Eliot Mason wrote:Mike:
For posterity and internet search engines I'm going to say that an RMH in a boat could work... it would be awesome to build some mass below one of the berths or the settee. But in your case ... this is tough.
Consider this 4" core : http://www.dragonheaters.com/4-dragon-burner-rocket-heater-core/ This ALMOST fits your space (just a little long ... maybe if it were mounted higher up?). This style of construction makes sense in your situation, but this is of course just one of the components. To burn cleanly and provide a radiant surface you need a "bell" - often a repurposed steel drum. This core recommends a 30 gallon drum ... which is roughly 20" in diameter and 28" tall. For your mounting spot I don't see how to integrate a 20" barrel ... but maybe the barrel could be cut in half, stacked and a back welded on ... making a semi-circular tunnel, 10" in radius and up to 28" tall.
HMM ... the geometry of the pieces is important. Not sure if it matters, and I'd have to dig around to find the answer, but that semi-barrel might be too tall for the riser. Maybe a kind and more knowledgeable person will read this and chime in.
Anyway... hat would give you a nice radiating surface - but no mass. Generally the mass is charged by running the exhaust through it (largely a conductive process), but I don't see how to easily add a mass in this case - best option might be just radiating off a boat load (hah!) of heat and trying to get everything warm that way. Alternately, a panel of just plain firebrick between the bulkhead and the half-drum would provide some mass. Hmmm... a thin sheet of metal with air gap mounted on the builkhead, a frame holding bricks against that, and then the barrel. Could work...
Uncle Mud has a "Cottage Rocket " (https://permies.com/t/150536/Uncle-Mud-CottageRocket-Paper) design that packs all of the normal features of an RMH into a single 55 gallon drum, sacrificing both mass and radiative surface for compactness. I think the exhaust is probably hotter than in a classic heavy-bench system (but it might not be hotter than what your current stove puts out). Anyway, the point is that his design is for small spaces so I think there is something to learn from his work for this application.
BUT - and this is a big one - while everything above is a "might work - let's see" there is no way this will work if you stay with the 3" exhaust. You'll get smoke in the cabin, temperatures will be too low, you might get creosote in the exhaust (and risk a chimney fire). So if you can't/won't enlarge that through-deck fitting, consider if there is a window you could modify.
And maybe if I say his name out loud three times we'll get Ernie Wisener involved ... he may be the one person in the world who can tell us how to do a rocket on a boat!