Are you sure you need that much power? If you size your genset for air conditioning
startup loads, it will be woefully underloaded 99% of the time, which will shorten its life. You can finance startup loads with an inverter/charger with power boost (Mastervolt and Victron make these, maybe others) which will use inverted battery
power to cover the momentary surges when you're starting up AC. I can't imagine how you could use 15kW on a boat that size. We have 6.5kW on our 54 foot boat and rarely manage to load it to more than 3.2kW, which is a whole shore power
connection, by the way.
A smaller generator
will be loaded better, will last longer and work more efficiently. If I were you I would analyze that very carefully. A smaller generator will be much cheaper, to boot, and LIGHTER, which will be especially good in a cat.