Need help sizing a boat heater

[Roy M]

I have a Searunner trimaran, 40 feet long, in which I'd like to install a diesel heater for liveaboard conditions in mild to chilly (above freezing) temperatures for several months a year (San Diego in winter, Puget Sound in late summer). The boat is effectively divided into two cabins by the center cockpit. The engine room, where I would mount the heater/blower unit is located below the center cockpit. I would like to duct the heat aft, into an approximately 500 cubic foot living space, and forward into about 200 cubic feet of cabin/head area. If the unit had a water heater capability, that would be dandy, as well.

Given that a heater needs to be correctly sized for an appropriate application, so as to reduce or avoid carbon buildups and excessive maintenance, I'm looking for some advice on brand and capacity from folks who actually have them and use them. I have previously used a Dickenson solid fuel stove for heating the aft cabin. It was adequate for the heating needs of this space, but the forward cabin and head could not be heated by the air flowing through the crawlspace connecting the two cabins beneath the cockpit. Also, solid fuel is too demanding of time for replenishing and cleaning. I'd like something a bit more self-tending. Since this is a multihull, weight is an issue which will affect my decision regarding size and water heating capacity. I have a variety of alternate installation sites, should other factors (venting, physical volume of the unit) need consideration. Any ideas?