[ILikeRust]

Interesting point - to me, anyhow, because my boats' setup is the same - the engine sits a fiberglass bed, which sits directly above the deepest part of the bilge.

I would lead the exhaust duct down into the bilge, if this causes you concern.

Another thing to consider, which I have partly setup, is to purchase at least one, and preferably two, Nicro solar "day/night" vent fans. Set one to pull air into the boat, and the other to draw air out. Install the "inbound" one forward in the boat - e.g., over the main cabin; install the "outbound" one in the lazarette or similar area. This will cause a constant, gentle current of air to flow through the boat and help not only keep everything dry inside the boat, but also help prevent an accumulation of vapors.

I also have read of people attaching a duct to the aft fan and leading the duct down into the bilge. This setup is even better - you get a flow of fresh outside air coming into the cabin, and then being directed down into the bilge and ducted up and out the aft end of the boat.

Just as a little data point, my boat is a 1968 and has an Atomic 4 in it since new, and the exhaust duct ends just behind the engine - i.e., not all the way down in the bilge. Evidently, it hasn't been a problem yet. Not to make light of the possibility, but is it possible that your concern of having sufficient gasoline vapors in the bilge to cause an explosion may be greater than necessary? Assuming, that is, that the fuel line and associated fittings are in good condition and working order and well-maintained...