Water Lubricated Stern Tubes

Water Lubricated Stern Tubes

• A shrunk-on bronze liner is placed over the tailshaft to prevent steel wastage.
• The liner is sandwiched between the propeller hub and the end of the liner using a rubber seal.
• A stuffing box gland on the inboard end minimizes seawater ingress.
• Sand or sediment can cause excessive weardown and early re-wooding.
• Replacement of wooden staves may be necessary after about 18 months due to aforementioned weardown.
• A coefficient of friction of about 0.005 is achievable when water lubricated.
• Short length-to-diameter ratios can be designed for high loading applications.

Water Lubricated Stern Tube Materials

• Lignum vitae staves are fitted with their end grain vertically beneath the shaft for better wear resistance
• Grooves are present between the staves to allow water access and accommodate debris.
• Staves are secured in a bronze bush with bearing length equal to four times the diameter of the shaft.
• Water absorption causes swelling, with the greatest expansion direction normal to the laminate.
• Expansion will not exceed 1 mm in 40 mm material thickness.
• Diametric clearance of about 2 mm is recommended for a 500 mm shaft.
• Physical properties suitable for tailshaft bearings include:
• Ultimate Tensile Strength (UTS): 62 MN/m²
• Compressive stress: 290 MN/m²
• Shear strength: 100 MN/m²
• Some designs use rubber-bearing surfaces.
• Impregnated plastic resin compounds on plastic type bases are used as replacements for lignum vitae.
• Tufnol, a thermosetting laminate produced from cotton fabric and phenolic resin, is one such type.
• The fabric is impregnated with the resin then pressurized under heat until fabric laminations are bonded.
• This material has uniform density, hardness, swelling, good wettability, low friction coefficient, and compressive strength (flatwise) approximately twice that of lignum vitae.

Oil Lubricated Stern Tubes

• Ends are sealed, and oil is supplied under pressure.
• Any present water should be regularly drained off.
• White metal weardown should not exceed 2 mm to avoid hammering out with inspections at 5 year intervals.
• Highly resilient reinforced plastic material is often used in place of white metal with good load-carrying capabilities, high fatigue resistance and good lubrication properties.
• Provision for oil cooling is needed.
• A gravity tank is fitted to supply lubricating oil to the stern tube, located above the full load waterline with a positive head.
• Oil sealing glands must be suitable for seawater temperatures.
• Static lubrication systems for vessels with moderate draught changes have header tanks 1-3m above maximum load waterline.
• Small differential pressure ensures that water is kept out.
• Cooling of simple stern tubes requires keeping the aft peak tank water level at least 1m above the stern tube.
• Ships with large draught variations may have two oil header tanks for ballast and fully loaded draughts.

Hydrodynamic or Hydrostatic Lubrication

• Steaming at slow speeds gives lower fluid film or hydrodynamic pressure in stern tubes.
• This has led to the introduction of forced lubrication for hydrostatic pressure independent of shaft speed.
• Supplied oil pressure gives adequate lift to separate the bearing and shaft, and provides sufficient oil flow for cooling.

Dry-dock Weardown Inspection

• First measurement should be checked immediately after fitting the propeller shaft and seal.
• Every measurement must be taken in the same place, marked by a line on the aft seal casing corresponding to markings on the liner "0".
• After propeller shaft removal or seal removal, take new measurements:
• Remove screw plugs with sealing washers
• Screw weardown gauge firmly into the holes
• Read the measured value and note on the inside lid of the weardown gauge case
• Re-seal the screw plug and sealing washer
• Attention must be given to connection locations for measurements

Shaft Seals

• The shaft sealing arrangement is that of the Simplex type shaft seal.
• Outboard and inboard seals attach to the spherical seating ring and diaphragm, respectively.
• The spherical carrier ring is bolted to the flange on the after end of the bearing tube and is supported by the spherical seating ring.
• The diaphragm is bolted to the flange on the inboard end of the bearing tube, and is bolted to the stern frame casing.
• O-rings on the carrier ring and diaphragm seal the oil space around the bearing tube.
• The bearing tube is spheroidal graphite cast iron lined with white metal.
• The tube is split horizontally, and the two halves are bolted together.
• The stern frame is fully machined before welding to the hull.
• The bore for the seating ring can be machined for adjustment, with controlled welding used to maintain alignment.
• The shaft is installed from the outboard end, with its rotating liner and carrier ring assembled.
• The shaft inboard end is fitted with an oil injection coupling, sometimes termed a sleeve or 'muff' coupling.

