Appendix 3 - Stopping Ability of Very Large Ships

  1 It is stated in the Standards for ship manoeuvrability that the track reach in the full astern stopping test may be modified from 15 ship lengths, at the discretion of the Administration, where ship size and form make the criterion impracticable. The following example and information given in tables A3-1, 2 and 3 indicate that the discretion of the Administration is only likely to be required in the case of large tankers.

  2 The behaviour of a ship during a stopping manoeuvre is extremely complicated. However, a fairly simple mathematical model can be used to demonstrate the important aspects which affect the stopping ability of a ship. For any ship the longest stopping distance can be assumed to result when the ship travels in a straight line along the original course, after the astern order is given. In reality the ship will either veer off to port or starboard and travel along a curved track, resulting in a shorter track reach, due to increased hull drag.

  3 To calculate the stopping distance on a straight path, the following assumptions should be made:

• .1 the resistance of the hull is proportional to the square of the ship speed.

• .2 the astern thrust is constant throughout the stopping manoeuvre and equal to the astern thrust generated by the propeller when the ship eventually stops dead in the water; and

• .3 the propeller is reversed as rapidly as possible after the astern order is given.

  4 An expression for the stopping distance along a straight track, in ship lengths, can be written in the form:

where:

• S : is the stopping distance, in ship lengths.
• A : is a coefficient dependent upon the mass of the ship divided by its resistance coefficient.
• R : is a coefficient dependent on the ratio of the ship resistance immediately before the stopping manoeuvre, to the astern thrust when the ship is dead in the water.
• C : is a coefficient dependent upon the product of the time taken to achieve the astern thrust and the initial speed of the ship.

  5 The value of the coefficient A is entirely due to the type of ship and the shape of its hull. Typical values of A are shown in table A3-1.

  6 The value of the coefficient B is controlled by the amount of astern power which is available from the Dower plant. With diesel machinery, the astern power available is usually about 85% of the ahead power, whereas with steam turbine machinery this figure could be as low as 40%.

Table A3-1

  7 Accordingly the value of the coefficient B is smaller if a large amount of astern power and hence astern thrust, is available. Typical values of the coefficient B are given in table A3-2.

Table A3-2

  8 The value of the coefficient C is half the distance travelled, in ship lengths, by the ship, whilst the engine is reversed and full astern thrust is developed. The value of C will be larger for smaller ships and typical values are given in table A3-3.

Table A3-3

  9 If the time taken to achieve the astern thrust is longer then 60 seconds, as assumed in table A3-3, or if the ship speed is greater than 15 knots, then the values of the coefficient C will increase pro rata.

  10 Although all the values given for the coefficients A, B and C may only be considered as typical values for illustrative purposes, they indicate that large ships may have difficulty satisfying the adopted stopping ability criterion of 15 ship lengths.

  11 Considering a steam turbine propelled VLCC of 300 metres length, travelling at 15 knots, and assuming that it takes 1 minute to develop full-astern thrust in a stopping manoeuvre, the results using tables A3-1, 2 and 3 are:

• A = 16,
• B = 1.5, and
• C = 0.8

  12 Using the formula for the stopping distance S, given above, then:

which exceeds the stopping ability criterion of 15 ship lengths.

  13 In all cases the value of A is inherent in the shape of the hull and so cannot be changed unless resistance is significantly increased. The value of B can only be reduced by incorporating more astern power in the engine, an option which is unrealistic for a steam turbine powered ship. The value of C would become larger if more than one minute was taken to reverse the engines, from the astern order to the time when the full-astern thrust is developed.