sailthentrawler, this is an actual event, meant as a warning, only. Three years ago, my neighbor at the yacht club decided to mount his RIB
on the canopy above his stern deck
. He had a davit already mounted, so decided it was the obvious place to stow this bulky item on his trawler
. As a result, his exhaust mufflers, which had been half submerged, were now fully submerged. One afternoon, deciding to warm up the engines, he started one engine, heard a "Pop", and being curious, shut down the engine and went to the engine room hatch
on the aft deck
to investigate. It took about twenty seconds to get there and open the hatch
. He saw water rapidly rising in the engine room and called to his wife, who was down below, to get off the boat. By this time, less than a minute, the boat had begun to list heavily. He grabbed his cell phone
and helped his wife off the boat, just as the stern went down, about two minutes into the incident. Having several large capacity pumps at our disposal, we went off to the emergency
locker for the needed equipment
. It took about five minutes to return with a Coast Guard type P-6 pump, a couple of electric
submersible pumps, and all the other toys normally used for an incident as this. The stern was fully submerged, the cabin
was filling, and within another fifteen minutes, only the tip of the bow was still floating, everything else was submerged, with the transom sitting on the bottom, fifteen feet down. Within a few minutes more, the dock
was crowded with Harbor Police pump boats, salvage
team and responding members. Late that afternoon, after multiple air bags had been deployed, the exhaust ports blocked, and continuous pumping of five high capacity pumps, the boat was raised and towed to a boatyard where it could be lifted on slings and set on the hard
The back pressure of the exhaust was great enough to cause a loosened exhaust hose to pop off the exhaust flange at the transom. The resulting 6 inch hole below waterline was sufficient to immediately flood the engine room. As the stern sank, the pressure increased, speeding up the flow of water into the boat. Within minutes water was flowing through the scuppers, then into the engine room hatch and the rest of the boat. Nothing could have been done, short of blocking the exhaust outlet, which in minutes was ten feet below the surface and not a good place to physically have one's body. The good news, besides no one being hurt and the boat being fully insured, was that the dinghy floated off the coach roof and was recovered.
Back pressure is not a good thing for marine engines. The exhaust ports may be partially submerged, but that is merely (and probably) a symptom of bad design, weight management, or in this case, a bad decision. It can occasionally have freak consequences, such as this one. Something to keep in mind, though.