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engines overheating at 20+ft swell
Posted: Friday, February 6, 2009 12:42 PM
Joined: 31/01/2009
Posts: 4

Subject: hi

maybe you could help?

have two seachests feeding raw water to all on boat.

both clean and clear

keep loosing both main engines to high temp

cleaned heatexchanges and checked pumps, all good.

waterpumps keeps getting air in them and cavatate.

must keep bleeding pumps to stop engines overheating.

think picks up air at inlets when roling in 20 + foot swell!?

is there any thing i can do to stop this?


Posted: Friday, February 6, 2009 1:15 PM
Are the sea chests, raw water inlets, on the ships centre line ?    Are the chests dedicated to the main engines ? Are you feeding air into these chests via other equipment  sucking off the  chests.
Posted: Friday, February 6, 2009 1:22 PM
are you running the sea chests in parallel  ?   If so the windward pickup might be feeding air in the system...close it off and run off the leeward side and see.
Posted: Friday, February 6, 2009 1:42 PM
Are the sea chest filters PERFECTLY clean ?   or do they 25 years of antifouling bogging them down ?
Posted: Saturday, February 7, 2009 7:12 PM
Joined: 03/07/2008
Posts: 9

Hi flyingfish,

Very little you can do regarding air locks in the raw water system during heavy seas, as the inlet sea chests invariably beome uncovered during the peaks. Most merchant vessels are fitted with air syphons on the sea chests, so that any air entrapped in the inlet sea chest, when it becomes uncovered during large swells, is expelled before causing too much of a problem. If you don't have these fitted, all I can suggest is to slow down and "ride with the swells" so you keep the inlet sea chest covered with sea water at all times.

Safe Seas, yottekgordon

Posted: Saturday, February 7, 2009 7:44 PM
Air siphon ?  What is an air siphon ? On my chests I have an air vent, simply a ball valve plumbed from chest to a cockpit drain that allows  air in the system to vent.  Never seen a siphon break ?  Does it break the water head pressure ? or does it holds the head pressure ?  Logic says that you are trying to maintain the water coloum...not break it ?   On the Dutch yacht that I sail, during gale condition battery charging with  generator, I will pull air and overheat.  The only way to overcome this is to shut off all Sea chest auxillary feeds..main engine, refer , air con... maintain the water column in the sea water system and keep the gen fed with seawater.  Id be interested to see how this air siphon functions...might save me some work.
Posted: Saturday, February 7, 2009 8:38 PM
Joined: 03/07/2008
Posts: 9

That is what an air syphon is. Simply an air vent with a ball valve that vents out entrapped air.

I did not say that a syphon break was fitted.

Anyway, why are you anonymous ?

Posted: Sunday, February 8, 2009 7:03 AM
Joined: 14/01/2009
Posts: 1024

Opps...sorry about that, forgot to log on. Syphon break.  You could be correct.  One of these days I'm going to bolt a clear plastic lid on that sea chest and obsever whats going on in heavy weather. I believe that as the boat rolls, vacuum is formed and you momentarily loose positive head pressure. Without positive pressure your water pump will be sucking air thru the syphon breaks and other attachments.   Of course once the sea chests recharges with water pressure you will have an air void in the system.  Ive always felt that with positive displacement this air is just pushed thru the system and eaten.  I could be wrong.  Going to spend some time on my hands and knees in the engine room next gale...sounds fun !!!!
Posted: Sunday, February 8, 2009 1:45 PM


THanks for all the feedback ...the weather has imprved and the pumps no longer have air in them. The chests are still picking up air but we are getting rid of it by makeshift bleedpipes on the chest.

thanks again


Posted: Sunday, February 8, 2009 1:48 PM
Joined: 03/07/2008
Posts: 9

Thanks for the continuing discussion.

Basically, what has to be done is to expel any air before it gets into the raw water circulation pump suction. Whatever device is fitted to do this is beneficial. If your yacht is sailing in heavy sea areas a lot of the time, I would politely suggest fitting a compressed air type priming device on the pump inlet piping, similar to those fitted to bilge pumps. These are reasonably cheap and work very well. One manufacturer is Hamworthy. This priming device discharges any air in the suction piping and ensures that only liquid reaches the pump.

Safe Seas.

Posted: Monday, February 9, 2009 12:24 AM
Joined: 18/05/2008
Posts: 9

Do you happen to have a link to Hamworthy's web site?
Posted: Monday, February 9, 2009 12:24 AM
Joined: 31/01/2009
Posts: 4

We used all the advice to get the engines to run sucessfull without air...Not sure if this is related...but steam is coming out of the port main engine exhaustpipe.

There is a notesable temp. difference between the port main engine and starboard main engine exhaustpipe outlets.

The portside is very hot, steam is comming out of the exhaust and the exhaust is giving off a strong burn smell.

The port side sea chest is clear, raw water pump was opened and looks ok, heatexchanger was stripped down and cleaned, but on inspection looked clear.

Both engines run at 1000rpm's and have 12psi raw water pressure, jacket water temp is 90 deg cellsius and oil temp is 90 deg. cellsius too.

Any ideas were to look for the problem?

having a bit of a shakedown cruise....appreciate the feedback

Nico - Flyingfish

Posted: Monday, February 9, 2009 2:26 AM
Joined: 03/07/2008
Posts: 9 , write air ejector primers in the search box.
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