Servicing Your Trawler's Cooling System

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Senior Member
Oct 6, 2007
Most used boats with inboard engines are provided with fresh water cooling systems.* These fresh water systems in conjunction with the raw water system cool the engine during operation.* The fresh water circulates within the engine and transfers the engine heat to the raw water system as it passes through heat exchangers.* This article will explain the step by step approach to servicing your yacht's raw water cooling system
But raw water cooling systems often become plugged with scale, calcium carbonate deposits, creating an unhealthy condition for the engine.* Heat exchanger cooling tubes and piping will become totally blocked if left unchecked.
How does calcium carbonate form in the engine you ask?* When hard water comes in contact with heated surfaces, the minerals in the water fall from suspension, Minerals, primarily calcium will then cling to any surfaces that might be there.* The same occurs inside hot water heaters and in industrial power plants that use untreated water.
Become familiar with your engines
Prior to servicing the engines on your used trawler, it is important that you take some time to familiarize yourself with the raw water cooling system of the engines.* Get drawings and parts diagrams if at all possible. Examine your engines and trace the raw water flow from the intakes to the exhaust.* Make a mental note of each component.
There are two methods to service your used boats engine.
Method 1 - Disassemble the Engine Cooling System
Using your parts manual as a guide, disassemble each component of the cooling system.* You will need new seals and gaskets when you put it back together so keep a running inventory as you remove components.* The principle sections of the raw water system are the raw water pump and impeller, the oil cooler, perhaps an aftercooler, a heat exchanger and transmission oil cooler.
Clean the cooling system
After the sections have been removed, each section must be examined.* Oil coolers and heat exchangers will possibly have a calcium deposit inside them.* A professional radiator shop can clean these for you but a cheaper way is to mix a 4-1 solution of Muriatic Acid and water.* Immerse the components into the solution and allow it to "boil" until all activity is complete; your components will be clean.* Use care to protect your eyes and skin as the acid is very hazardous.
Re-assemble the cooling system.
Once your system is clean replace all the sections back on the engine using new seals and gaskets where needed.* Replace older rusted bolts too.* Now is a good time to replace the impeller too
Test for leaks and proper operation
When you are refilling your engine with anti-freeze, be sure to bleed the system of trapped air.* You should find that information in your operators manual. *Following the re-assembly, the only remaining task is to start up the engine and check for water flow and stop any small leaks by tightening bolts
Method 2 Clean in Place
Inspect your cooling system and locate your raw water pump on your used*trawler* Next, locate an intake in the raw water system downstream from the pump where you can connect a hose.* On my Volvos, I have a hose that runs from the water pump to the oil cooler that I can temporarily remove.* Then locate an outflow from the raw water cooling system where the water leaves the engine.
Assemble the following:
  1. <li class="MsoNormal">50 gph bilge pump<li class="MsoNormal">About 20 feet of wire to connect the pump to your batteries<li class="MsoNormal">A 5-gallon bucket<li class="MsoNormal">About 10-15 feet of hose sized to fit connections<li class="MsoNormal">1 gallon of Ph-Ospho-Ric (Home Depot paint department) phosphoric acid
Connect a portion of the hose to the bilge pump and the other end to the intake you have located.* Place the bilge pump into the bucket and fill ½ full with water and ½ of the Ph-Ospho-Ric.* Connect another portion of hose to the outflow of the engine and route back to the bucket.* At this point remove the engine zincs and replace the holes with plugs.
What you now have created is a closed loop where the acid can be circulated through the engine.* Start the bilge pump and begin circulating the water and acid.* The water will turn a dark gray and bubble as it neutralizes the calcium deposits.* You may have to add more Ph-Ospho-Ric as you continue the process.* Finally, after you are confidant the deposits are cleaned out, reassemble the engine, install new engine zincs and start the engine to flush the remaining acid.
Mike Dickens is a live aboard trawler owner.* He also operates as Paradise Yachts Broker in Florida USA.
"What you now have created is a closed loop where the acid can be circulated through the engine."

There are far more kindly chemicals for this treatment.

Be sure to replace the engine water pump after this acid wash , as you will have removed all the lubricant in the water pump, and probably have dissolved the seals.
Change out the thermostat and gaskets too.

The radiator shop will also pressure test the heat exchangers, and repair any leaks they find. I find it's well worth it to take the parts to them.
I doubt the thermostat will be affected since it's in the fresh water side of the engine, not the raw water side.* Also, I think he came in upstream of the seawater pump and didn't run the solution through it.

I assume that you probably want to set up the flow to run reverse from normal to help flush out any non-dissolvable stuff??
"I doubt the thermostat will be affected since it's in the fresh water side of the engine, not the raw water side."

"When you are refilling your engine with anti-freeze, be sure to bleed the system of trapped air".

Unless its for a winter of non use I have never heard of anti-freez in the SEA water side?
That note was on the "disassemble the cooling system" method, not the "flush in place" method.

Also - note that some (Perkins, at least) engines have heat exchangers on both sides of the raw water pump.* Mine has the aftercooler and oil cooler downstream, and the main exchanger upstream.* So you'd have to go through the process twice in that case.
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