BOWTHRUSTER

1. BOWTHRUSTER

2. Use of Bowthruster / Steering aid Thrusters can be used : • to assist steering at very slow speed • to keep a ship’s position in the channel without headway. (this capability is put to good use when the ship is unable to proceed to the berth immediately because of an unexpected obstruction in the channel or a delay in tug availability, etc). • When navigating stern first, a bow thruster is more useful than a tug, provided the ship is not allowed to gather too much sternway.

3. Position of bowthruster: as far as possible forward

4. Use of bowthruster / Steering aid • Altough slow to move a ship, thrusters provide a helpfull cushion when landing alongside • They are useful when a ship has to berth in a very tight corner with little manoeuvring space for a tug • Thrusters are useful when anchoring • Thrusters may be accepted in lieu of a tug, reducing the costs in port.

5. 2 thrusters to increase the power:

6. Reliability of bowthrusters • Older units are proned to problems: • Outdated electrics and hydraulics • Situated in fore part of ship and subjected to heavy weather,vibration and damp. • Negligence of maintenance • lack of use on long sea passages • New units: • More reliable and robust • Sometimes underpowered and inadequate

8. Power of thrusters • More common: • reversible electric motor driving a fixed pitch propeller • Reversible electric motor + hydraulically operated variable pitch propeller • Limitation of Design parameters: • Restricted location • Size of tunnel • Ballast draft • Hull strenght • Limitation of power → Twin bowthrusters • Many thrusters are equivalent to 10-15 tonnes bollard pull

9. Counteracting forces near the tunnel These forces will vary with the vessel moving ahead, astern or stopped. The effect of the thruster is the greatest when the ship is stopped

10. Counteracting forces near the tunnel In a ship moving ahead, pressure fields will develop near the entrance and the exit of the tunnel The suction, acting over an area of plating, creates a force which is opposing the bow thruster. This can reduce the effectiveness by 50% at a speed of 2 knots .

12. Speed limitation • Most thrusters loose effectiveness as the ship increases speed and are little or no help at speeds of more than 3 or 4 knots • They may still exert some force until a speed of between 5 and 10 knots has been reached , but the rudder would then be far more effective for turning the ship

13. Wind speed / Thruster • Thruster A: 1000kw • Thruster B: 2000kw • Container ship 210m: • Thruster A enough for • a beam wind of max. • 3 to 4 Beaufort (11kn) • b) Thruster B enough for • a beam wind of max. • 4 to 5 Beaufort (16kn) • 2. Car carrier 200m: • a) Thruster A enough for • beam wind 4 Beaufort • (13kn) • b) B enough for beam • wind up to 5 beaufort • (18kn)

15. Thruster effectiveness

16. « Rudder action » versus « thruster action » • DEAD SLOW AHEAD • Rudder to starboard • Pivot Point moves forward • Center of gravity moves to port

17. « Rudder action » versus « thruster action » DEAD SLOW AHEAD 2. Thruster action Pivot point moves astern Center of gravity moves to Sb The ship « turns on her heels »

18. « Rudder action » versus « thruster action » • FULL SPEED AHEAD • Rudder action • Pivot point moves forward • Center of gravity moves to Ps

19. « Rudder action » versus « thruster action » FULL SPEED AHEAD 2. Thruster action Pivot point forward Center of gravity moves to Ps due to difference in pressure fields at the bow. No effectiveness of thruster with speed ahead

21. Lateral motion to Port using rudder and bowthruster • With short kicks ahead • and rudder hard to Sb. • Moderate amount • of thruster to port to compensate swing to Sb • B. With astern power • Transverse thrust of propeller • Moderate thrust of bowthruster to port to compensate possible swing

22. Lateral motion to Sb using rudder and bowthruster • Short kicks ahead + Rudder • hard to port + thruster to Sb • No problems in lateral motion • B. With astern power • Bow swings to Sb→ compensate • with Bowthruster to Port • Thruster effect + transverse • thrust cause unintentional • lateral motion to port. • This movement can be of • considerable magnitude

23. Specific aspects of bow thruster work: • thrusting when stopped • thrusting with headway • creating lateral motion • working the thruster with sternway

24. Thrusting when stopped Pivot point position Thruster is working on pivot point located one ship’s beam from stern (due to underwater profile of the ship) → large turning lever. Turning moment with ship stopped is better than with: - headway - sternway

