Norwegian Workboat Develops Rising Stern-Bearing Temperature at Cruise Despite Acceptable Shaft Alignment as Cutless-Bearing Water Supply Is Investigated

Case Background

A Norwegian workboat developed a rising temperature near its stern-bearing area during sustained cruise.

Propeller-shaft alignment was checked and did not show a clear major error. The bearing clearance also remained within its serviceable evaluation range.

Because the support geometry itself appeared acceptable, technicians investigated whether the water-lubricated bearing was receiving enough cooling and lubricating water.

A Water-Lubricated Bearing Depends on the Lubricant Supply

Many marine cutless bearings use water between the shaft and bearing surface.

The intended process is:

Water enters bearing → Shaft rotation establishes lubricating film → Friction is controlled → Heat is carried away

The bearing can therefore be dimensionally correct but still overheat if water supply is inadequate.

Possible supply paths vary by vessel design and can include natural seawater flow or a dedicated water-feed circuit.

Why Cruise Load Increased Bearing Temperature

At harbor speed, shaft rotation and bearing load were relatively low.

During sustained Norwegian workboat cruise:

• shaft speed increased;
• propeller loading increased;
• bearing frictional heat increased;
• water supply remained below the required operating condition;
• temperature gradually rose.

This made the problem appear load-related even though the shaft and bearing were not immediately mechanically damaged.

Possible Water-Supply Restrictions

Technicians reviewed:

• water-feed hose;
• small inlet fittings;
• supply passages;
• seawater source;
• debris;
• marine growth;
• hose collapse;
• and routing.

They also verified that the bearing design actually required a dedicated supply before applying this diagnostic path.

Not all stern bearings use the same lubrication arrangement.

Why Alignment Was Still Important

Misalignment can increase bearing load and produce similar heating.

Therefore, the correct conclusion was not:

“temperature is high, so water supply must be low.”

Instead, the diagnostic sequence compared:

bearing geometry

with

lubrication/cooling availability.

Once alignment and basic clearance were acceptable, the water path became more significant.

Difference From a Worn Stern Bearing

A mechanically damaged stern bearing can create friction because of poor surface condition or excessive shaft contact.

In this case, the focus was on a bearing that could be structurally acceptable but lacked the fluid conditions needed to operate correctly.

Norwegian Marine Operation Added Cold-Water Complexity

Cold seawater does not automatically guarantee effective bearing cooling.

If flow is restricted, low water temperature cannot help a surface that receives insufficient water volume.

Actual flow path matters more than ambient water temperature alone.

Why Bearing Replacement Could Repeat the Problem

A new cutless bearing supplied by the same restricted water circuit can develop abnormal temperature again.

The lubrication source should therefore be verified during bearing diagnosis.

Technical Lesson

Marine bearing reliability depends on both:

mechanical geometry

and

lubrication medium delivery.

A normal shaft alignment result does not prove that a water-lubricated stern bearing is receiving adequate water.

FAQ

Can a cutless bearing overheat even if shaft alignment is acceptable?

Yes. Inadequate lubrication-water supply can increase friction and temperature.

Do all marine stern bearings require external water supply?

No. Bearing and vessel designs vary.