Fuel tanks,  Page 2 Another interesting situation that I observe on occasion is the presence of huge bacteria colonies in diesel fuel tanks. The colonies present themselves as gelatinous blobs that float at various levels in the fuel, some close to the bottom and some in the middle. In this unusual case I was operating a 1982 42 Atlantic Offshore with twin 3208 Cats. The sea trial  was to take us to Newport, a distance of about twenty miles. The sea was almost calm for about half the trip and the vessel performed better than expected at all engine RPM’s.  About half way the wind and sea picked up a bit, small sea, about two feet or so. I had already checked out the fuel  delivery system before getting under-way, and it was in great shape. As soon as we hit chop of about two feet the starboard engine went back to an idle while the throttle lever was at half throttle. Incidentally, the wind and waves began to increase as soon as the engine decided not to operate properly. We decreased speed and went along for a while on the port engine,  which would have been a pain without the auto pilot. Both fuel gauges indicated full,  and I knew the delivery system was in good condition, so what could cause the engine to idle. I left the throttle lever in the same position it was in when the trouble began. Believing that the remainder of the trip was going to be a drag I just sat back and watched the sea grow.  
Five or ten minutes went by and the engine was back to full RPM. What magic is this, I thought.  Another few minutes went by and the same engine went back to idle again. I was navigating from the Sound into the Bay then to Newport harbor. As I approached the harbor the engine revved up and shut down a couple of times, and I thought how nice it would be to have both engines running as docking around the million dollar yachts is a bit tricky in all this wind. Well, luck was with me, and as soon I reached the harbor and the waves went away the engine came back to its normal operating mode  and docking was not a problem. I let the new owner know about the details of the day and he was upset that I did not pick up on this problem during the previous days original sea trial, which was conducted without a wave in sight. As he was with me the first day, he admitted, the vessel operated as it should and I could not have noted a problem if there were no symptoms. I had a real interest in this problem, so I decided to find out why and what. After several calls to local Marine Diesel mechanics  who suggested everything from water in the filters to clogged injectors, I talked the new owner into another sea trial, no charge, and I would remain in the area of the fuel manifold so that I could change tanks when the problem arose. Sure enough, as soon as we hit some mild chop the engine RPM dropped back to an idle. I switched tanks via the fuel manifold and the engine picked up speed and remained steady all the way back to the dock. The new owner had the fuel tank in question cleaned and the cleaning crew discovered several large blobs of what looked like black Jell-O. It appears that the blobs were floating at a level where they could get under the pickup tube which had a screen to prevent large particles from entering the system, and partially shut off the fuel flow.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             ©  Cross Systems 2016