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Old 23-05-2011, 09:30   #1
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Join Date: Mar 2010
Location: Full time cruising
Boat: Southern Cross 39 - Nemo
Posts: 39
A Tale of Engine Overheating or Not

I bought my sailboat 20 years ago. It has a Perkins 4108 with a V drive transmission. The entire time I have owned the boat I have had a problem with the engine overheating.

I have changed out the heat exchanger, re-plumbed the coolant lines, tried different pressure caps, installed a coolant expansion tank, bleed the coolant system, checked the mixing elbow, installed an overheat alarm, tested the thermostat and replaced the temperature gauge, etc. I put in controls to turn the high output alternator off, as it heavily loads the engine. I don't even remember everything I did. I also bought a feathering prop with adjustable pitch and have experimented with pitch over the years. These steps made small incremental improvements, but there has always been a point where as the speed and RPMs go up, the engine goes over 200 degrees.

Over the years I have dealt with this by running slower and we have rarely been able to go over 5 knots. Since I have been cruising full time, I have noticed several anomalies in the temperature readings including the fact that the temperature changes suddenly when the alternator goes on. I have an alarm sensor on the block that is supposed to trigger at 200 degrees. It has never tripped, although the gauge has been over 200. (Note that the Perkins manual says the water leaving the block could be 200 with a 180 thermostat) I started to suspect the gauge, even though I had put a new one in. I started researching temp gauges and found that cars have devices that supply a constant voltage to the gauge as the batteries vary in voltage. I do not have such a device on the boat. Also, I started to suspect the ground connection. I put a new ground from the gauge to the temp sensor and the temp dropped significantly.

I found that there are gauges that aren't electrically based at all (except for the light). The sensor screws into the block conventionally, but connects to the gauge with a flexible copper tubing that transmits the reading via a gas. Can't fully articulate how they work. They comes with sensors permanently attached with tubing of different lengths. The one I got is made by "Autometer" though there are other brands. They come in different sizes and styling and the one I got was around $100.

The new gauge shows radically different readings. I get 5 knots @ 2400 rpm in flat water (Based on my current prop pitch which seems right for the 4108) and am under 180 degrees. I get 6 knots @ 2600 rpm with 185 degrees and 7 knots at 2800 rpm with 190 degrees. Note again this temperature is under what the manual predicts. I have made so many improvements in the system that I probably have the cooling system at peak efficiency. All these years I have not been overheating at all. There are so many times when I wish I had more power, but was afraid to crank it up (like fighting tidal currents). It turns out that I can now routinely run at a higher speed than I used to. Its like having a new boat.

I report this story, as I did a lot of researching of list serves and forums and read hundreds of posts about this problem. Maybe This will help others.
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Old 23-05-2011, 09:49   #2
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Join Date: Aug 2008
Location: Wash DC
Posts: 3,169
Re: A Tale of Engine Overheating or Not

hand held ir thermometer can be your friend.
can tell you a lot about what is going on. great tool on a boat.
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Old 23-05-2011, 10:09   #3
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Join Date: Dec 2009
Location: South Florida
Boat: 43' Vista Aft Cabin 'Victoria"
Posts: 91
Re: A Tale of Engine Overheating or Not

I would believe the mechanical guage you just installed. An IR gun is helpful but the mechanical guage should be more accurate. Since the electrical guages rely on resistance, there are a lot of reasons for inaccurate reporting from the guage. The sensor could be mis-matched, there could be too much resistance in the wiring (corrosion, etc) there could be an intermittant ground, and on and on. Sounds like every is just fine temperature wise based on the mechanical guage and the 200 degree warning alarm not sounding.
Keep the dirty side down!
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engine, overheating

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