Question: I have recently purchased a used catamaran that was part of a charter rental fleet and I have a question about the exhaust system from the AC generator on the boat. In the photo you can see that the exhaust exits the hull on the port side just below the opening porthole. The porthole is to provide ventilation for one of the staterooms on the boat where people sleep.

It is a diesel generator, but I’m still wondering about the possibility of carbon monoxide getting into the state room if the port is open and the generator is running. What do you think? Should I be worrying about this?

boat exhaust placement

Placing a generator exhaust port directly beneath an opening porthole on a boat is poor design.

Answer: The photo you sent in reinforces a thought I have more and more often as I look at boat systems installations, and that is that “common sense” is not always that common. Your doubts are very well founded, and I’m really glad you sent in your question.

Even though your generator is diesel-fueled, there is still some carbon monoxide, as well as other rather obnoxious and harmful gasses like nitrogen oxide, phenol, formaldehyde, and polynuclear aromatic hydrocarbons (PAH) in the exhaust. And, even though a 2008 report from the American Academy of Forensic Sciences claims that lethal poisoning from inhalation of diesel fumes is unheard of, I suggest that your instincts are not without merit. The ABYC standard that addresses exhaust system installation (ABYC P-1) is a bit vague on this, and simply makes this statement:

“The exhaust system shall be gas-tight to the hull interior.”

That sentence is certainly subject to a great deal of interpretation, but that said, I’m on your side. I think the installation you show is a poor design, and personally I would make sure that the port is closed any time the generator is running. Or I would re-position the exhaust farther aft on the hull so that there was no danger of the exhaust recirculating into the cabin.

See other key questions answered by Ed Sherman, like Is my compass accurate? and Is my boarding ladder safe?.