CoC Oral Exam Preparation (Part – 12): Stern Tube
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The stern tube is a hollow tube-like structure at the stern or rear part of the ship. A ship needs the propeller to drive it forward in water. The propeller, located outside the ship, needs to be connected to the engine inside the ship’s engine room. The propeller shaft is used for connecting the ship’s engine and the propeller. The stern tube is a narrow hole in the hull structure at the rear end (aft peak) of the ship, through which the propeller shaft passes and connects the engine and propeller.
Stern tube bearings serve two main functions:
1. To properly connect the propeller to the ship
2. To keep water from leaking into the stern tube (and lubricant from leaking out)
There are three types of stern tube bearing arrangements that are used to accomplish these goals:
1. Stuffing boxes, which uses packing material to seal the area
2. Lip seals, which are connected to the tail shaft to prevent lubricant or water from passing through the shaft
3. Radial face seals, which are supported with specially fitted springs around the shaft, aft bulkheads and near the back end of the stern tube
Stresses in tail shafts:
1. Due to the considerable weight of the propeller, the tail shaft is subject to a bending stress.
2. There is a torsional stress due to the propeller resistance and the engine turning moment, and
3. Compressive stress due to the propeller thrust.
Stern Bearing Wear-down Measurement:
1. Use Poker Gauge or Wear down Gauge or Vernier Caliper.
2. Remove Rope Guard.
3. Take out Check Plug and Drain Plug.
4. Turn Tailshaft until ‘0’ marks on periphery of aft chrome steel liner flange and mating marks on Simplex seal and Sterntube are in line.
5. Measure at measuring plug (top) and drain plug (bottom) through 180°at same radial position as previous docking.
6. Compare with previous readings.
Stern Bearing Wear Measurement:
1. Measured at same radial position at every dry dock.
2. Look for reference mark left usually on propeller boss when measured during last docking.
3. The same mark should be used again, so that reading can be taken at same radial position.
4. Sometimes measurement had been taken when No. 1 Unit was at TDC, and important thing is that it should be at same radial position as last docking.
5. Measuring at top telltale hole or check plug is enough and bottom check plug is only used for counter check.
6. As a CE, witness the measuring procedure when vessel is on dock
Examining a tail shaft and stern tube:
1. Before the periodic inspection the bearing weardown should be measured.
2. After shaft removed given thorough examination.
3. On water lubricated shafts the integrity of the fit of the bronze liner should be checked by tapping with a hammer along its length listening for hollow noise indicating a separation.
4. Measure wear of shaft.
5. Examine key way for cracks especially the nut thread area.
6. Replace rubber rings.
How to check Stern tube wear down? (Lignum Vitae, water cooled stern tube)
1. Measurement can be taken by Wedge Gauge or Feeler.
2. If Wedge Gauge is used, the side of the Wedge contacting the bearing is chalked and inserted into the clearance space between top of the screw shaft and bearing.
3. Gauge is pressed home and withdrawn.
4. Clearance is measured on the Wedge at the point where the chalk marking is scrapped-of by bearing.
For Oil Lubricated bearing,
1. 2 mm clearance.
2. When the ship is on Dry Dock, release system oil and remove plug on the end of the stern tube, and insert Poker Gauge to measure the distance from the datum to the top of the shaft.
3. The difference between new reading and original measurement is bearing wear down value.
All types of Stern Bearing:
1. Fit a Dial Gauge on the Rope Guard or Sterntube Nut so that the Gauge spindle is vertical and touching the Propeller Boss.
2. A hydraulic jack is placed on the Stern Frame Skeg at same point over a Keel Block so that the Skeg is supported.
3. A wood shoe is placed between the jack and the Propeller Boss.
4. The jack is then used to lift the Propeller until the Screw Shaft contact the upper parts of its Stern Bearing.
5. The lift recorded in Dial Gauge gives the bearing clearance.
Skeg: An arm extending to the rear of the Keel to support Rudder and protect Propeller.
Tail end shaft taper: ¾” to 1″ per foot length.
Stern tube bearing length:
- Aft bearing 4 D Forward Bearing 1 – 2 D (Water-cooled)
- Aft bearing 1.5 – 3 D Forward Bearing 0.6 – 1.25 D (Oil cooled)
How you check Stern tube Sealing in Dry dock?
– LO header tank is topped-up and checked for leakage for 24 hours.
What action to be taken when SW leaks into Sterntube oil system?
– Higher up the Stern tube oil tank level to counteract SW force.
Stern tube Leakage Test:
- Remove screw plug and fill up the stern tube oil (normally using main engine system oil). Oil pressure must amount to only 0.2 – 0.3 Kg/cm² more than SW pressure.
- Preheated to 60 – 70°C when viscose oil is used or low temperature prevails
- Remain in this condition for several hours (says overnight) and turn engine by Turning Gear to change the shaft position 3 – 4 times.
- Check oil leakage from Drain Plug and if it is all right, fit back Drain Plug and fill oil from venting and Filling Plug and close.
- Forward Seal is fitted.
Constant Air flow type Stern Tube:
Fig: Constant Air flow type Stern Tube (Source: KOBELCO Air Seal, Japan)
- . A constant quantity of air supplied from the air source, which then passes through the #2/3 seal chamber and is then spouted into sea. The air pressure is always maintained about 0.01 Mpa (0.1 bar) greater than the seawater pressure so to slightly exceed the tightening force of the #1 and #2 sealing rings.
- The air pressure is added to stern tube oil tank, which is installed at 3m (0.03 Mpa) above a shaft center. The stern tube oil pressure becomes “air pressure in the #2/3 chamber + 0.03 Mpa and it also follows the draft change.
- Any draft change can be automatically detected and both the air pressure in the #2/3 chamber and the stern tube oil pressure follow the draft change instantly. Accordingly all pressure differences on the aft sealing rings are always negligible.
- Leaking oil and seawater can be drained from the #2/3 chamber.
- In order to drain any leaking liquid smoothly, a small quantity of air is always blown through a flow controller on the drain tank.
There are two advantages of the constant air flow system:
- One is that seawater rarely comes into the #2/3 chamber because the air pressure is always greater than seawater pressure.
- The life of sealing rings, especially the #1 and #2 rings, increases because of the small pressure difference maintained.