat. With the VFD, if I have a 20a circuit that is dedicated I can run it at full speed, and if I am on an extension cord with other stuff running I can turn it down as low as I need to so it doesn't trip the breaker. Part of the beauty of the Rix is it doesn't care what RPM it runs at since it doesn't have splash lubrication vs. your Bauer JR that has a range of RPM but you really need to keep the pump turning in the 1500-2100rpm range so the lubrication system works. Technically the VFD can "overspeed" the motor and if I was on a 30a circuit I could certainly do that, but I just make sure I'm patient and let it cruise. If you go to split-phase you can usually find VFD's up to about 10hp that are designed for single-phase input. Any of them will do phase conversion, but you have to double the HP rating so you don't overload them and the ones that are designed for phase conversion just make the install a bit less intimidating.
A 25hp motor would have required a 50hp VFD and those have been available for well over 30 years if not longer. They were horrifically expensive up until about 10 years ago which is why you see/saw so many recommendations for roto-phase converters. The issue with a roto-phase is that they still don't solve your soft-start conundrum and with a 25hp motor that is a massive inrush current to get it started so by the time you add a soft start for the motor and the roto-phase you may as well get a VFD that does soft start and soft stop, gives you some speed flexibility if you hit a weird harmonic resonance frequency, and all sorts of other controls that make the motors last longer. Roto-phase is great if you are trying to run a machine shop that has a dozen 3-phase tools that aren't really all that big so instead of buying a dozen VFD's you can get the one roto-phase. This makes sense if you have a bunch of lathes, mills, etc. that you got for cheap from an old machine shop, but if you only have a one of each for your home shop then the VFD is the more appropriate tool. For a large compressor they are definitely the right tool and buying a roto-phase for a single motor is ridiculous with the prices of VFD's as low as they are today. A 25hp roto-phase is around $3k, so is a 50hp VFD. One of them gives you soft start/stop, fine control of speed, and can shut the motor off if it senses imbalance somewhere or bad power coming in, and is the size of a small desktop computer and does not require rewiring your shop. The other does none of those things, weighs 600lbs and takes up a 3ftx3ftx3ft corner of your shop and requires you to run a separate 3-phase panel and 3-phase outlets. Which one are you going for?
In terms of brand, I would buy a pair of these from Invertek at $2500 before I bought a single $5k unit. The components are very similar and with a dive shop compressor that you can't have down for 2-3 days while a capacitor is replaced you have a spare unit that is already programmed and takes about an hour to swap over. Dive compressors are also not really tasking on a VFD because they aren't running 24hrs a day, they aren't doing a lot of fine motor control, etc. so about the only thing that will take them out is a brown-out but they usually shut themselves off pretty quickly when they sense that.
VFD, 230V, 3 Phase, 50hp, 138A, VT, NEMA 1, Fan and Pump, DC Bus Link Reactor
www.wolfautomation.com
