Propeller
.propeller material
Material
| Alloy | Nominal Composition | Stress(N/mm2) |
| Nickel Aluminium Bronze | 80Cu/10Al/5Ni/5Fe | ±87 |
| Manganese, Aluminium, Bronze | 74Cu/13Mn/8Al/2Ni | ±62 |
| High Tensile Brass | 0.5Mn/1Al/0.6Sn/Rem. Cu (25% alpha) | ±42 |
Types of propellers:
Classification by Number of Blades Attached
- Three blades
- four blades
- Five blades
- Six blades
Classification by Pitch of The Blade
- Fixed Pitch Propeller
- Controllable Pitch Propeller
Checks after repair work in propeller:
Measures to be taken before propeller is fitted back:
- Ensure M/E is not turned.
- Test report of seal assembly to be checked.
- Report of tail shaft cone polishing and crack test to be checked.
- If propeller is repaired, then repair report.
- Propeller polishing and crack test report.
- Visual inspection of threaded part of shaft and pilgrim nut.
- Radial reference mark on both propeller and shaft to coincide
- Final push up pressure according to the manufacturer’s instruction.
- Loading ring should not come out more then 1/3rd of the ring width.
- Distance travelled by the propeller boss.
- Nut and bolt to be wire lashed.
- When fixing ensure maker instruction to be followed.
Construction diagram of controllable pitch propeller:
A controllable pitch propeller is made up of a boss with separate blades mounted into it. An internal mechanism enables the blades to be moved simultaneously through an arc to change the pitch angle and therefore the pitch. A typical arrangement is shown in Figure below.
When a pitch demand signal is received a spool valve is operated which controls the supply of low-pressure oil to the auxiliary servo motor. The auxiliary servo motor moves the sliding thrust block assembly to position the valve rod which extends into the propeller hub. The valve rod admits high-pressure oil into one side or the other of the main servo motor cylinder. The cylinder movement is transferred by a crank pin and ring to the propeller blades.
The propeller blades all rotate together until the feedback signal balances the demand signal and the low-pressure oil to the auxiliary servo motor is cut off.
To enable emergency control of propeller pitch in the event of loss of power the spool valves can be operated by hand.
Keyless propeller:
- With the hydraulic mounting method using pilgrim nut, keyless propeller is now a standard feature in propeller mounting on fixed pitch propeller on cargo ships.
- Final push up pressure and travel distance according to instruction manual.
- Loading ring should not come out 1/3 of the ring width.
- Uniform stress distribution.
- Boss made of bronze, shaft made of alloy steel.
- Keyless fitting relies on the good interference fit between the propeller boss and the taper end of tail shaft
- This method removes problems associated with keyways and facilitates propeller mounting and removal using hydraulic nut.
- The reliability of keyless propeller depends on the accuracy of the hub and shaft tapers and correct grip from the stretched propeller hub on the shaft.
- The degree of stretch (or strain) is controlled by the amount of push up. It must ensure adequate grip despite temperature changes and consequent differential expansion of bronze hub and steel shaft. It must also avoid over stressing of the hub and in particular any permanent deformation.
- Lloyds require that the degree of interference be such that the frictional force at the interface can transmit 2.7 times the nominal torque when the ambient temperature is 35°C.
- Lloyds also require that at 0°C the stress at the propeller bore shall not exceed 60% of the normal stress.
Withdrawal:
