TST Engineering Services | Module 33 Stern Tube Seals, Propellers & Shafting Infographic

Module overview

What this module must make you confident with

The oral focus is not only naming components, but linking design, operation, maintenance, inspection evidence and fault response. You should be able to explain how the shaft line transmits power from the main engine to the propeller, how the stern tube and seals protect the sea interface, how the thrust block carries axial thrust, and how alignment and oil condition determine reliability.

Core learning outcomes

Explain stern tube sealing arrangements, including lip seals, mechanical face seals, seal running surfaces and leakage monitoring.
Describe how EAL oils are selected, monitored and trended as part of condition-based maintenance.
Explain Michell / tilting-pad thrust block operation and the creation of the hydrodynamic oil wedge.
Compare FPP and CPP propeller systems, materials, defects, hydraulic pitch control and dry-dock checks.
Describe alignment checks, bearing wear-down measurement, coupling face / rim readings, and when a jack-up test is used.

Stern tube Thrust block Intermediate bearings CPP hydraulics Oil analysis Dry-dock checks

Oral answer structure

For most questions, answer in this order:

1. Define the item – what it is and where it is fitted.
2. Explain the principle – how the component does its job.
3. Show what you monitor – temperatures, oil quality, vibration, leakage, wear-down, alarm points.
4. State likely failures – misalignment, contamination, seal wear, cavitation, loss of oil film.
5. Finish with actions – immediate precautions, investigation, class / maker limits and record keeping.

Section 1

Stern tube sealing arrangement, monitoring & EAL lubrication

The drawing below should be used to explain the entire sea-to-engine arrangement: propeller and shaft, aft seal housing, multiple lip seals, chromium liner, stern tube bearing, forward seal, oil header / monitoring tank and the drainage / leakage path. The key principle is continuous rubbing contact between the sealing element and the rotating running surface, with contact force maintained by garter springs, elastomer geometry or spring / bellows loading depending on seal type.

High-detail stern tube sealing arrangement infographic — Use the image to walk left-to-right from seawater aft of the propeller to the inboard oil side. Emphasise the outboard sealing interface, bearing lubrication supply, leakage collection, level monitoring and the role of the chromium liner as a hard, smooth, low-wear running surface.

Arrangement & operating principle

The aft seal prevents lubricating oil escaping to sea and resists seawater ingress from the outboard side.
Multiple lip seals are common. Each lip runs in continuous contact on the liner, providing redundancy and staged containment.
The stern tube bearing supports the shaft in the tube and is supplied with filtered lubricant.
The forward seal prevents leakage into the machinery space and helps isolate the bearing chamber.
Any leakage from the seal chamber is routed to a leakage / collection space and then to a monitoring or header tank.

EAL oils & monitoring tank logic

EALs reduce the environmental impact if leakage reaches the sea, but they must be compatible with seal materials, bearing metals and coatings.
The monitoring tank provides a visual level check; abnormal level rise or fall can indicate oil leakage or seawater ingress.
Routine sampling should trend water content, viscosity, wear debris, TAN, oxidation and additive health.
Drainage to bilge must be managed correctly and, where applicable, routed through approved pollution-prevention arrangements.

Typical failure signs

Milky oil, free water or chloride contamination in the sample.
Increased top-up demand or visible external leakage.
Rising stern tube bearing temperature or abnormal noise / vibration.
Scoring, corrosion, liner wear or seal lip damage during dry-dock inspection.
Header tank level drift without obvious operational reason.

Chief engineer response

Verify levels, alarms, trend history and recent oil consumption.
Take and analyse a representative oil sample before disturbing the system unnecessarily.
Inspect accessible leakage points, filters, cooling arrangements and local temperatures.
If contamination is confirmed, identify the source before a full oil change so the fault is not repeated.
Record findings, actions and any deviation from normal operation in the engine log / PMS.

Exam tip: If asked how the seal works, state that the sealing line is created by continuous rubbing contact between the rotating liner / face and the stationary sealing element. The required contact pressure is maintained by garter springs, elastomer geometry, axial springs or bellows depending on the design.

Lip seals

Simple, compact and widely fitted. Several lips are arranged in series to separate seawater, drain space and oil chambers.

Mechanical face seals

Precision lapped faces run against each other. Axial force from springs or bellows maintains contact and minimises leakage.

Chromium liner

Provides a hard, corrosion-resistant running surface. Inspect for scoring, pitting, turbulence polish and wear steps.

Section 2

Thrust block operation & intermediate shaft support

The thrust block carries the propeller’s axial thrust and transmits it into the ship’s structure, while intermediate bearings support the shaft line and keep bending loads under control. In the Michell / tilting-pad design, thrust pads pivot so a converging wedge of oil forms between pad and thrust collar. Relative motion drags the oil into the wedge and develops hydrodynamic pressure that fully separates the surfaces.

High-detail thrust block operation and intermediate shaft support infographic — Study the thrust collar, tilting pads, pivots, oil inlet / distribution, sump and the step-by-step development of the oil wedge. Use the right-hand side to revise shaft support, alignment checks, wear-down, sag and gap, and jack-up testing.

How a Michell thrust bearing works

At neutral condition the pads sit nearly level with an initial oil film.
When axial thrust is applied, the collar loads the pads and each pad tilts about its pivot.
The converging clearance drags oil in, generating a hydrodynamic wedge and a pressure field capable of supporting the load.
So long as the oil film remains intact, metal-to-metal contact is avoided.
Local film pressures can become very high under load; always refer to maker design data and class guidance rather than quoting a single universal value.

