Propulsion Shaft Bearing Overheating Alignment Case Study

Propulsion shaft bearing overheating is a release-critical condition, not a nuisance alarm. A vessel can still reach shaft speed while the stern-tube or aft line-shaft bearing is carrying too much reaction, losing lubricant-film margin, generating excess heat and wiping bearing metal at one end of the contact patch.

This case study follows a single-screw coastal vessel after drydock work on the shaft seal, stern-tube bearing and propeller. During the post-refit sea trial, the aft bearing oil outlet temperature climbs toward the alarm limit, while the propeller appears clean and delivered power is normal. The engineering task is to decide whether the vessel can continue the trial, whether the symptom is cavitation, and what shaft-line evidence is required before release.

The case is simplified for engineering education. Real propulsion shafting work must follow the shafting design report, bearing vendor requirements, classification society rules, drydock alignment procedure, jack-up or strain-gauge method, lubricant requirements, sea-trial plan, safety limits and qualified marine-engineering judgement.

Case Context

The vessel left drydock after stern-seal work, bearing inspection and propeller polishing. Static alignment readings were acceptable in the dock, but the first loaded sea-trial run produced a sustained temperature rise at the aft oil-lubricated bearing.

The central question is:

Is the high bearing temperature an acceptable transient after maintenance, or does it indicate an overloaded propulsion shaft bearing that must be corrected before unrestricted service?

The correct decision is to hold release, verify bearing reactions and shaft-line alignment in the relevant afloat and hot condition, inspect the bearing and lubricant, correct the load distribution, and repeat the sea-trial evidence.

Field Data

Use the following representative data for a screening review.

Quantity	Symbol	Value
shaft speed during reviewed run	n	210\ \text{rpm}
shaft diameter at aft bearing	D	0.240\ \text{m}
effective bearing length	L_b	1.10\ \text{m}
design aft bearing reaction	R_{design}	95\ \text{kN}
measured hot aft bearing reaction	R_{hot}	155\ \text{kN}
corrected aft bearing reaction target	R_{corr}	105\ \text{kN}
projected bearing-pressure review value		0.60\ \text{MPa}
bearing oil mass flow rate	\dot{m}_{oil}	0.24\ \text{kg/s}
oil specific heat	c_p	2000\ \text{J/(kg K)}
suspected mixed-friction coefficient	\mu_{hot}	0.025
normal hydrodynamic-friction screen	\mu_{corr}	0.014
oil supply temperature during run		47^\circ\text{C}
propeller blade count	Z	4

The reaction values are simplified. A real shaft-line review would separate vertical and horizontal bearing reactions, hull deflection, bearing offsets, shaft sag, thermal growth, propeller immersion, thrust bearing condition, tank state, draft and trim.

Field Evidence

The evidence points toward shaft-line bearing overload rather than propeller cavitation.

Evidence	Engineering interpretation
bearing temperature rises with sustained shaft load	friction heat and lubricant-film margin are involved
dominant vibration is near 1x and 2x shaft frequency	alignment, rub or shaft-line reaction is credible
blade-rate vibration is not elevated	propeller cavitation is less supported
diver inspection finds no fresh cavitation erosion	the propeller is not the leading suspect
oil sample shows light bearing-metal debris but no water ingress	bearing distress is plausible without seal flooding
jack-up readings show high aft reaction and light adjacent bearing load	load distribution is not as designed
drydock alignment record was taken in a different support condition	cold dock readings do not prove afloat hot alignment

The diagnosis should not depend on temperature alone. It should connect shaft speed, bearing reaction, projected pressure, heat generation, lubricant temperature rise, vibration frequencies, oil evidence and post-correction response.

Step 1: Convert Shaft Speed to Surface Speed

The rotational frequency is:

With:

the result is:

Shaft surface speed at the bearing is:

Substitute:

Engineering Comment

Surface speed matters because friction power scales with both bearing reaction and sliding speed. A small increase in reaction can create a large temperature consequence when lubricant-film conditions deteriorate.

Step 2: Calculate Projected Bearing Pressure

Use the projected bearing area:

With:

and:

the area is:

The design pressure is:

The measured hot pressure is:

After correction, the target pressure is:

Engineering Comment

The measured hot pressure is close to the 0.60\ \text{MPa} review value. This is enough to stop treating the alarm as instrumentation noise. The corrected reaction returns the bearing pressure close to the intended design range.

Step 3: Check PV Severity

A simple severity screen is the product of projected pressure and surface speed:

For the hot condition:

For the corrected condition:

Engineering Comment

PV is not a universal acceptance criterion for every stern-tube bearing material and lubrication regime. It is still useful for comparing two states of the same bearing: the hot reaction creates about 48\% more pressure-speed severity than the corrected state.

Step 4: Estimate Friction Heat

Use a first-pass friction heat screen:

For the suspected hot condition:

For the corrected hydrodynamic condition:

Engineering Comment

The heat generation is not only higher because the load is higher. The friction coefficient is also worse because the bearing is closer to mixed lubrication, edge contact or local wipe. That combination explains why a modest-looking reaction error can create a strong temperature symptom.

