Light Gen Set Running

The friendliest place on the web for anyone who enjoys boating.
If you have answers, please help by responding to the unanswered posts.

Roger Long

Senior Member
Jul 14, 2015
Vessel Name
Gypsy Star
Vessel Make
Gulf Star 43
I would like some real world experienced ground truthing of theory. Most of us know that running a diesel generator at light or no load is bad for it. I used to do generator load analysis (a quarter century ago) to properly size gen sets. However, things always look different in the real world.

We put a new Westebeke 7.5 kw unit in our trawler last year. As a result of my load analysis experience, I would have preferred to put in a much smaller one and done load management but we knew that we would only own the boat for a few years and took the advice that selling it would be difficult if it didn’t have one of the size expected on boats of comparable size. One of our AC / Heat units is inoperative and we haven’t replaced it yet because we don’t need it much on our cruising schedule. We were just barely able to get the unit up to the minimum recommended load for break in by doing things like running a portable heater in the stateroom with the AC on and letting the hot fresh water run slowly.

I monitor the output frequently and almost always find the amp meters (split buss for service and AC) at zero. The water is hot, the reefer and fridge are cold, and the AC or heat thermostat has tripped off. I shut down and restart a lot but this often results in the ice cream getting soft and the fridge not being cold enough to make it through the night with power off. Most of the running time, this 7.5 KW unit is putting out only about 10 amps, 15% load. A lot of the time, it is just free wheeling. I still find myself starting up the portable heater in the stateroom with the AC going.

I’m sure from this experience that lots of trawlers are running their generators very, very lightly. So, my real world questions: Are any of you seeing the bogy men I was told about back when I was doing load analysis, wet stacking, maintenance problems due to blow by from light piston ring loading, etc.? Are we likely to see problems with this unit or are we just shortening its life by some indeterminist amount way down the road after we are doing our cruising in Barco loungers in front of the nursing home TV?

I know there are solutions to this issue such as a golf cart battery bank with an inverter for the fridge but we are cruising on a frayed shoestring and I’m trying to get a handle on the issue to prioritize with other needed upgrades.
Investing in an small charger inverter and a few golf carts may well improve the sale ability of your vessel and provide a better set of conditions for your genset. That said, I've had two gas vessels without inverters and the gensets ran a fair amount lightly loaded.
Years ago after having a 5 KW Northern Lights generator installed in our new boat by Bayshore Marine in Annapolis, Jeff the owner advised me about loading the genset.

He said that they do a nice business overhauling generators that spent months in the Caribbean with the genset running 24/7. At night there are no loads other than A/C and the A/C only runs 15% of the time. The no/light load situation lets unburned diesel and lube oil bake on the cylinder walls forming a glaze. The glaze isn't hard to get off, just takes a few passes with a hone, but it does require removing the engine and pulling the head.

Why are gensets prone to this condition whereas propulsion engines routinely run at light loads. It is because gensets must run at full rpm all of the time. That means that lots of air is being sucked in with little fuel burned which results in incomplete combustion and fuel condensing on the cylinder walls causing the glaze.

So I always try to load my genset to 25% to avoid this.

Hopefully Ski will describe his experiences which include running his genset all night with no problems but only for a week or so a couple of time a year. I think it takes extended light load running to cause the glazing.

And a good way to avoid the situation is to run hard for a couple of hours every once in a while. That will help burn off the incipient glaze.

The under loading problem is real, BUT mostly for far larger noisemakers than yours.

"I think it takes extended light load running to cause the glazing."

There are different theories as to how the problem is caused.

My belief is the tiny combustion pressure from the unloaded engine is not strong enough to get behind the rings and seal them properly.This leaves oil on the cylinders and the lightly loaded rings simply slide , rather than seal as they were built to do..

This causes burnishing , smooths the cylinders and the hone or oil dimples are gone.

The folks that put noisemakers together realize that just battery charging , or reefer cooling is done frequently , so select engines that come from the lawn tractor or farm implement sector.

These are not heavy duty engines like a 100KW or more set would have , so are less effected by light loading.

A slightly more often oil change (same filter) will help with carbon buildup and blowby in the lube oil.

Nothing beats a long hard run , for propulsion engines or noisemakers , just before an oil change to heat the detergents in the oil, and give them time to capture sludge for removal.

On our boat we consider a 4 hour hard run a minimum , and prefer 6 hours to capture in the oil as many of the fines that the filter can not capture .
Last edited:
I have a 4.4 kw Westerbeke genset. It has 2200+ hours. All we really have is fridge and battery charger running 90% of the time.
It seems fine at this point in time.
I usually try and keep at least a portable heater (1.5kW) on the whole time my 8kW set is running.

That way when the battery chargers drop offline, it is still loaded up near 20 percent with the other parasitic outlet chargers, tv, etc.

Getting near 6000 hrs of which only about 1000 is mine but I will bet the PO lightly loaded it much of the time too.
Ok, here is my theory: At low load, soot gets into the hone mark crevices and not enough radiant heat from flame to burn it out. Especially in indirect diesels the flame is mostly hidden.

In 20yrs of engine business, only had a few instances of this on gennies and it was both on NL units. Never seen it on Kubota (onan, et al) or Mitsubishi (Westy). And as PSN posted, he has a fair number of hours on his mitsi.

I've got about 5000hrs on my NL with many a night spent on the hook with AC or heat on. Once those cycle off, basically zero load. I let it run all night anyway because I am too lazy to get up and shut it down. In the morning I make coffee, charge up air tank and run water heater to get it up to over 50% load for at least 15min. And about once a month I load it up to like 85% for an hour or so using heat pumps and a cube heater.

Rings and pistons have never been out. I did have to pull head and do a valve job, but that was related more to a period of looonnnng term storage unrelated (I think) to my particular duty cycle.

Never have to add oil in 200-250hr intervals.

So I think occasional light load for several hours is ok, just load it up once in a while.
Last edited:
I found this good discussion of cylinder glazing which is the primary issue with light loads. It doesn't mention the primary mechanism which is the piston rings being forced against the walls by combustion pressure acting on the the oil in the piston grooves behind the rings. If the piston is coasting on the later part of the down stroke due to no load, the rings lose their seal.( I know this from breaking in aircraft engines as a maintenance officer. We had to fly the plane for hours at the lowest possible altitude and WOT).

The article (soon to disappear according to the site) has one good take away: Glazing isn't likely to be a problem once the engine is properly broken in.
It's no-load-at-all that causes the problem, over very extended periods of time. I agree that an inverter is a wonderful thing if you have light or occasional loads like AC- only refrigerators.
Part of the glazing problem is the rpm. Bigger diesels run at 1200 rpm (for 60hrz) and rarely have glazing problems. My theory is small engine piston speed is high and diesel supplies some lube even in combustion. But there's little fuel at no load. From what I have seen and measured, the cylinder is worn away and becomes shiny, but not coated with some oil/fuel baked on mixture. My current boat came with 2 14 kw generators. One Perkins 4108 and Onan MDL3. PO ran 24/7 one of them whenever away from the dock. Both were glazed, both run at 1800. Both put oil in the exhaust and used excessive oil.
I overhauled the Perkins, but Onan doesn't support their small engines and only wanted me to either buy an engine or a whole new set. I had custom oversize rings made, honed the cylinders (no sleeves), same pistons, made my own gaskets, and it runs fine. I'll never own another Onan.
One solution for boats running AC at night is have the generator auto shut off and on as needed, and set the temperature spread wider. An option for loading the generator is a relay on the main engine block heater that turns on when the generator runs. You can't overheat an engine with a block heater and the heat is all in one place.
Top Bottom