Intermediate shaft support

Intermediate bearings maintain shaft line geometry between stern tube and engine.
Wear-down is checked so that load sharing remains even and the shaft does not run off-center.
Coupling face / rim readings and sag / gap checks verify correct angular and parallel alignment.
A jack-up test may be used to assess bearing loading and shaft support balance.

Failure signs

Rapid rise in thrust pad temperature, alarm or thermal trip activity.
Discoloured oil, metallic debris, vibration or rumbling in the axial direction.
Wiping, scoring, fretting, excessive end float or abnormal pad wear pattern.
Increased noise or heat after alignment work, grounding, heavy weather or repair activity.

Checks you should mention orally

Monitor oil pressure, temperature, cleanliness and alarm response.
Inspect pad condition, thrust collar surface and oil distribution arrangements where accessible.
Record bearing wear-down at all intermediate supports and compare with historical data.
Repeat alignment checks after any machinery movement, foundation work or suspected distortion.

Section 3

Propeller design, materials & CPP systems

You should be able to describe the geometric features of a propeller blade, compare FPP and CPP arrangements, name common propeller materials, explain the hydraulic actuation path inside a CPP hub, and discuss typical dry-dock inspection points such as cavitation, pitting, root cracking, edge damage, oil leakage and pitch response.

High-detail propeller design materials and CPP systems infographic — This image combines an annotated FPP, a cutaway CPP hub, a material guide, an FPP-vs-CPP comparison, a vane / Grimm wheel concept and a dry-dock inspection list. Use it as your visual answer map.

FPP vs CPP

FPP: fixed blade angle, mechanically simpler, lower initial cost and lower maintenance burden.
CPP: blade pitch is varied hydraulically while the shaft rotates, giving rapid manoeuvrability and allowing the engine to remain near constant speed.
CPP is especially useful for ferries, tugs, offshore vessels and DP applications, but it introduces hydraulic, seal and feedback-system complexity.

Materials & defects

Nickel-aluminium bronze is common because of strength, cavitation resistance and corrosion behaviour.
Manganese bronze and stainless / duplex steels are also used for specific service demands.
Typical defects include cavitation erosion, pitting, bent blades, edge damage, cracks, corrosion, imbalance and boss / hub movement.

CPP hub internals

Hydraulic oil passes through the hollow shaft into the CPP hub.
A servo piston moves a crank / yoke or link arrangement that rotates each blade about its axis.
Return oil travels back from the hub to the control system.
Critical concerns are oil leakage, contaminated hydraulic oil, piston wear, linkage damage, poor feedback and sluggish pitch response.

Dry-dock inspection points

Inspect faces and backs of blades for cavitation, pitting and erosion.
Check leading and trailing edges, root fillets and blade thickness.
For CPP, inspect hub seals, leakage evidence, hydraulic pipes, control response and full-ahead / full-astern pitch change.
Confirm boss fit, taper integrity, key / nut security and signs of fretting or movement.

Remember: if asked about the vane / Grimm wheel, explain that it is a freewheeling vane fitted aft of the propeller. The inner region extracts energy from the propeller wake like a turbine, while the outer region acts as a propulsive surface that improves overall efficiency.

Pitch

The theoretical distance a propeller would advance in one revolution through a solid medium.

Rake

Fore or aft inclination of the blade relative to the radial line, affecting clearance and loading behaviour.

Skew

Angular sweep of the blade planform, often used to reduce vibration and pressure pulses.

Section 4

Shaft alignment, bearing inspection & condition monitoring

This section pulls together the whole shaft line from propeller to engine. It shows where to measure wear-down, how to check run-out and coupling alignment, what tools to use, and how oil analysis links shafting condition to the broader condition-based monitoring strategy covered elsewhere in the TST course.

High-detail shaft alignment bearing inspection and condition monitoring infographic — Walk through the shaft line left-to-right, then use the lower panels to revise chromium liner condition, oil sample analysis, alignment principles, measurement tools, misalignment sources and investigation triggers.

Measurement methods

Wear-down / poker gauge: measure bearing clearance or journal lift at prescribed positions such as 12, 3, 6 and 9 o’clock.
Dial indicator: used for shaft run-out checks and for coupling face / rim readings.
Laser / optical alignment: provides rapid and accurate angular / parallel alignment data.
Infrared thermometer: useful for comparative checks, but surface readings must be interpreted carefully.

Condition-based monitoring links

Trend oil analysis for wear particles, water content, viscosity, TAN and additive depletion.
Monitor vibration, bearing temperatures and any change in noise signature.
Use trend review rather than isolated readings wherever possible.
EAL lubricants still require the same disciplined monitoring approach as mineral oils.

Misalignment sources & evidence

Foundation movement, hull deflection, poor installation, thermal growth or worn bearings.
High face or rim readings, uneven wear-down, excessive vibration, seal wear or abnormal wipe patterns.
Overheated or discoloured bearings, metallic debris in oil / filters, or repeated leakage problems.

When to investigate immediately

Any sudden increase in vibration, noise, temperature or oil contamination.
After grounding, stern impact, heavy weather, dry-dock work or significant repair to the shaft line.
Following coupling work, engine movement, bearing renewal or seal replacement.
When trend data shows progressive deterioration even if alarms have not yet activated.

Final oral link: alignment, seal life, thrust bearing health and oil condition are all connected. A ship can have a good seal design yet still suffer leakage or wear if alignment is poor, loads are uneven, lubrication is contaminated, or operational trends are ignored.