Page 274 - Introduction to Naval Architecture

MANOEUVRING 259

Figure 103 Zig-zag manoeuvre

the initial response of the ship to the rudder being put over that can be
vital in trying to avoid a collision. This initial response is studied in the
zig-zag manoeuvre. In it the ship proceeds on a straight course at a steady
speed, a rudder angle of 20° is applied and held until the ship's head
has changed by 20° and then the rudder is reversed to 20° the other way
and held until the ship's head has changed 20° in the opposite
direction. The manoeuvre is repeated for different speeds, rudder
angles and heading changes.
The important measurements from the manoeuvre, Figure 10.3,
are:
(1) the overshoot angle. This is the amount the heading increases by
after the rudder is reversed. Large angles would represent a ship
in which the helmsman would have difficulty in deciding when
to take rudder off to check a turn. Values of 5.5 and 8.5° would
be reasonable aims for ships at 8 and 16 knots respectively,
varying roughly with speed. The angle does not depend upon
ship length.
(2) the times to the first rudder reversal and the first maximum
1
heading change. It has been suggested that for reasonable
designs, times to change heading by 20° would be of the order of
80 to 30 seconds for a 150 metre ship over the range 6 to 20
knots. The time would be roughly proportional to length.
(3) the steady overshoot angle and the period of the cycle once a
steady condition is reached.