SHIPHANDLING (Pivot Point, Wind and Transverse Thrust)

Q. Is the transverse thrust blessing or curse?
Transverse thrust is aptly used by mariners to their advantage. A few instances being as follows:

  1. While anchoring the port anchor, transverse thrust while using astern propulsion opens the stem out and the cable is nicely laid.
  2. Alternately giving stern kick with rudder amidships with dead slow ahead with hard starboard can turn the vessel to starboard in a limited area. 3. Since the transverse thrust is maximum when starting from rest, the plot while berthing uses this feature to turn the bows or stern without causing a fore and aft movement. The person at telegraph must be extremely cautious.

4. On the ships with twin screw propellers the ship can be made to turn swiftly using the transverse thrust from both the propellers in addition to their individual off centre effect.
5. Transverse thrust is often used to turn the vessel and make her parallel to berth as the berth is approached.

Q. Why in case of a twin screw propeller set up right handed propeller is placed on starboard side?

By placing the left handed propeller on port and right handed propeller on starboard, there are four different causes which together help vessel turn swiftly to port as well as starboard. The transverse thrust as well as off centre effect from both the propellers combine. This will not happen if the propellers are placed the other way.

Q. In the following diagram, find the position of centre of windage assuming that the wind is from beam.
In the following case, let us assume that the wind is from the beam and the moment of the windage areas is taken about the aft. Taking the moments of areas about aft can give distance of centre of windage about aft. Centre of windage from aft = \dfrac{\left( 850\times 85\right) +\left( 300\times 12\right) +\left( 60\times 160\right) }{1210}=\dfrac{85450}{1210}=70.6m

Q. How does wind affect differently on a ship during sternway?
This is due to the size of lever changing as shown in the diagram. The vessel under consideration has aft accommodation. The wind is on the beam. The large area of superstructure and funnel directly face the wind. The centre of windage is at effective centroid of the areas offering an obstruction to wind.
This now needs to be compared with the underwater profile of the ship and the position of the pivot point (P). With the ship initially stopped in the water this was seen to be close to amidships. The centre of windage (71 m from aft) and the pivot point (approx. 85 m from aft) are thus quite close together and therefore do not create much turning influence upon the ship.
Although, it will vary slightly from ship to ship.
With ship making a headway and the wind on the beam, the centre of windage remains where it is but the pivot point moves forward (113 m from aft). This creates a substantial turning lever between PP & COW (42 m) and, depending on wind strength, the ship will develop a swing of the bow into the wind.
With the wind on a ship making sternway and assuming that the COW remains in the same position, with the wind still on the beam, the shift of PP to a position 42 m from aft creates a turning lever (29 m) that will encourage the bow to fall off the wind.

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