Withdrawable Stern Gear System

• Withdrawable systems allow inspection, re-alignment, or repair without disturbing the propeller or uncoupling the shaft.
• The system can be used with fixed or controllable pitch propellers and most shaft seals.
• The bearing withdrawn, the propeller and shafting weight is supported on a ring secured to the stern frame.
• The whole unit, including the outer seal, can be moved along the shaft inboard for inspection.

Fixed Pitch Propellers

• Although usually described as fixed, the pitch does vary with increasing radius away from the boss.
• Consists of a boss with several blades of helicoidal form.
• When rotated, the propeller thrusts through the water by giving momentum to the water passing through it.
• A propeller turning clockwise viewed from aft is a right-handed propeller.
• Most single-screw ships use right-handed propellers.
• Twin-screw ships usually have a right-handed starboard propeller and a left-handed port propeller.
• Propeller material needs to be resistant to seawater corrosion, cavitation erosion and shock loading.
• The material should be castable into intricate shapes, repairable and capable of achieving a good surface finish.
• High tensile strength is required for thin blade sections.
• High-tensile brass construction (manganese bronze) was used for many years.
• Manganese bronze is a copper-zinc alloy with manganese, aluminum, iron, tin, and sometimes nickel, with tensile strengths of 40-50 MN/m², depending on composition.
• Modern applications use aluminum bronzes (8-10% aluminum, 10-12% manganese, 2-3% iron, 2-5% nickel).
• Aluminum bronzes have greater tensile strength (65 MN/m²) and corrosion resistance, with lower specific gravity.
• Stainless steel propellers are also in service.
• The common method is to cast the blades as a one-piece unit with the boss.
• The tapered bore is machined along with boss faces.
• Blades are profiled by hand with datum grooves or with an electronically controlled profiling machine.
• The blades are ground and polished to a smooth surface finish.
• As manufacturing improved, propellers with separately cast blades secured by studs/nuts were succeeded by one-piece castings.
• Built-up propellers had the advantages of easily replaceable damaged blades and adjustable pitch.
• However, they had restricted blade width at the root, greater thickness for strength, and a larger hub diameter.

Methods of Securing Propellers

• Fixed pitch propellers were fitted to tail shafts using a key and taper arrangement, with a tightening nut.
• The key was intended as a safeguard against poor fitment or reduced grip at high seawater temperatures.
• Keyless fitting has become the preferred method for propeller fitment, removing the problems associated with keyways (stress raisers).
• Many fixed propellers are flange mounted for easy tailshaft removal through use of a muff coupling.

Keyed Propellers

• Torque under ideal conditions is transmitted by the interference fit, with the key as a backup.
• Incorrect conditions can cause fatigue cracks at the forward end of the keyway, or more serious cracks due to fretting damage.
• This is particularly noticeable in high-powered single screw vessels.

Keyless Propellers

• Keyless designs rely on the accuracy of the taper.
• The amount of strain depends on the 'push up' of the propeller onto the taper.
• Adequate grip is required regardless of temperature changes and overstressing of the hub must be avoided.
• Propellers can be solid castings, built up, or of the variable pitch type.

Contra-Rotating Propellers

• Rotational exit losses can be in the region of about 8% to 10%.
• Co-axial counter-rotating propellers can increase efficiency by up to 6%.
• The aft propeller should have a slightly smaller diameter to avoid cavitation problems.
• Advantages of contra-rotating propellers:
• Compensated propeller induced heeling moment
• More power can be transmitted for a given propeller radius
• Increased propeller efficiency
• Disadvantages of contra-rotating propellers:
• Expensive, complicated installation and intensive maintenance
• Hydrodynamic gains largely lost through mechanical losses in shafting

