10. EFFECT OF WIND FORCE ON MANOEUVRING

10.1 Wind force

The aerodynamic force acting above the ship's waterline represents wind effect on manoeuvring ship. The aerodynamic force and its moment influence ship's trajectory and cause heel angle.

The force and moment due to wind action depend on:

• relative wind direction;

• wind force;

• windage;

• shape of superstructures (streamlined or sharp-edged)

• position of superstructures on the ship.

The superstructures location on the ship determines position of the centre of wind pressure - for different types of ships the position of CP is situated in different points of a ship (fig. 10.1).

Fig. 10.1 Windage and centre of pressure for different types of ships

Magnitude and direction of the wind force depends on the relative wind direction:

Fig. 10.2 Wind velocities and force components

The wind force and its moment for a given ship can be calculated using experimental wind tunnel tests results of this ship, however in general such data are not available.

Thus in practice the aerodynamic force is calculated:

• Using experimental results of wind tunnel tests choosing data of a ship having parameters close as much as possible to the own ship; Some examples of so called aerodynamic coefficients for a tanker both in full load and ballast conditions and a passenger/ car ferry are given below (fig.10.3, 10.4, 10.5). For a given apparent wind angle, the aerodynamic force and moment can be calculated using the following equations:

for longitudinal force:

for transverse force:

for yawing moment:

where V_A - apparent wind velocity;

A_L -lateral projected area of the ship ( above the waterline)

A_T - transversal projected area of the ship

ρ - the air density (1.23 kg/m³)

Fig. 10.3 Wind force coefficients depend on the direction of the apparent wind

Fig. 10.4 Wind force coefficients depend on the direction of the apparent wind

Fig. 10.5 Wind force coefficients depend on the direction of the apparent wind

• Using approximate formulae for side and longitudinal force coefficients, for example:

Cy=1.05 sin(apparent wind angle);

Cx=0.8 cos(apparent wind angle)

10.2 Ship in beam wind

10.2.1 Ship stopped

• Wind force is large.

• There is no longitudinal component.

• Behaviour of the ship depends on the centre of wind pressure, which could be located in front or behind of the point of application of the transverse resistance force (pivot point). This point is approximately at midship.

• Ship is drifting and turning either way, depending on the relative position of these points.

Fig. 10.6

10.2.2 Ship with headway

• Point of application of wind force is behind the pivot point.

• Ship has tendency to swing towards the wind line.

Fig. 10.7

1. Ship with sternway

• Point of application of wind force is in front of the pivot point.

• Ship has tendency to swing out of the wind line.

Fig. 10.8

2. Wind from bow quarter

1. Ship with headway

• Point of application of wind force is behind the pivot point.

• Ship has tendency to swing towards the wind line.

Fig. 10.9

1. Ship with sternway

• Point of application of wind force is behind the pivot point.

• Ship has tendency to swing towards the wind line.

Fig. 10.10

3. Following wind

10.4.1 Ship with headway

Fig. 10.11

1. Ship with sternway

Fig. 10.12

2. Effect of transverse thrust when backing

• Swing in both directions is possible.

• Behaviour of the ship depends on whether the moment of the wind force (Fw . a) is greater or smaller than the moment of the transverse thrust (Tt . b). ]

Fig. 10.13

Wind force- approximate formula:

Assuming that approximately coefficient C_L is varying from 0.8 to1.2 for beam wind we may assume that in average C_L = 1.0 and with air density 1.28 kg/m³, in in kilograms the wind firce (beam wind) is equal to:

kgf

To allow 20% safetu margin, the formula could be written

kgf

10-8

Chapter 10- Effect of wind force on manoeuvring

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