EFFECT OF SHALLOW WATER

·         Speed for same constant engine rpm is reduced

·         Diameter of turning circle increased

·         Rate of turn is decreased

·         Effect of astern manoeuvre is reduced

·         Effect of rudder in reducing ship speed is reduced

·         During a turn the bridge (i.e. aft) will tend to follow the same path as the bow instead of following a wider turn (i.e. less sideways skid)

·         Increase in bow wave and stern wave

Magnitude of such changes is varied, but effects increase as UKC decreases

FACTORS AFFECTING TURNING CIRCLE

Rudder Angle – Increase Causes

·         Diameter of turning circle reduced

·         Rate of turn increased

·         Speed reduced

·         Angle of heel increased

Approach Speed – Increase Causes

·         Rate of turn increased

·         Squat increases (varies as speed2)

·         Angle of heel greater (\ increase in draught)

RPM – Increase During Turn Causes

·         Rate of turn increased

·         Diameter of turning circle reduced

Direction of Turn

A turn to port will result in a slightly smaller turning circle than a turn to starboard

Depth of Water

A change in the depth of water during the turn can have a marked effect on ship’s turning circle

Condition of loading, trim, wind speed and tidal stream can all have a marked effect on turning circle

MANOEUVRING DATA

Wheel Over;              The point at which the change of course is initiated.

Advance;              The distance which the ship has moved in direction of initial heading.

Transfer;              The distance which the ship has moved perpendicular to initial heading.

Distance to new course;              The distance from the intersection of initial and final heading to the wheel over point.

Change of heading;              The angle between initial and actual instantaneous heading.

Rate of turn;              The rate of change of heading expressed in degrees/minutes.

Drift angle;              The angle between the ships fore and aft line and the tangent to the turning circle (constant on circular part of turn).

·         Data should be presented in simplest form.

·         Ideally data should be presented in graphical and tabular format.

·         Diagram should state ship condition at time the information was compiled. (see below).

·         When comparing different sets of graphical data, the scale should always be the same.

·         Data should contain a warning that data will be different for other conditions .

·         Manoeuvring data is usually compiled where depth of water is at least twice the ship’s draught.

·         Manoeuvring diagrams usually indicate path of ship’s bridge – make allowance for bow and stern.

Information required:

1. Loaded Condition,

2.      Direction of Turn,

3. Initial speed,
4. Rudder angle,
5. Change of heading,
6. Instantaneous speed,
7. Rate of turn,
8. Advance,
9. Transfer,
10. Distance to new course.