Ship's propulsion is done by electrical power or prime movers run by diesel engines.Normally ships have single or double propeller system .Number of propellers depends on thrust requirement and propeller submergence. Tanker,bulk carriers and container ships where thrust requirement are moderate but propeller efficiency requirements are high,and draft in operating range is less variable,Single propeller is much efficient. For ships with higher speed requirement,like passenger ships have dual large propellers.Smaller vessels like AHTS,PSV,boats etc where sufficient propeller submergence is restricted by lower operating drafts,double propeller is more suitable. For high speed small vessels which operate at even lower drafts,number of propellers must be increased to achieve required speed.Commercial vessels have fixed pitch monoblock system.Shafting consist of propeller shaft,tail shaft and intermediate shafts. Propellers are normally build of bronze and shafting of forged steel.
SHIP CONVENTIONAL PROPULSION SYSTEM
Conventional propulsion system can be categorized in following components.
RPM and number of blades influence vibration onboard and the resonance frequency of the ships.Most small ships use single screw 4 bladed propellers while 5 bladed propellers are common with bigger ships.Normally number of propeller blades are same as number of motor's cylinder to avoid reverberations.
- 1. Engine
- 2. Engine Shaft and flexible Coupling
- 3. Reduction Gear box
- 4. Shaft Generator
- 5. Stern Tube with bearing
- 6. Propeller shaft
- 7. Propeller
RPM and number of blades influence vibration onboard and the resonance frequency of the ships.Most small ships use single screw 4 bladed propellers while 5 bladed propellers are common with bigger ships.Normally number of propeller blades are same as number of motor's cylinder to avoid reverberations.
PROPELLER CAVITATION
Speed of the incoming water creates under pressure at forward area of propeller
( suction side ) and pressure on the aft side of blade ( pressure side ).If propellers move to fast that will generate negative pressure at suction side so bubbles will form. Once these bubbles will implode,it will create erosion on propellers.This is called cavitation. Cavitation will create,blade roughness,low efficiency,increased vibration and noise in the ship.
Speed of the incoming water creates under pressure at forward area of propeller
( suction side ) and pressure on the aft side of blade ( pressure side ).If propellers move to fast that will generate negative pressure at suction side so bubbles will form. Once these bubbles will implode,it will create erosion on propellers.This is called cavitation. Cavitation will create,blade roughness,low efficiency,increased vibration and noise in the ship.
FIXED PITCH PROPELLER
Propellers blades will have fixed position So to stop or reverse the direction of propellers,reversing clutch or reversing gears are used for small ships. For bigger vessels engine rotation direction is reversed. In this kind of propeller required thrust is achieved by variable RPM.
Propellers blades will have fixed position So to stop or reverse the direction of propellers,reversing clutch or reversing gears are used for small ships. For bigger vessels engine rotation direction is reversed. In this kind of propeller required thrust is achieved by variable RPM.
VARIABLE PITCH PROPELLER
Propeller pitch is made variable by a mechanism fixed in bossing.This pitch can be remotely operated from bridge. These propellers rotates in single direction so reversible engine and clutch are not required.If hydraulic system in bossing fails,propellers can be locked in ahead condition.it can propel ships at very low speed compared to fixed pitch where only range of RPM are allowed.With this kind of propellers,shaft generators can supply power with integration of fixed RPM engines.Pitch can be adjusted for required thrust while shaft generator will absorb the rest of energy.
Propeller pitch is made variable by a mechanism fixed in bossing.This pitch can be remotely operated from bridge. These propellers rotates in single direction so reversible engine and clutch are not required.If hydraulic system in bossing fails,propellers can be locked in ahead condition.it can propel ships at very low speed compared to fixed pitch where only range of RPM are allowed.With this kind of propellers,shaft generators can supply power with integration of fixed RPM engines.Pitch can be adjusted for required thrust while shaft generator will absorb the rest of energy.
- Propeller Blade
- Boss or Hub
- Watertight/Oil tight seal
- Stern Frame
- Propeller shaft
- Stern Tube
AZIMUTH THRUSTERS
These thrusters can rotate full 360 degree with help of underwater gearbox which is driven by vertical power shaft.
These thrusters can rotate full 360 degree with help of underwater gearbox which is driven by vertical power shaft.
- 1. Shaft from Engine
- 2. Vertical drive shaft
- 3. Propeller shaft with gear
- 4. Kort Nozzle
- 5. Rotation point
- 6. CP propeller
- 7.Hydraulic lines of CPP
- 8.Oil fixed gearbox
DIESEL ELECTRIC DRIVEN POD THRUSTER
Electric motors with variable RPM is placed in a pod attached to the bottom of ship.For push and pull type of thrusters, propellers are attached to the back and front respectively.Diesel engines can be placed anywhere on the ship which will generate electricity through generators. These thrusters require high voltage input,so main switchboards and transformers are used. Frequency converters help to change RPM of motors.
Electric motors with variable RPM is placed in a pod attached to the bottom of ship.For push and pull type of thrusters, propellers are attached to the back and front respectively.Diesel engines can be placed anywhere on the ship which will generate electricity through generators. These thrusters require high voltage input,so main switchboards and transformers are used. Frequency converters help to change RPM of motors.
PROPELLER SHAFTING
Stern tube contains bearing in which propeller shaft rotates.Aft bearing is the water tight seal which protect inner part and oil from sea water.Front side of bearing is welded to aft peak bulkhead and aft part is to propeller post.Normally water or oil is used as lubricating agent between shaft and shafting.
Water inside stern tube always has higher pressure compared to outside sea water.
Stern tube contains bearing in which propeller shaft rotates.Aft bearing is the water tight seal which protect inner part and oil from sea water.Front side of bearing is welded to aft peak bulkhead and aft part is to propeller post.Normally water or oil is used as lubricating agent between shaft and shafting.
Water inside stern tube always has higher pressure compared to outside sea water.
