Greek Workboat Shows Reduced Raw-Water Flow at Cruise Despite a Clean Strainer Basket as Strainer-Lid Seal Air Ingress Is Investigated

Case Background

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.

A Suction Leak Can Pull Air In Without Letting Water Out

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.

Why the Strainer Looked Normal

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:

• flattened O-ring;
• damaged lid;
• dirt on sealing surface;
• loose cover;
• incorrect seal;
• cracked transparent lid;
• or distorted housing.

Cruise Operation Increased Suction Demand

At low engine speed, the pump required less water.

A small air leak might have limited effect.

During Greek coastal cruise:

• raw-water demand increased;
• suction at the strainer increased;
• more air entered through the weak seal;
• pump efficiency declined;
• cooling-water delivery became unstable.

The bubbles therefore increased under the exact condition when more seawater was required.

The Diagnostic Path Separated Restriction From Air Ingress

Technicians reviewed:

• sea-chest condition;
• seacock opening;
• strainer lid;
• O-ring condition;
• suction hoses;
• hose clamps;
• pump inlet;
• and pump output.

The presence of air upstream of the pump was particularly important.

Difference From an External Sea-Chest Restriction

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.

Why Pump Replacement Could Miss the Fault

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.

Technical Lesson

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.

FAQ

Can a sea-strainer lid leak air without leaking water externally?

Yes. Suction pressure can pull air inward while no water drips outward.

Can this reduce raw-water pump performance?

Yes. Air entering the suction line can reduce effective water delivery.