sea water intakes

powerskipper

powerskipper

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I was on a shaft drive boat today and we had an overheat alarm incident, the filters had been checked an hour before we left,
we turned off the engine, then restarted her to see if level remained high while on tick over or whether she would cool down, she cooled down, she had expelled about 1/2 a litre of coolant in the bilges while hot, we noticed that the water level in the filter was almost below the intake to the engine and was showing no water movement [it a clear toped one]
we moored up and I did some more investigation, I opened the filter again, pushed a ratchet handle down the intake pipe, and put it back together again, started her again and she ran great for the rest of the day. Now the only thing I noticed was that the bolts holding the cover on were not as tight as I would have expected, now to the point!

could she have been sucking in air if the top was not on tightly and over heated when the power was applied or was it as I suspect something had blocked the intake valve.
 
More than likely the top of the water filter being slightly loose allowing air to be sucked in. There is a bit of a vacuum to get the water from the intake through the filter to the engine.

It happened to me a few years ago, when I replaced the screw-on filter top, and the seal was not fitted properly. Once I refitted the top with the seal correctly fitted, no problem.
 
i agree with philip i think with the bolt down cover type of weed filter housing you need to be carefull to do up the bolts, wingnuts or what ever uniformly. i soppose if you crank oneside down tightly to begin you could cause a small gap opposite!
 
Yes that can happen, and worse:

A few years ago I was on a delivery where this happened:

Boatyard had fitted a new strainer, then checked raw water impellor, but in re-fitting the cover pinched the O ring seal. As the new strainer was dry, they primed it with water.
Engine started fine and we happily motored out to sea, found wind and sailed for six hours before the wind died. When we re-started the engine we didnt know that the badly fitted O ring had admitted air and the system had unprimed itself.

30 minutes later, BANG auto fire extinguisher went off. (overheat alarm not working!) and there we were without an engine 40 miles out in the north sea at 2 a.m.

Actually this story is worse. We didnt have any steering either, the boatyard had overtightened the saddle bolts on the quadrant, so after a while it just slipped. Oh yes and the emergency tiller didnt fit (that at least might not have been their fault)

Finally the lifeboat radioed to say they would be 45 minutes longer as one of their engines wasnt working.

Yes, and where was the lifeboat recently serviced - same boatyard.

Name of boatyard? I have NO hesitation in telling you it was AMBLE
 
I had (and may still have an overheating problem) I thought it may have been growth in the intake pipe.
What I dont understand is if the top of the strainer is leaking then why doesn't sea water just leak out. I know I must be missing something basic here but I will post anyway!!
regards
 
The impellor relies on a fully sealed system in order to draw sufficient raw water into the cooling system. If the system is not air tight, the impellor is drawing in air as well as raw water hence the reduction in water volume and the resulting loss of cooling effect.

Hope this helps
 
If the seal is not fitted properly the when the engine is running the pump is sucking air in through the bad seal as well as water from the through hull fitting. Therefore not enough cooling water is being passed through the engine. In an extreme case the leak can be so bad as to only allow air into the system with no water being sucked through. When the engine is stopped, there should be a leak at the seal, but the pressure in the system is much reduced therefore the leak of water out of the system could be negligible.
 
Thanks guys, you got through to me at last!
I hadn't realised that the impellor was so powerfull. My intake is about 3 feet below water level so I thoght that there would be enough pressure at the seal to show a significant amount of water.
 
You may find the impeller is now pretty "knackered", I had this problem travelling up the Seine to Paris. I decided to check the strainers daily and failed to get a good seal on the lid of one engine. Motored a few hundred yards and the alarm on one engine went off. Re-seated the lid OK, but at the next service it was very easy to spot which impeller had been made to suffer!
 
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