Types of Propulsion Sytems

Following types of propulsion systems are in common use:-

  1. Fixed Pitch propeller with Internal Combustion Engine.

  2. CPP (Controllable Pitch Propeller with Internal Combustion Engine)

  3. Diesel Electric (Fixed Pitch Propeller connected with Huge Electric Motors)

  4. Steam Turbines (Fixed Pitch Propeller connected to steam turbine)

  5. Azipod

1. Right handed Fixed Pitch Propeller

Right handed FPP usually connected to an internal combustion Engine is most common types of propulsion system. Here when looking from stern, Propeller is turning clockwise and ahead movement and antilock wise in astern movement.

Transverse Thrust:

                               Ahead Movement- Turn to Port

                               Astern Movement- Turn to Stbd

 

(Doubt Clearing: V Imp

Lots of time transverse thrust is taught like “Ahead-Bow cant to port, Astern- Bow cant to Stbd”. Here I would like to emphasize that the both transverse thrust and rudder forces are forces acting on stern. So actually it is stern which will move due to transverse thrust or rudder action.

Understanding above is particularly important when using engine kicks close to berth, as while running astern, one would expect bow will move to stbd, but actually stern will move to port much faster and may result in stern landing heavily on port side jetty similarly an ahead kick will make stern move away from port side jetty.)

 

Availabilty of power: Immediate (Diesel Engine normally will give immediate thrust.)

Number of Kicks: Limited (Minimum 12 kicks as per regulations)

Running Astern: Engine may miss astern kick at higher speeds (generally when ahead speed more than 7 knots).

Steerage with Engine stopped: reasonably good steering is vessel has some speed through water even if propeller is stopped.

 

2. CPP (Controllable Pitch propeller)

As name suggest here a variable pitch propeller is connected to diesel engine. Variable pitch means that the pitch/Angle of blades can be adjusted using hydraulic mechanism. This propeller is connected to a continuously running engine, Ahead or astern movement is achieved by adjusting the pitch as required.

Transverse thrust in CPP:

  1. Find out which way shaft/propeller is turning (Left handed or right handed)

  2. In CPP, the shaft will continue to rotate in same direction both for ahead and astern movement. So transverse thrust will also be same for both ahead and astern.

  3. So if CPP left Handed

                               Ahead- Turn to Stbd

                               Astern- Turn to Stbd

   4. Right Handed CPP

                               Ahead- Turn to Port

                               Astern- Turn to Port

      (Usually vessel are fitted with left handed CPP to emulate the movement of Right hand fixed pitch propeller when going astern)

   5. As a matter of fact since propeller is continuously rotating, even when stopped, there may be a slight effect of transverse thrust and eddies even when pitch is set to zero.

 

In case of CPP, Transverse thrust is usually not very strong and most of the time one may not notice at all.

 

Availability of power: Immediate (Diesel Engine normally will give immediate thrust.)

Number of Kicks: No Limit, You can give as many kicks as required since, engine is never stopped during maneuvering, and there is no limitations of engine kicks due to low starting air pressure. Also you can adjust pitch to any setting to achieve desired speed.

Running Astern: No Limitation.  

Steerage with Engine stopped:

This is most important precaution when handling a CPP vessel.

If speed is not reduced gradually, and vessel has still very high speed and CPP is put on stop, this will result in closing the face of CPP blades, hence they will allow only a very small flow of water through propeller, thus it will result in virtually vacuum and strong eddies at rudder, thus rudder will be ineffective in steering the vessel and vessel may loose steerage. Hence it is strongly advisable to reduce speed gradually so that vessel speed over water and propeller speed is almost similar. However whenever a situation arises that vessel loses steerage, short kick ahead with rudder hard over may be used as many times as required. Only caution here is using too many kick ahead may cause you not able to reduce speed in time.

Using Minimum Ahead: Most of the CPP can be set to Minimum Ahead, where it will give speed about 2 to 4 knots and can be steered as well.

Mooring Lines fouling CPP: It has to be kept in mind that propeller is all the time rotating at a rather high RPM (usually about 40 rpm), even when you have ordered stop. So one must be careful while handling mooring lines close to propeller. If necessary, Propeller must be de-clutched while handling lines.

Mooring boats must be warned about CPP, and should not be allowed to come close until propeller is declutched and stopped.

Creep at zero pitch: Some vessels may have slight creep fwd or aft when propeller pitch is set at zero. Pilot must confirm same with vessel’s master before undertaking maneuver.

  

3. Diesel Electric

Few vessels have conventional fixed pitch propeller powered by huge electric motors. These electric motors are fed power mostly by steam turbine based generator or fuel oil operated generators. They are usually found on LNG vessels.

 

Availabilty of power: Immediate (As electric motor can deliver instant power and torque)

Number of Kicks: No Limitation.

Running Astern: No Limitation

 

Steerage with Engine stopped: Same as conventional right handed FPP.

 

4. Steam turbines

Steam turbines are found on LNG vessesl powering conventional right handed fixed pitch. All characterstics are similar to conventional Right handed fixed pitch propeller except as below:-

Availability of Power: Once steam turbine is started from stop, it take little longer to build up RPM and Power as compared to Diesel engine. It is very important to keep in mind that short kick ahead are not so easily effective in steam turbine.

 

Number of Kicks: No limitation however keep in mind slow build up of power.

 

Running Astern: Again allow time for turbine to completely stopped build up astern rpm.

 

Steerage with Engine stopped: Same as conventional right handed FPP.

 

5. Azipods

Azipods are mostly found on tugs, Off Shore vessels and passenger vessels. Azipod unit include a propeller mounted on a protruding structure which can be rotated full 360 Degrees. Usually vessel will be fitted with twin azipods. Will discuss more details in Twin screw vessel handling.

Mumbai, India