What I recommend
I recommend you consider the unit from Flagship Marine. They make self-contained units from 6,500 btu to 120,000 btu and are the provider the US Coast Guard. The units have rotary engines and are so quite, efficient and smooth that I cannot feel it running under by V bert. I have 16,500 btu unit and it really cools my Catalina 310 - even in direct Texas sunlight on 100+ degree days (the heat is also nice in the winter). Units are very afforable and some of the Catalina 310 folks have installed themselves. The manufacturer sells direct to users and the package is complete down to the controls. See their web site at http://www.flagshipmarine.com/selfcontained.html