A coastal workboat operating in Greece developed reduced raw-water cooling flow during cruise.
The sea strainer basket was clean and the raw-water pump impeller appeared serviceable.
No major external water leak was visible.
During loaded operation, however, technicians observed bubbles in the suction-side water path.
This shifted attention toward the sea-strainer lid and its sealing O-ring.
The raw-water path is:
Sea chest → Seacock → Strainer → Pump → Heat exchanger
The pump creates suction upstream.
If the strainer lid is not sealed correctly:
Pump lowers pressure inside strainer → Outside air enters through lid seal → Air mixes with seawater → Effective pump delivery decreases
Because the internal pressure can be below atmospheric pressure, the fault may not produce an obvious outward water leak.
The basket itself contained no meaningful blockage.
The water path was open.
The problem involved the pressure boundary around that path, not the flow area through the basket.
Possible causes included:
At low engine speed, the pump required less water.
A small air leak might have limited effect.
During Greek coastal cruise:
The bubbles therefore increased under the exact condition when more seawater was required.
Technicians reviewed:
The presence of air upstream of the pump was particularly important.
A sea-chest blockage reduces the amount of water available.
A lid-seal fault allows water to reach the strainer but introduces air into the suction stream.
Both reduce effective raw-water delivery, but the causes and evidence differ.
A new pump connected to the same leaking strainer lid can still draw air.
The pump may even generate stronger suction and make the leak more noticeable.
Marine cooling diagnosis should treat the suction side as both a water-flow path and an air-tight pressure boundary.
A clean strainer does not guarantee a sealed strainer.
Yes. Suction pressure can pull air inward while no water drips outward.
Yes. Air entering the suction line can reduce effective water delivery.