Kort Nozzles

• Kort nozzles are a fixed annular forward extending duct around the propeller.
• Principle is to maximize water quantity and minimize additional velocity for thrust generation at good efficiencies.
• Advantages of Kort nozzles:
• Better efficiency at high thrust loads, with efficiency raised by 20% and bollard pull by around 30% for tugs
• Small reduction in efficiency in a seaway
• Improved course stability
• Nozzle replaces rudder in 'steerable' versions
• Disadvantages of Kort nozzles:
• Impaired course changing ability going astern
• Damage during ice conditions
• Cavitation occurring earlier due to pressure drop

Propeller Fitment in Dry Dock

• This task is to be witnessed by a surveyor.
• Keyed taper shafts would use marking blue to ensire good fit.
• The use of hydraulic nuts to tighten propellers produces high axial forces, therefore care must be exercised not to overstress the material.
• Manufacturer procedures should be followed, including crack detection after fitment (dye penetrant test).
• The propeller cone needs to be properly sealed with a rubber ring for ensuring no water access.
• Keyless fits rely solely on a good interference fit, largely superseding keyed arrangements

Keyless Propeller Fitting

• Effectiveness depends on shaft and propeller taper accuracy, and correct propeller hub grip.
• Grip is controlled by 'push up' allowance.
• Overstressing the hub causes permanent deformation.
• Lloyds Register requires interference capable of transmitting 2.7 times the nominal torque at 35°C.
• At 0°C, stress at the propeller bore must not exceed 60% of the 0.2% proof process of the propeller material, measured under test conditions.
• One method is an the oil injection system with axial and circumferential grooves.
• High-pressure oil is injected between the tailshaft and propeller, reducing friction, and a hydraulic jacking ring pushes the propeller until positioned.
• When the oil pressure is released, the oil runs back, leaving the shaft and propeller securely fastened.
• Data stamped on the propeller boss outer surface:
• Oil injection type
• Start point load
• Axial push up 0° C
• Axial push up 35° C
• Identification mark on screw shaft

Pilgrim Nut Method

• Provides pre-determined frictional grip between propeller and shaft.
• A threaded hydraulic jack is screwed onto the tailshaft.
• A steel ring receives thrust from a hydraulically pressurized nitrile rubber tyre, which is applied to the propeller to force it onto the tapered tailshaft.
• The engine torque may be transmitted without loading the key (if fitted).
• Before use, calculate hydraulic pressure, monitor hub movement up the taper via dial gauge.
• Propeller removal is completed by reversing the pilgrim nut.
• This is fastened to the propeller boss by studs while pressurizing the tyre, and the propeller is removed from the taper.

Dry-Docking Inspections & Maintenance

• Cracks near blade edges are particularly dangerous.
• Consult the makers if cracks are within 0.45 of propeller radius because any repair results in high residual stresses resolved by annealing.
• Temporarily stop spreading of small cracks by drilling a hole of at least 10mm in diameter at the extremity, then plug the hole.
• Damage ranges from slight deformation (striking submerged object) to severe breaking/bending.
• Causes disturbance of flow, loss of efficiency, vibration and erosion.
• Impact may create cracks on blade edges.
• Distortion can be straightened.
• Breakages are repaired by cutting out the damaged section and welding a replacement piece in its place.
• Corrosion (electro-chemical attack on the metal surface) causes pitting and dezincification.
• Severely pitted surfaces break down and become rough, causing a loss of efficiency and reduction in blade thickness.
• Smooth surfaces need to be restored by grinding.
• Erosion is a mechanical attack on the metal by disturbance of the water flow or an incorrect blade design, thus producing cavities needing welding repair.

Methods of Blade Repair

• Repairs while the propeller is on the shaft are limited to fairing minor edge damage, light corrosion and erosion by surface grinding.
• Localized heating is not allowed due to stress relieving not being possible.
• Ideally, the propeller should be returned to the workshop for assessment, pitch checking, and assessment of required repairs.
• Straightening: bent portion is slowly heated to a low red heat, distortion corrected by weights. After slow cooling, the pitch is re-checked, and the process is repeated.
• Burning: damaged portion is cut out, cavity enclosed by a sand mould and molten metal poured until full, providing a permanent repair.
• Welding: cracks are repaired via oxy-acetylene or electric arc welding. A coated aluminum-bronze or phosphor-bronze electrode is used (manganese bronze is not possible). The area is pre-heated, then the weld is annealed to maintain correct alloy micro-structure. The repaired area is then polished, carefully checked, and is statically balanced.

