Marine Diesel Cooling Systems

Marine Diesel engines are often viewed as the ideal choice for main engine power for cruising boats of 40 feet and upwards for planing hull vessels, and most all displacement vessels. 

Beyond regular servicing such as engine oil, coolant, gearbox oil, and items such as impellers and belts, a very important part of keeping a marine diesel in a reliable condition and to minimise the risk of failure or major damage to the engine is the cooling system.

Let’s start with what makes up the cooling system on a typical marine diesel engine, in this scenario, for clarity we have referenced a 6BTA , some of the key components of the cooling system are listed here:

1/ Raw water intake from a fitting through the hull. The valve needs to be operable and in good condition to allow for the rest of the system to be serviced
2/ Sea strainer, to filter debris from the seawater. This needs to be periodically inspected for leaks, corrosion and cleaned to allow proper flow of water to the rest of the system
3/ Raw water pump on main engine, to distribute the seawater to and around cooling system. Impeller and pump needs to be serviced at planned intervals and inspected regularly for leaks
4 /Fuel cooler. Diesel engines return some fuel to the fuel tank. During the process of supply and return the fuel absorbs heat. The purpose of the fuel cooler is to cool the fuel. Coolers need to be serviced at scheduled intervals
5/Gearbox or transmission oil cooler. The transmission, whilst in operation generates heat.  The transmission oil cooler exchanges the transmission fluid heat with seawater and needs to be serviced at scheduled intervals
6/ Closed cooling system. Typically this is comprised of coolant circulating around the engine cooling system and through a heat exchanger. Coolant needs to be changed at scheduled intervals
7/ Heat exchanger. Exchanges the heat from the cooling system with seawater and needs to be cleaned and inspected for correct performance
8/ Charge air cooler or more often referred to as an after cooler. On a boosted engine (such as a high power density engine mentioned here) this cools the boosted air before it enters the combustion chamber to increase power and efficiency
9/ Exhaust. The raw water circulating around the engine exits via the exhaust which serves to keep the exhaust system cool. The exhaust system can be adversely affected by lack of cooling water due to restrictions from the heat exchanger / saltwater cooling system


Here’s some thoughts or typical viewpoints to consider;
1/ My engine has never overheated so I don’t need to worry about servicing the cooling system yet
Temperatures are one scenario. Your everyday cruise where the engine never overheats may well change if you need to change the power such as a bar crossing, in adverse weather. The last place you want overheating engine and potentially limited power

Charge air cooler, you have air under pressure coming into the engine combustion chamber on one side, and seawater on the other side. Without regular servicing you have the distinct possibility of seawater being ingested into the engine under pressure. Result being almost instant major damage to the engine requiring rebuild or replacement with no warning.

Fuel cooler. This fails and you have salt water in your fuel and the engine can suffer significant damage

Internal condition of heat exchanger and other coolers can not be known without dismantling and inspection. As there is salt water on one side and coolant, oil fuel or air on the other, it follows that any internal failure of one of these components will have serious consequences to the equipment it relates to

2/ I’ve had the system “flushed” and its fine now
Flushed typically means an acidic solution has ben put through various parts of the system. A couple of relevant points;

This can not tell you about the condition of cores due to corrosion or other issues

This does not address condition of seals / gaskets / O rings. These can be a point of failure and lead to problems mentioned.

This can not deal with build up or restrictions on the gas side of the charge air cooler which would serve to reduce the effectiveness of the charge air cooler.

The consequences of not keeping your cooling system in good condition include

1/ Major damage / engine failure due to water ingress

2/ Accelerated wear and failure to engine raw water pumps due to restrictions downstream 

3/ Damage to exhaust components such as fibreglass piping, rubber piping, due to lack of raw water flow creating excessive temperatures. NB This can occur over a longer period of time.

4/ Reduced performance and efficiency via increased temperatures

To keep your vessel running reliably and efficiently it is advisable to follow manufacturers guidelines

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