Danish Workboat Develops High-Load Coolant Temperature Despite Normal Pump and Thermostat Operation as External Keel-Cooler Fouling Is Investigated

Case Background

A Danish workboat developed gradually increasing engine coolant temperature during sustained high-load operation.

The engine water pump circulated coolant normally, the thermostat responded correctly and no significant internal coolant restriction was identified.

The vessel used a closed-loop keel-cooling system rather than relying entirely on raw water passing through an internal engine heat exchanger.

Technicians therefore inspected the external surface of the keel cooler exposed to seawater.

Internal Coolant Flow Is Only Half of Keel-Cooler Performance

The heat path is:

Hot engine coolant → Keel-cooler internal passage → Metal wall → External seawater

For effective cooling, both sides must work.

The internal circuit may have correct coolant flow while the external heat-transfer surface becomes covered by:

• marine growth;
• shell deposits;
• sediment;
• paint buildup;
• protective coating deterioration;
• or other fouling.

This adds thermal resistance between the coolant and surrounding seawater.

Why High Load Exposed the Reduced Surface Performance

At light propulsion load, the remaining effective cooler area could remove enough heat.

During sustained Danish workboat operation:

• engine heat rejection increased;
• hot coolant reached the keel cooler normally;
• fouled external surface transferred heat less effectively;
• coolant returned warmer;
• engine temperature gradually increased.

The water pump and thermostat therefore appeared healthy because they were performing their own functions correctly.

The Vessel’s Cooling Architecture Was Important

Unlike a conventional raw-water heat exchanger system:

seawater does not necessarily need to be pumped through the engine room cooler.

Instead, closed-loop engine coolant passes through tubes or structures mounted externally against seawater.

This means a raw-water pump inspection may be irrelevant on some keel-cooled systems.

Correct system identification comes first.