generator overheat question

[trogers]

Has anyone had issues with a generator overheat that occurs only while on the hook/at dock?

I had a new generator put in a couple of years ago and it has proved "annoying" in that it works well while underway but overheats while at anchor or at dock.

My supposition is that the water flow through the raw water intake is increased by the flow across the through hull while underway and correspondingly diminished somewhat while stopped.

Any thoughts on how I can prove this supposition. To look over the side at the exhaust/raw water that is passed through the system I cannot make a visible distinction between the volume of water while underway or while stopped. I'm sure I looked a bit like the villiage idiot hanging over the port side bow cap rail trying to guage water volumes while underway.

I'm looking for (1) some ideas on testing, (2) alternative hypothesis on what might be causing the overheat.

 

[Traveler 45C]

How's the RW impeller look? When was it changed last?

 

[fissioneng]

I agree with your hypothesis. I think there is a minor obstruction somewhere. A few things:

1) Has it done this since Day 1 of the new generator installation, or did it start afterwards?
2) Do you have a scoop on your intake? I don't think generators are supposed to have them. (prevents water ingestion if not running the generator and the boat is moving)
3) Could your intake holes be partially blocked with bottom paint?
4) Is your intake/strainer clean inside and out? If slightly dirty, a little extra pressure may be all that is needed to get by the clog.
5) What shape is your RW pump impeller in? It could be weak and needs the extra pressure from the movement through the water to keep up.

If you have another thru hull/strainer nearby, you could swap hoses to see if it has something to do with the thru hull. If that checks out and it still overheats, I would check the RW impeller and all the lines in the RW system for some sort of minor obstruction.

 

[trogers]

RW impellor looks fine.

Yes it has done it from the get go.

The bottom paint hasn't been changed in a while, the last time I was down there I didn't notice anything appreciable, but in truth, I haven't made a specific trip to look at the intakes. I'm thinking it will be worth the 30 minutes of time to check it though. Just to take it off the consideration list.


I've checked the sea strainer a few times and it has been spotless when this happened. I have thought about trying to test this (gasp) without the strainer basket in place - just while I'm sitting at the dock.

Any ideas on how to measure the impediment that the various turns, bends , petcocks, etc. add to the equation. I worked a couple of summers for my father as an electrician's "helper" and I recall the "rule" that said a J-box had to go after every 4 or so "90's" so you could grease and fish the wire through without it bogging down in the pipe. The intake and sea strainer are in the amid ships stateroom and there is a modest distance between the generator room and the intake that might have some build up/bends that are accounting for the challenge.

On my old boat I built a kit to flush the heat exchangers based on some insight that I gained from this web site. Has anyone ever used a similar technique on the intake hoses in a situation like this?

 

[Brian Degulis]

Have you checked the temp both at the dock and under way? Have you looked at the ambient temp in the machinery space under both conditions? Assuming the gen is in the same space as the main engines Then does the gen overheat at the dock if the mains are cold? It would seem to me that any change in water flow caused by the effect of a scoop being prressurized underway is going to be very small if everthing else is OK. Is it possible you have a small air leak on your raw water suction side that's being eliminated by water pressure under way? Just some thoughts it should be pretty easy to nail down with a step by step aproach.

Brian

 

[TopHattandTails]

Are you docked over a bed of weeds? I occasionally dock at 1 of 2 docks when transiting. My A/C intake is always over a bed of weeds. SOmetimes it shows up in the intake strainer - other times, no so much, but an un-docking and running clears it out.

 

[REBrueckner]

My first thought was weeds as well...or maybe even an old piece of winter cover flapping down there....is there an external strainer...if slotted or if there is none you'd think some weed pieces would show up in the interior strainer....if a pinhole type which is best at keeping stuff outside, maybe you'd not see anything inside.

If you have a scoop outside, could it be keeping a weak suction hose from collapsing when underway?? I'd visually check the suction hose with the genny running dockside.....look for a flat section...

What happens when you anchor in deep water and run the genny?? That should give you strong evidence about weeds dockside...

 

[Pascal]

i'd take a look under the boat first. check for barnacles, plastic bag, etc...

since the genny is new, you would know if an impeller had been changed and a bit left behind, but you may want to check the heat exchanger inlet just in case.

you shouldnt' have a scoop on a genny so being underway shoudln't make a difference. Most genny manufacturer warn agaisnt them. that said, we have them on the 70 footer i captain, factory installed and no issue with the northern lights (6 year old boat).. so go figure.

an easy test is to open the strainer and seacock adn see how much water comes in. you should get a pretty strong flow. a few weeks ago, one of the NL 20 here on Charmer was running fine, not hot, but i coudl tell by the sound that tehre wasnt' enough water. new impeller, clean strainer. then i noticed that there wasn't a lot of "static" flow in the boat with the strainer open. turned out to be a bunch sea weeds stuck in teh hose, near the sea cocl

that showed me that most modern gennies have a lot of excess cooling capacity, so if i were you, after checking the flow, i'd look at the cooling system. belt, circualtion pump, etc... not necessarilay the raw water side.