SHIPHANDLING (Calculations)

Q. What is pivot point, as referred to in shiphandling?
Pivot point is a point about which vessel pivots or turns during maneuvering. When the ship is on even keel in calm conditions and still water, the pivot point is generally found as follows:
1. When standstill or simply underway; approximately midships.
2. When making way through water; approximately, one third the length from forward.
3. During the stern propulsion; approximately, one fourth the length from stern.

Q. How does the pivot point shift as the ship starts from rest?
In standstill mode pivot point is about midway. With the start of head propulsion, ship makes her headway against water. This is similar to a toy car which finds a wall ahead. The pivot point may be assumed at one eighth of length from forward. Equilibrium is achieved between the forward momentum and longitudinal resistance to forward momentum, shifting the pivot point to a position approximately from forward..

During the stern propulsion, the situation is reversed. The momentum of sternway balances longitudinal resistance. The pivot point moves aft and establishes itself approximately ¼ L from the stern.

Q. Why is the Rudder most effective in turning the vessel, when starting from rest?
There are two reasons to this:
1. Head resistance is maximum.
2. The turning lever is biggest.
Thus, forward movement is initially resisted. Right at start, thus, a pivot point is created which is well forward and is approximately L/8 from the bow. The importance of this is absolutely vital because at this stage, with the ship just beginning to make headway and the pivot point well forward, we have the optimum rudder force. In fact, the turning lever is best at this time.

Q. If you want to pull a vessel bodily away from wharf, in what ratio, you must apply the tug pull when:
1. The vessel has a headway.
2. The vessel has a sternway.
The turn lever plays an important role here. Because of the position of pivot point in respective case the lever of forward and aft tug will become smaller and larger than the lever for aft tug, respectively. Thus, in 1st case the pull required by aft tug will be half of the forward tug. In 2nd case the pull required by the forward tug will be one third of the pull required by aft tug.

Q. How is transverse thrust created when a ship moves in forward direction?
Transverse thrust can be attributed to the following two reasons:
1. Different in fullness of the ship’s hull, forward of upper half of propeller and lower half of propeller.
2. The helical discharge from the propeller.
Fore and aft components add up giving a forward movement to ship. Of the athwart ship components a larger thrust is created on the port side of the rudder, pushing stern to starboard. The importance of transverse thrust when using an astern movement is of much greater significance to a ship handler. The helical discharge, or flow, from a right handed propeller working astern splits and passes forward towards either side of the hull. In doing so it behaves quite differently. On the port quarter it is inclined down and away from the hull whilst on the starboard quarter it is directed up and on to the hull. This flow of water striking the starboard quarter can be a substantial force in tonnes that is capable of swinging the stern to port giving the classic ‘Kick Round’ or ‘Cut’ of the bow to starboard.

Q. How may generally the force due transverse thrust be compared to the thrust available in astern propulsion?
If we assume that a ship has a full ahead power of 20,000 SHP. The full astern may be only 50% of this. Transverse thrust may be only 5 to 10% of the applied stern power and thus, in this case a maximum force of 750 SHP or 7.5 tonnes.

Q. What is the relationship between SHP and the thrust in tones?
The relationship may be assumed as follows:

Q. A ship 180m in length of 20,000 SHP, maneuvering in calm waters, has a RHSC propeller. Assuming that the stern power is 50% of head power and transverse thrust is 7% of stern power, how will you determine the following:
1. Turning moment if astern movement is given with vessel still making some headway.
2. Turning moment if astern movement is given with vessel stopped.
3. Turning moment if astern movement is given with vessel making sternway.

Q. A ship 180m in length of 16,000 SHP, steering a course of 000oT, with beam wind on starboard side (wind force of 30 kn), has a RHSC propeller. Assuming that the stern power is 50% of head power and rudder force on dead slow ahead is 10% of ahead thrust, is used to counteract the wind force. Transverse thrust is 1% of ahead power. Can you show by drawing the moments of forces acting on ship. Ship’s transverse windage area is 2000m2. Assume centre of windage be amidships.
Forward thrust = 160t. Rudder force on D S Ahead = 16t. Though the turn moment will depend on thrust normal to the plane of rudder multiplied by perpendicular distance from pivot point. In this situation however, we will assume rudder thrust to be acting perpendicular to ship’s line and lever equal to 2/3rd the ship’s length.
Transverse thrust = 1.6 t.
Approx wind speed = 15m/s. Thrust per 1000m2 = 152/ 18 = 12.5t. Total thrust = 12.5 x 2 = 25t.

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