Bow thruster

Bow or Stern thrusters are very helpful for ship handling.

 

A typical bow thruster is installed into a Transverse tunnel in forward underwater hull section of ship. Bow thruster consist of an electric motor whose powering an impeller whose pitch can be varied through a hydraulic system. So main function of Bow thruster is to suck water from one side (say port side) and throw onto other side (say stbdside), which will cause bow to move toward port side and vice versa.

So before we proceed we will again recap on below conversion of power.

 

  1. SHP=BHP(approx.)

  2. 100 SHP=1 T BP

  3. 1 HP = 746 W = 0.746 KW

  4. So A bow Thruster of 1000 HP = 746 KW = 10 tons Bollard pull.

               Bow Thruster of 5000 HP = 3730 KW = 50 tons Bollard pull.

 

As we can see above to get power equivalent to a 50 tons tug, we need Bow thruster of 5000 HP. But in Practice a 5000 HP Bow thruster will be much more effective as compared to a 50 ton BP tug for the following factors:

  • Bow thruster’s entire power is available for maneuver, whereas in case of tug, it has to use a significant amount of power to keep itself moving along with the movement of vessel.

  • The efficiency and skills of tug handler

  • BT power is almost immediately available on either port or stbd side, but in case of tug, you will have to allow tug to change position from push to pull mode or vice versa.

  • The bow thruster and stern thrusters are located almost at extreme forward or extreme aft along the length of vessel hence they can produce a very good turning lever, whereas if you are making fast tugs on side shoulder/break of accommodation, the turning level of tug is less hence it will produce lesser turning moment.

Few Important precautions and mistakes to avoid when using Bow thruster:

  1. Generally vessels are fitted with very weak bow thrusters (Ex: A small chemical tanker of 150 mtr may have BT of 700HP, A large container vessel of length 3000 mtr may have BT of abt 2000 HP). As you can see this force is not sufficient in comparison with tug, hence if vessel needs to turn around in harbor these thrusters will not be adequate to execute the maneuver. On can only manage berthing/unberthing without securing tug at bow with such bow thrusters, but if vessel need to turn around, tug assistance must be sought at bow.                                                        (I have experienced generally such underpowered bow thrusters of chemical tankers have hardly any effect at drafts  more than 9 mtr.)

  2. When vessel is deeply laden, the Bow thrusters may be totally ineffective.

  3. Bow thrusters are generally ineffective at speed over water more than 3 to 5 knots. Thus if you are handling laden vessel in strong tide, you cannot rely entirely on bow thruster.

  4. When Bow thruster is used for long time, it produces a slight forward creeping movement of vessel.

  5. Be cautious while working with mooring lines near bow thruster to prevent fouling of ropes with Bow thruster.

  6. If mooring boats are used for passing lines, Bow thruster should be stopped when boat is working close by.

Mumbai, India