25. Thrusting when stopped Ship develops small amount of headway due to a flow of water around the bow creating a low pressure ahead of the ship

26. Thrusting with headway / Straight line • performance falls off rapidly wit speed> 2knots • pivot point position reduces the turning lever • → poor turning moment

27. Thrusting with headway / Perfomance loss

28. Thrusting with headway / Turning 1. When the rudder is applied with a kick ahead and no speed, pivot point moves briefly forward to P, close to the thruster position → no turning lever / bow is only pushed sideways 2. When vessels gets some speed: pivot point moves to P1 but adverse effect of speed

29. Thrusting with headway / Turning 3. Thruster may be very useful to develop lateral motion in conjunction with propeller and rudder. for example: rudder to port and thruster to starboard → bow and stern tends to move to Sb and ships moves sideways

30. Lateral motion to port with kicks ahead Frequently used when berthing: driving a ship « by the seat of the pants » On large vessels moving sideways, the kinetic energy can carry on for quite some time Sometimes difficult to balance the forces of rudder and thruster: Full thruster of 13t is only equal to kick ahead with dead slow

31. Lateral motion to port with astern power • When putting propeller astern ,anticipated swing of bow to Sb can • be stopped by applying port thruster. • Very effective with large tonnage ships / some caution required to • avoid landing too fast and too heavily

32. Lateral motion to starboard with kicks ahead Same way as for a lateral motion to port. Problem arise when the engine is reversed (see next slide)

33. Lateral motion to starboard with astern power • More problematic to starboard • (with single screw) • When astern power is applied • (to control headway): • bow swings to Sb • tendency to give thruster to • Ps will bring ships back to initial • position. swing

35. Thrusting with sternway • pivot point moves aft : big turning lever for the thruster / possible to • steer the vessel with the bowthruster • beware forpoor thruster power and sluggish response of vessel • avoid the development of a big rate of turn: difficult to control

36. Thrusting with sternway / Turning moments of propeller and bowthruster • If the engine is going astern (200m ship/ transverse forces respectively 7t - propeller and 13t- thruster) : • transverse thrust of propeller pushes the bow to Sb • turning lever of propeller = 30m → thrust = 7t x 30m = 210 tm • bowthruster turning lever = 125m → 13t x 125m = 1625mt (contd)

37. Thrusting with sternway / Turning moments of propeller and bowthruster Therefore it is possible to work the vessel astern by overriding The effect of transverse thrust with the bow thruster. → It may often be preferred to back up a waterway or to a berth

38. Bowthruster at work

39. Working astern in a channel Lack of experience ends up with the ship in a difficult position (pos6)

40. Working astern in a channel The process ofbacking a ship up a waterway with the aid of a bowthruster can be dangerous: The inexperienced can unintentionally allow the ship to become a victim of unexpected influence of transverse thrust and insidious drift →For this manœuvre, large ships use often the assistance of a tug secured aft to assist steering and positioning of the stern / the bowthruster controls the bow.

41. Position 1 • Ship has entered the channel • Good position with bow canted to • port to allow for prolonged use of • stern power and transverse thrust

42. Position 2 • Insufficient correction of transverse thrust → stern has dropped away from centre of channel • If propeller and thruster are now used simultaneously: lateral motion to port • this is not intentional and may be unnoticed

43. Position 3 • ship steady on correct heading but off • centered • at this stage: use bow thrustervigorously / or: kick ahead with rudder to port to bring stern back to the centre of the channel

44. Position 4 Sequence of events in position 2 is repeated → ship keeps drifting to port On very large vessels, the kinetic energy keep the vessel drifting for a long time. A light wind on the Sb side will increase the drift.

45. Position 5 The ship is getting too close to the edge and is running out of manoeuvring space There is insufficient water to port to enable the bow to be thrusted to port

46. Position 6 The ship ultimately is hitting an obstruction on the port side.

47. To avoid this kind of manœuvre: • Get the ship moving smartly astern initially and keep it moving • Use kick aheads with caution: it takes the way off the ship which can drift out of position with leeway • No attempt in transverse winds overriding the thruster’s power • When sufficient sternway: stop the engine to avoid transverse thrust • Watch the heading carefully at all times and use thruster vigorously to keep the stern on the intended track / the rest will follow.

49. Working astern to a berth The position of the pivot point: ships goes slowly astern and turns to Sb.

50. Working astern to a berth With engine ahead it is easy to correct a too high approach speed or an unaccurate positioning of the stern