Waterlock burst, engine over heated. Why?

[pcatterall]

Sailing towards the IOM on Saturday. This is a 'new boat' to us but we had run the engine around the harbour for several hours no probs.
Not much wind so after an hours sailing we decided to use the engine again ( it had performed perfectly for 2 hours against the current coming down the Ribble)
After 2 hours, lots of smoke and overheating, engine switched off. Checked engine oil and water ( Perkins 4108) oil fine and just half a pint of water needed.
Sailed on and when cooled down retarted engine. No water being pumped lots of smoke in engine compartment. Changed impellor but original seemed fine. Dark now so decided best solution to head for Liverpool as wind conditions would allow us to get there under sail with engine available for short time if needed. Made it to Liverpool and bagged a mooring for what was left of the night. We had found by then that we could run the engine without overheating at low revs water and fumes were entering the engine room and we surmised that the sea water cooling and exhaust system was working only as far as the engine room but tha provided we kept the hatches open and pumped out every few minutes we could run the engine safely.
In daylight we finally identified the problem, a hole had been blown in the top of the water lock. We made into the marina and ordered a new waterlock but thought that we still needed to understand the reason for the overheating and why the waterlock burst.
We checked the plumbing from the water lock through to the stern, all was clear. We put a bung in the old water trap and fitted it. Everything worked fine, Water pumping out fine and temp stable at 70 degrees.
All seems good but we are still concerned to understand why the over heating? did this 'cause' extra pressure which blew the water lock and if so why over heat.
Any suggestions please?

 

[Bosun Higgs]

MOst likely you had a blcokage that stopped water flowing. This would cause the exhaust to run dry and the plastic water lock to melt where it got hot.

Why the blockage? Could be at the water input ie sea cock and sometimes here you get temporary blockages where some rubbish like a piece of plastic gets sucked against the hole and is held there by the pumps valiant efforts to suck. Or it could be some lose rubbish, maybe even an impellor vane, stick in the water passage ways. Or even funnier problems like sheared impellor drive or impellor centre coming unbonded from the vanes.

All pretty noddy type problems but unfortunately only you can investigate bit by bit going through the cooling water system. Its all common sense so be assured there is nothing you cant work out for yourself as you think your way through it.

 

[Billjratt]

you don't say why you know it was overheating. I assume you had an alarm - a buzzer, light or gauge- or did you touch the engine, was the heat exchanger the same temperature at both ends?.
What triggered it? normal overheating would be sensed around the thermostat region on the cylinder head - on my 4108 there isn't a buzzer but we do have a temperature guage which will go ballistic if the fanbelt is removed so the fresh coolant isn't circulated. It would take longer if the raw water stopped and the whole system reached trigger temperature.
However, if there is a sensor for the exhaust coolant - usually where the raw water is injected, and the raw water goes elsewhere, then you will get "an alarm" long before overheating takes place.
To precis, I don't think (from your description) you actually had an overheat, as all the enginny bits were still protected, only the output side of the exhaust would be jeapordised, as the rubber hose would not be cooled.

 

[pcatterall]

Plastic over inlet did cross our minds ( we checked inside the strainer first thing) As soon as we stopped the engine the 'item' could have fallen off. We rely just on the engine temp guage which stays around 70 degrees at cruising revs. First sign was the whiff of smoke then the engine temp was 100 degrees.. Once the water lock burst we eventually realised that the engine was essentially ok (but with the sea water discharging into the boat!!) We wondered at some sort of blockage beyond the water lock but nothing there upon our later investigation.
The impellor looked fine but was changed ( thank goodness for the 'speed seal' system as it was dark and a bit bumpy).
Just a worry not knowing the cause as, of course it could happen again.
In discussion afterwards we realised that we had the tools,skills and knowledge to have carried the temp. repair that we eventually managed, we assume that we were just to tired ( some were sick) to think it through properly.

 

[Billjratt]

Does sound like a temporary blockage of the raw water system then.
Looks like you (and anyone else with a plastic bit in the exhaust system!) should have a sensor at the raw water injection point, like some mobos. That way, you get a bleep at the first loss of raw water. before the plastic melts and the rubber delaminates (Which you should check by the way)