Page 272 - Introduction to Naval Architecture

MANOEUVRING 257

(3) the transfer which is the lateral displacement of the ship's centre
of gravity from the original path. Usually transfer is quoted for
90° change of heading.
(4) the tactical diameter which is the value of the transfer for 180°
change of heading although this is not the maximum transfer. It
is usual to quote a tactical diameter to length ratio, TD/L. Modern
frigates at high speed and full rudder turn with a TD/L of about
3. For smaller turning circles such as may be required of a mine
countermeasures vessel lateral thrust units or azimuthmg
propellers would be used. A value of 4.5 would be regarded as
good for most merchant ships but a value greater than 7 as very
poor.
(5) the diameter of the steady turning circle. The steady state is typically
reached at some point between 90° and 180° change of
heading.
(6) the steady speed on turn. Due to the fore and aft component of the
hydrodynamic forces the ship slows down during the turn.
Unless engine power is increased it may be only 60 per cent of
the approach speed. The steady speed is reached as the diameter
steadies. If a ship does need to reverse direction, as might be the
case of a frigate hunting a submarine, the time to turn through
180° is likely to be more important than a really small diameter
of turn. Because of the loss of speed on turn such ships would
choose a lesser rudder angle to get round quickly and to avoid
the need to accelerate so much after the turn.
(7) the turning rate. The quickest turn might not be the tightest. A
frigate would turn at about 3° per second. Half this rate would be
good for merchant ships and values of 0,5-1 would be more
typical.
(8) the pivoting point. This is the foot of the perpendicular from the
centre of the turning circle to the middle line of the ship,
extended if necessary. This is the point at which the drift angle
will be zero and it is typically about | of the length from the
bow.
(9) the angle of heel during the turn. A ship typically heels in to the
turn as the rudder is initially applied. On the steady turn it
heels outwards, the heeling moment being due to the couple
produced by the athwartships components of the net rudder
and hull hydrodynamic forces and the acceleration force
acting at the centre of gravity which is caused by the turning
of the ship. It is countered by the ship's stability righting
moment.
If the steady radius of turn is R, Figure 10.2, and the steady
heel is tp and the transverse components of the forces on the hull