Stern Tube is like hollow tube at the aft of ship. It’s small narrow hole in the ship’s hull at the aft of ship. It’s the part of rear end shaft of propeller shaft and connects propeller at sea side. It’s carrying tail shaft, bearing, lubrication system and sealing arrangements.
On the basis of bearing lubrication system and sealing system, Stern Tube is of two types: Water Lubricated Type and Oil Lubricated Type.
Water Lubricated Type
This design is now superseded by oil lubricated type.
SW is used for cooling and lubricating the bearing.
Open aft end allows SW to flow in with supplementary SW connection, piped into stern tube from ship’s main.
A soft packing gland fitted at forward end and slight leakage is allowed to ensure cooling of packing.
Tail shaft is protected by corrosion resistance bronze liner, shrunk or pressed on tail shaft, extending from propeller hub to forward of forward gland seal (continuous or single piece).
Liner may be continuous or in two pieces.
Liner thickness, dictated by Classification rules, is 23 mm for 500 mm shaft.
- Very hard and wear resistant.
- Natural lubrication assisted by SW.
- Low swelling due to SW absorption.
- Predictable wear rate allows scheduling of docking in advance.
- No sophisticated ford/aft seals required.
- Higher wear rate due to large clearance and use of SW as lubricant.
- Less load carrying capacity due to staved surface.
- Shaft needs extra liner for SW corrosion protection.
- Fatigue crack generating from corrosion pits could be the outcome, as galvanic action between shaft and sleeve (liner) is possible.
- More shaft movement and vibration due to larger clearance.
- More shaft movement may cause fretting at shaft coupling bolts.
- Packing grips at forward end wears out liner unevenly.
- Oil is better vibration damper than water.
- Abrasives enter the bearing.
Normal clearance: 0.003 to 0.004 of shaft diameter.
Maximum Clearance: Varies between 6 – 10 mm.
Checking clearance (or) Wear Measurement
It is done by inserting small wooden wedge or feeler gauge between the shaft liner and lignum vitae, once the rope guard has been removed, when ship is in dry-dock.
Wear rate: Cargo ship with engine amidship = 0.5 mm to 4 mm / year.
Tanker or Ship with engine aft = 1 mm to 13 mm / year.
Lignum vitae size: typical length 9″ x 2″ width x ¾ ” thick.
Clearance at which re-wooding required varies between 6 – 10 mm.
Survey interval of tail end shaft:
Single liner tail shaft to be exposed for examination every 3 years.
Double liner tail shaft to be exposed for examination every 2 years.
Oil Lubricated Type
Two bushes of white metal lined, grey or nodular cast iron, are pressed into sterntube.
Mechanical seals are provided at both ends and stern tube space is filled with oil.
Oil pressure is maintained slightly above seawater pressure by means of static header tank, keeping the static head pressure, 0.30 bar higher than seawater pressure.
- Less wear is experienced.
- Very less power loss at bearing.
- Less heat is generated
- Hydrodynamic lubrication can be established.
- No bronze liner required in way of bearing.
- No abrasives enter the bearing
- Oil is superior lubricant and good vibration damper.
- Low clearance reduces shaft movement and vibration.
- White metal debris may choke and restrict oil supply, speeding up failure.
- Contaminated oil supply, causes abrasive wear.
- Prolonged low speed operation may allow only boundary lubrication.
- Poor bonding of white metal to bush may exist.
- Bearing metal failure due to fatigue.
- Lack of oil supply, due to low level in header tank, obstructed flow, damaged pipework.
Continuous length of bearing metal = 1.5 to 2.0 x shaft diameter, for aft end bearing.
= 0.6 to 1.25 x shaft diameter, for forward bearing.
Thickness of bearing bush: Varies according to Classification Society: 3.8 mm thickness for 300 mm diameter shaft and 7.4 mm thickness for 900 mm shaft.
Oil clearance: Depends upon Class and LR recommends 0.0015 – 0.002 of shaft diameter.
Maximum clearance: 2 times original clearance.
Stern Bearing Wear Measurement:
- Measured at every dry-dock and always should be measured at the same radial position with usually 4 readings taken at 90°intervals.
- ‘0’ marks are stamped on the periphery of aft chrome steel liner flange. (Ideally chisel or similar marks should be left on the propeller boss. Alternatively the readings can coincide with the propeller blades.)
- Measurement is taken at plugged oiling hole (Top Check Plug) and drain hole (Bottom Check Plug) of aft seal intermediate ring, through the seal housing. (To give maximum accuracy, it is recommended that readings be taken at top and bottom if possible, so that any eccentricity of the shaft is taken into account.)
- Mating marks are curved at top and bottom Check Plugs.
- In measuring, bring the ‘0’ mark on chrome steel liner to fixed position at all times.
- Screws down the wear down depth gauge into the hole tightly and align the mating
- mark and ‘0’ mark.
- Measurement is recorded in record sheet.
Survey interval of tail end shaft: Special Survey at 4-years interval.
Stern tube oil: Oil is a compound type with sp. gr. 0.95 and viscosity 300 RW No.1 at 60C.
Temperature sensor and pressure gauges are usually fitted.
Oil pressure fluctuation with respect to ship draught, means leaking of oil seal.
3 types of sealing arrangements:
- Simple stuffing box
- Lip seal type
- Radial face seals.
Sealing Arrangement will be discussed later.