Hi Boby, does your boat have a Genset? If so no worries, go with a six gallon tank. Our boat came equipped with a six gallon hot water heater and a Genset. We've done many extended (two month plus) cruises and the six gallon has always been adequate. Then again if you have crew unaccustomed to taking navy style showers, then go with the 12 gallon.It is a 45 DS