Once upon a time I was doing on and off electronics work for a small dealership. Along comes a new boater with a check, and he buys a little 26" foot cruiser, but the sale is conditional. The conditional part of this purchase is he has to have air conditioning installed on the boat for his wife to use while on the hook. "No way" I say, "You can't get a genset into this little craft, you can barely get into the engine room in the first place. The customer comes back and says there must be a way. My response is, "Well I can get the air conditioner to fit in the boat, and maybe it could be powered by a portable Honda genset that could ride on the swim platform." His response is "He will check with his wife."
The next day he comes back and says his wife didn't want the portable Honda genset on the boat becuse it would be ugly, and hands me a piece of paper filled with scribbled numbers, and says an inverter could be used. I look at the numbers. "It's theoretically possible", I say, "but the battery bank will be huge, and the weight increase on the boat will be notable." The salesman literally begs me, and against my better judgement, I reluctantly I build it.
The only place the batteries would fit was on the starboard side where the three gallon hot water heater was, so out it went, and the batteries went in. Using four ought wire, I assembled the system, installed the air conditioner along with a large inverter, and it worked for the 6 hours I had promised in between charges. "What happened to the hot water heater? My wife must have hot water", he says. My response was "Where else could you put all of the batteries? Even the rats are hunched backed in this little boat. And there is no such thing as cold water on a boat in Florida. Do you see how large all the batteries are? Where did you think they could go?" I didn't listen closely enough to my inner voice that kept telling me this is a stupid thing to do, whether it was technically feasible or not. I swore I would never do an inverter based air conditioning system ever again. I changed my mind this week about all of this.
I just finished installing a lot of gear on a very shiny, and brand new Boston Whaler 32" Outrage Cuddy Cabin (the installer liked this boat). Much to my surprise the vessel had a factory installed Dometic (Marineair) 12V cuddy air conditioning kit. These 3500 BTU units are specifically designed to run in an inverter, and 12VDC battery environment. The system is turn key, includes the inverter, and should be able to be installed by a modestly skilled boater.
The kit normally comes with an inverter, but Boston Whaler has opted to use the sturdy ProMariner True Power Combi QS 2500w inverter/charger in its place (the Dometic supplied inverter does not charge). In my opinion Boston Whaler made several good design decisions in configuring this system. The air conditioning inverter, and battery system is isolated from the ship's house, and starting battery system. This insures that the air conditioning system can't inadvertently pull down the starting batteries in particular. The other good decision was have the AC system battery bank chargeable only by shore power. Although it would be technically possible to recharge the air conditioning battery bank via the Verado alternators, it would add an additional level of complexity, and impracticality to the system. Simple on a boat always works best. The True Power Combi QS inverter will shut the system down when the battery bank reaches 10vdc also insuring the battery bank can't be discharged (warning at 10.5vdc) down to damaging levels.
At 3500 BTU, this compact system won't cool down your entire Magnifico 50' yacht, but for smaller vessels, it is an attractive alternative for cooling without a genset set. The power draw is 29vdc amps, so eight hours of cooling will cost you about 240 (usable) amps, and you won't be bothering your neighbors in that quiet anchorage with a droning generator all night. So over all a good gig. The system comes turnkey, and looks like it's easy to install, at least as easy as anything on a boat ever is. There are other air conditioning system options available in the market place, including direct 12vdc systems. Do your homework well, double check the power requirements, and don't wire it with lamp cord.
Many thanks to Ian Goodnow from ProMariner, and Boston Whaler's tech support department for letting me bug them with questions, and the good answers I got.