VESSEL HANDLING EXERCISE TWO: NAME:
QUESTIONS
1 Define transverse thrust. Explain how transverse thrust would affect a left handed propeller vessel, if berthing port side too.
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2 What is shallow effect?
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How does it affect a vessel operating in shallow water?
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3 List the precautions prior to and during the taking of a tow.
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4 List the information that you would require to cross a river bar safely.
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5 List the advantages of a twin screw vessel.
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Vessel Handling ANSWERS
1. The propeller rotates and draws us along using axial thrust and produces a side effect called transverse thrust, also known as paddle wheel effect or prop walk. Transverse thrust is caused by the increase in water pressure and density with depth making the propeller blades more efficient at the bottom of their rotation. The water flow to the blades at the top of the rotation may also be adversely affected by hull form and obstructions.
Transverse thrust has the effect of trying to turn the vessel. A left handed propeller will ‘walk’ the stern of a vessel to port and the bow will swing to starboard when forward power is applied. A helmsperson automatically corrects for this by applying a small helm adjustment. In the same vessel when power astern is engaged the stern will ‘walk’ to starboard and the bow swings to port. Adjustments to the helm will not counteract this.
A left handed propeller is one that that turns in an anticlockwise direction when viewed from the stern. When going astern the propeller will turn clockwise.
2. The motion of a vessel causes an increase in water pressure at the bow and stern and a reduction in pressure amidships. Interaction occurs when the normal flow of water around the hull is restricted by the influence of shallow water, a breakwater, dredged channel or by the close passage of another vessel.
This change of trim is commonly known as ‘Squat’ (Shallow Water Effect, or Smelling the bottom) and has the effect of increasing draft when this is least desirable. Vessels with fine hull lines will squat by the stern and a barge shaped vessel will tend to squat by the head.
In shallow water, vessels may experience significant changes in manoeuvring characteristics including:
· loss of rudder effectiveness
· therefore increase the vessel’ turning circle
· reduction of output power, ie speed
· increase the vessel’s stopping distance
· increase in draft (reduced freeboard)
· change in trim
3. The following factors must be amongst those considered before taking another vessel in tow:
Does the exercise pose a danger to your crew or vessel.
Duration of the voyage.
Whether you have sufficient fuel allowing for the additional drag.
The effect of the delay to your own vessel.
The power of your engines.
Notify the owners and insurers of your vessel.
Enter into a Lloyd’s Open form of Salvage agreement.
4. - state of tide
- weather forecast
- latest condition report
- safest route
- best crossing time
- prevailing wind
- wave pattern timing
- course to follow
- bar traffic
- alternate route
5. Your answer should include information regarding the manoeuvrability, steering without a rudder and negating transverse thrust.
Ranger Hope © 2005 (contains reworked material courtesy of ANTA)