Controllable Pitch Propellers

• The use of these propellers has increased with the use of uni-directional engines, uni-directional gas turbines and multi-diesel drives.
• The engine room (or bridge) signal can be fed to a torque-speed selector that fixes both the engine speed and the propeller pitch (referred to as a combinator system).
• The input signal acts on the diaphragm in the valve housing and directs pressure oil via one piston valve through the tube to one side (left) of the servo piston, or via the other piston valve outside the tube, to the right of the servo piston.
• Movement of the servo piston, through a crank pin ring and sliding blocks, rotates the blades and varies the pitch.
• Emergency local pitch control, alarm and communication systems, as well as failsafe systems (navigable ahead pitch) are required.

Control of CPP(Controllable Pitch Propellers)

• Two lever control systems, (one for the CPP and the other for the engine speed), has simple control and allows experienced operators to get the best machinery under varying circumstances.
• Larger vessels are fitted with combinator systems to operate both the engine speed and the propeller pitch (either pneumatically or electronically).
• A closed loop is employed so feedback from the propeller pitch and engine speed balance off the control signal.
• Additional control stands can be mounted on the bridge wings (slave units).

Testing of CPPs

• The hand control must be tested throughout entire range.
• Before entering waters where navigation may be restricted, check that controls are responding normally.

Limitations of Combinator Controls

• Controls normally provide for the engine to be run at 60-70% of its rated speed when the handle is vertical (zero pitch).
• Not necessarily zero thrust, it may be slightly off zero to prevent the vessel inching forward.
• For CPP vessels movement on the lever astern should obtain zero pitch.
• Movement of the lever forward initially affects only the pitch until 1/2 lever movement. The engine speed starts to increase to full speed with further movement.
• It is frequently the case controls incorporate relay to 'back off' the pitch in the event of the engine becoming overloaded, with a trim knob controlling the range of pitch.
• It is usual to provide automatic starting of the standby pump in the event of hydraulic oil pressure.
• Interlocks prevent starting air from being admitted to the main engines unless the hydraulic pump pressure is available and gearbox clutches.

Periodical Survey Tasks

• Blade seals should be renewed.
• Blade bolts should be crack tested upon re-tightening (follow the manufacturers instructions).
• If removable blade bearing rings are fitted, they should be crack detected, replaced, and re-doweled.
• If water has caused rust in the hub, the crank rings need to be crack detected.
• Clean the CPP, oil tanks, recoat as required and pressure test. Flush the system on completion.
• Examine the pump sets to confirm that wear is within the limits.
• Remove the shaft flange protection sleeves, and check for corrosion.
• Check where on stern tube seal wear rings correct surface finish using manufacturers instructions.

In-service Maintenance

• Check CPP oil tanks:
• 4 hourly at sea
• 12 hourly in port
• Check CPP oil tanks for water contamination once a watch.
• Immediate effect of water contamination is wear on the sliding blocks, and sludging of the filters.

Blade Maintenance

• Blade maintenance is similar to that on fixed pitch propellers, however greater wear can be expected on vessels that are required to do a lot of maneuvering.
• Blades and hubs made of medium chromium stainless steel require crack inspection during dry-docking, and inspection of signs of corrosion in the crevices.
• Where CPP's are run at a constant speed can enable shaft driven generators which allows constant cavitation erosion of the blades to be significant or run at reduced pitch for prolonged periods.
• The blade and blade carrier form a strong integral component, and are mounted on the spherically shaped hub. Construction is such that the hub components have the same strength as a solid hub.
• The blades are sealed onto the hub with synthetic rubber O-rings, mounted on the blade foot.
• The seals enclose the lubricant, with the blades being individually replaceable and disc or pivot bearing arrangements.

Controllable-pitch Propeller Advantages

• Fast stop maneuvers are possible.
• Uni-directional engines can be used.

Controllable-pitch Propeller Disadvantages

• The fuel consumption and propeller costs are higher.