Genset Puzzle

[denpooch]

Hey folks. Here is one for the experts.
I have a 2005 vintage Kohler 7.3 generator. Its a well maintained unit, new impellers every year, new carb 2 seasons ago. Heat exchanger boiled out with new carb installation. It was running great until.... it stopped running great.
The issue that I have is when at the dock or idling about she runs great. With a full load, its smooth sailing. Sometimes 'searches' a bit if the compressor on one of the a/c units turns off, but that seems par for the course.
The issue happens when I bring the boat to cruising speed. Running at 3400 rpm, doing around 17 knots, the unit decides to shut down. When this happens, I try to restart right away and it doesn't wanna cooperate. I'll go to the boat the following day, start it up and she'll run for hours.... at the dock.
The generator feeds off of the port gas tank which also supplies the port engine. I was thinking fuel pump, but a mechanic friend suggested that if it is a fuel pump issue, it would also have problems at the dock.
Any thoughts of solving this puzzle?
Thanks.

[Viper]

I wonder if at that speed you are not drawing raw water. If you have a directional intake, some will face them aft so that when you're under way at high speed, you don't force water into a non running generator that might result in flooding the cylinders. Water at high speed rushing by a rear facing intake however might create a vacuum at the intake rather than push water through it and prevent the pump from drawing any. The unit then shuts down as a result of low raw water pressure. It's a thought. The sure way to tell is duplicate the problem and see if it starts emptying the strainer while underway or pull the hose at the pump and see if there is good volume there at that speed.

If you can run it all day long at slower hull speed, the above is what might be going on.

As long as you have over a quarter tank of fuel in that tank, it shouldn't be starving for fuel unless there is something else wrong or the tank pick up is at the front of the tank and the angle of the hull at that speed starves the pickup.

[tomschauer]

I would bet Viper has it, losing raw water to the genny at cruising speed.

[ColRon]

And me makes three. Losing raw water could cause an overheating shut down.

[tomschauer]

Hey, I just thought of something else. Maybe when you bring it up on plane the intake sees air when the bow is high and you don't notice the genny shut down until later. Maybe try keeping the the bow trimmed down and bring it onto plane slowly, or wait until your flattened out then start the genny.

Just a couple ideas.

[denpooch]

Yea good thinking. That is also what my mechanic friend suspects as well. But why now? Did not happen for the last two years that I have owned the vessel. The boat was designed with intakes where they are and I assume for the years prior to my ownership it worked.
The 'intake' for Genset is a bottom facing, round plate about 6 inches in diameter with a bunch of small holes. When I painted the bottom, I cleaned out every one of the wholes so that I would get good flow.
Viper's idea of checking flow at the pump or strainer is near impossible, especially at speed, due to the position of strainer and especially the pump, which is somewhere near the rear of the Genset. Tomschauer, I do pay attention to the trim and besides the intake is pretty far aft. I does not see daylight under any circumstances.
I am not quite sure how to proceed with this one. Again, why now and not before????

[bobkirkbride]

Someone above mentioned the tank pickup. The anti-siphon valve on the tank my not be opening fully at high rpm. If the genset and engine are using the same tank pickup on the port tank, a sticking valve may open enough for the engine to run but not open enough for both to run. Even if the genset has it's own fuel line the anti-siphon valve be something to check. I had a similar problem and after new fuel filter, carb rebuild, etc. it turned out to that $25 valve.

[denpooch]

Thanks everyone. Much appreciated.

[Viper]

The genny would be on its own pickup from factory unless someone modified it. Under the same load, if it were a faulty anti-siphon valve, the fault would occur at the dock as well. On its own line and pickup, it would be independent of engine operation. I have run into boats with modified genny feeds to other tanks with the option of switching tanks like you would for the engines. If they didn't do it correctly and tied into an existing engine line instead of it's own separate pickup, that's certainly a possibility. Barring all that, the only thing that seems to stand out here is that it happens while underway.

PS: check the vent line and fitting for that tank and make sure they're completely clear. To test if this has anything to do with it, remove the gas deck fitting cap while at that speed and see if the genny still shuts down. Only do this if you're sure you won't get any water splashing into the opening. Best to do it once you're on plane.

[bud37]

Source of the post I am not quite sure how to proceed with this one. Again, why now and not before????

Good question, why now.....considering that, has any work been done just before this started ? Perhaps you now have a restriction/marine life ....can you blow out the thru hull through the intake hose with a shop vac......check your hose fittings for loose clamps that may have been disturbed during any recent work.

The previous owner/owners may have never used the genny on plane, I know I don't, but if you have and there was no problem before now then the intake design is just fine, we all ( with the same boats ) would probably have the same design.

The gen running at the dock fully loaded will use the same water and fuel as other times, the gen has no idea how the boat is moving thru the water.

You really have to be sure why the gen is shutting down, could be as simple as vibration related electrical connection somewhere.

Just a few thoughts.....good luck with it, just try to be systematic in crossing things off the list, especially any recent work anywhere on the boat.