And like all boat heaters, I think it's all about the flue. We had the heater on a Cape Dory 28 for over a decade and used it spring and fall up and down the east coast and Maine.
I believe the 1" flue is only capable of so much draft. With propane, it was hard to detect when the flue was not removing all of the exhaust. But you could tell there was a problem by the air quality in the cabin. That was only 5,000 btu burner too, no more than a stove burner. Still, in a small cabin, this is a conern.
Once you get the large flame going in your K1 version, I think you simply have too little draft capacity to exhaust the burner.
A boat is simply not an ideal place to build a chimney. For starters, the flue will be short and not as capable of creating it's own draft like a masonary chimney in a home(even these experience an occasional back draft). And also, especially with the Force 10, the diameter limits it's capacity as well. Also, the flues terminate at the cabin top. Nice for not catching lines but not as effective as a raised pipe for combustion.
To add problems, it's not always easy to keep a positive pressure inside the cabin. That is, more air coming in than going out. When it goes negative due to wind gust/direction, the flue is inclined to reverse and pull air (and smoke) into the cabin.
I've dealt with alot of these problems in our current boat which has a wood burning fireplace. Totally different fuel but the exact same problems; maintaining a positive draft flow to exhaust fumes. In this case, you're immediately aware of a reverse draft and have to deal with it. It's a tricky heater but well worth the attention for the warmth it puts into the cabin on a cool Maine evening.
My advice is to try to maintain that positive cabin pressure. Wind blowing over a dodger often will pull air out of a cabin. Experiment with a hatchboard or two and cracking a forward hatch. Think about packing more air into the cabin than what is going out, that will help the draft and of course is the only safe way to keep exhaust going up the flue, not into the cabin.
One last thing; we kept ourselves warm and comfortable this past weekend on an overnight. Before leaving, I cleaned the flue. We never had a back draft and the fireplace drew perfectly for many hours of use. Your flue may need regular cleaning to work at it's best.
