10 years or so ago a new fiberglass fishing
boat in Seattle, on its maiden trip, suddenly caught fire in the
engine room. The crew ran her on the beach and were OK, but the
boat was destroyed. The cause they believe was excessive non-vented
heat from the dry stack.
In October 2000 Fred Hammonds ALCINA (shown elsewhere on Das Site and in The Troller Yacht Book) caught fire and was destroyed. We dont know what started it, but the fire apparently started in the engine room and that reminded me of the boat I just mentioned.
Engine exhaust gets very hot. Wet exhaust systems arent much of a problem because seawater is injected into it which cools it right down. The modern exhaust pipe is rubber (they used to be metal) and as long as your seawater pump functions everything is cool. The exhaust hose can run wherever you want it, and from what Ive seen, it appears that with water cooled exhaust engines in cramped and tiny engine rooms, even with very large engines, engine room temperature isnt an issue.
Dry stacks are a totally different issue. Back in the 1960s I had a sailboat with an aircooled Lister diesel. It had a dry exhaust that exited the hull right at the waterline, the hope being that water would keep the planks from catching fire. It worked, but barely; the planks around the exhaust were black from the heat. And the pipe itself was wrapped in a 4 cocoon of asbestos.
While the simplicity of this system has much to be said for it, I wouldnt do it today. If I was going to use an air cooled motor Id rig a seawater pump to inject water into the exhaust. But, Id still make the system out of pipe rather than hose, and exit it to split the waterline. That way if the impeller failed Id still be OK. Of course this is just my normal paranoia coming through because a water-cooled engine will melt the hose too if the impeller fails; Ive done it. But the difference is, the air cooled engine will run without the impeller, and the water-cooled engine wont. So, Id never build an exhaust system that on its own can stop you. But in this country larger aircooled engines are rare in boats. The heat is an issue, as is noise. But they do work good, and the problems are solvable. A guy I used to know had a 6 banger Duetz in a 55 powerboat, and liked it.
Anyway, back to the immediate subject; engine room and dry stack heat. As I said, a large water-cooled engine can be stuck almost anywhere, but our dry stacks as well probably want in our powerboats really raise the temperature around them and this heat must be got rid of.
First, the stack. On my designs youll note the stack is inside a wooden box to hide it. The box is big enough so the stack can be wrapped in heat insulating material, and still leave several inches of air passage around it. This box ends at the pilot house roof, and then there is an external chimney the stack runs through. This chimney is also bigger than the exhaust pipe, and has large air vents in the sides.
On the first several trial runs Id pay close attention to the temperature in this box. With good heat shielding heat shouldnt be an issue, but perhaps a blower might be needed to be installed up in the chimney to help the air flow.
This box can also, when a blower is installed at the base of it, vent the hot air in the engine room itself, but an idea I saw that really appealed to me was plywood box ducting, about 12 x 4, that came up inside the pilot house about 18 off the sole (floor, you lubber), on each side of the house. This ducting had closeable louvered vents that allowed air to flow into the pilothouse, and also, through the outside wall of the house.
In cold weather the vents are left open in the pilot house to provide heat. In warm weather the vents are open outside the house to get the heat away from the boat. A blower is installed in the base of each duct to force the air up and out.
I would also install a portlite in each side of the engine room. Aside from letting you see out, it brings in fresh air.