Step 5: Estimate Oil Temperature Rise

The oil temperature rise from friction heat is:

For the hot condition:

With a 47^\circ\text{C} oil supply temperature, the outlet temperature screen is:

For the corrected condition:

and:

Engineering Comment

The simplified heat balance matches the field symptom: the hot bearing approaches a typical alarm region, while the corrected load distribution should produce a stable outlet temperature with useful margin.

Step 6: Separate Alignment Evidence from Cavitation Evidence

The shaft frequency is:

The second harmonic is:

For a four-bladed propeller, blade-rate frequency is:

The trial spectrum shows elevated 1x and 2x components near the bearing, but no abnormal blade-rate peak near 14.0\ \text{Hz}. That does not prove the propeller is perfect, but it makes cavitation a weaker primary diagnosis than shaft-line reaction or bearing contact.

Engineering Comment

This is where the case differs from a propeller cavitation investigation. Cavitation diagnosis would depend on blade-rate pressure pulses, erosion pattern, wake, cavitation number and underwater noise. Here the strongest evidence is bearing temperature, reaction distribution, 1x/2x vibration and lubricant debris.

Step 7: Check Edge-Loading Sensitivity

Projected pressure assumes the full bearing length is sharing load. Misalignment can concentrate contact near one end. If only 75\% of the bearing length is effectively carrying the reaction:

The effective projected area becomes:

The local pressure screen is then:

Engineering Comment

This explains why a bearing can show wipe marks near one end even when the average projected pressure is only slightly below a review value. Shaft-line alignment is a load-distribution problem, not only an average-pressure problem.

Engineering Decision

The vessel should not be released for unrestricted service in the hot condition.

The reason is not a single alarm. The decision comes from the combined evidence:

Finding	Consequence
aft bearing reaction is 155\ \text{kN} instead of 95\ \text{kN}	the bearing is overloaded
projected pressure is 0.587\ \text{MPa}	it is close to the review value
friction heat screen is 10.2\ \text{kW}	oil outlet temperature is expected to rise
temperature rise screen is 21.3\ \text{K}	the measured alarm trend is physically plausible
1x and 2x vibration dominate	shaft-line reaction or contact is plausible
blade-rate evidence is weak	propeller cavitation is not the leading diagnosis
oil debris and wipe marks agree	bearing distress is already starting

The action is to suspend the full-power trial, keep the vessel inside a restricted rpm and temperature envelope if movement is necessary, and correct the shaft-line load distribution before release.

Corrective Work Scope

A practical correction package should include:

Work item	Engineering purpose
repeat bearing reaction measurement afloat	confirm the support condition that exists in water
review tank, draft and trim state	avoid comparing different hull-deflection states
inspect bearing surface and oil filters	identify wipe, edge contact or embedded debris
verify oil grade, flow and cooler performance	prevent a lubrication problem from masking alignment work
update shaft-line alignment model	include hull deflection, bearing offsets and hot condition
adjust bearing offset or support shims	redistribute reaction without unloading adjacent bearings
check propeller and stern clearance	exclude a secondary rubbing or fouling source
rerun vibration and temperature trend	prove the correction at operating speed

Correcting the alarm setpoint or accepting the drydock alignment record is not enough. The release evidence must show that the actual operating load path has changed.

Corrected Sea-Trial Result

After adjustment, the aft bearing reaction is reduced to:

The pressure becomes:

The friction heat screen becomes:

and the oil temperature rise becomes:

During the repeated sea trial, the outlet temperature stabilizes near 55^\circ\text{C} with no rising trend, the 1x and 2x components return near baseline, oil debris stabilizes, and adjacent bearing reactions remain positive.

Release Criteria

The vessel can be released only if the evidence package satisfies all gates.

Gate	Release evidence
bearing reaction	aft bearing reaction within approved tolerance and no adjacent bearing unloading
temperature	outlet oil temperature stable with margin to alarm at reviewed rpm
lubricant condition	oil sample and filters show no continuing bearing-metal generation
vibration	1x, 2x and blade-rate components inside reviewed limits
shaft-line alignment	documented afloat or corrected alignment state, not only drydock values
propulsion performance	speed, shaft power and fuel data remain consistent after correction
inspection	bearing wipe marks understood and dispositioned
configuration control	bearing offsets, shims, tank state and trial condition recorded

If any gate is missing, the correct status is restricted operation or retest, not unrestricted service.

Lessons Learned

Propulsion shaft bearing overheating is a system-level symptom. It may involve bearing geometry, shaft alignment, hull deflection, propeller load, lubricant flow, thrust condition, vibration and trial configuration.

The important engineering habits are:

1. Treat temperature as a physical consequence, not just an alarm tag.
2. Convert shaft speed into surface speed and frequency evidence.
3. Compare design, measured and corrected bearing reactions.
4. Use pressure and heat screens to test whether the symptom is plausible.
5. Separate shaft-line evidence from propeller cavitation evidence.
6. Validate correction with sea-trial temperature, vibration, oil and reaction data.

The final release decision should be based on a closed evidence loop: overloaded bearing reaction found, heat mechanism quantified, alignment corrected, bearing condition checked, and the repeated sea trial demonstrating stable temperature and vibration.