Page 296 - Practical Ship Design

254 Chapter 8

of the ship the pitching period will be longer and the accelerations less. Other
design constraints generally prevent any significant action to follow this counsel
but it should be kept in mind.
The heavily flared sections forward advocated to minimise water on deck can
increase pitching (and forward damage) so a compromise between pitching and
water on deck may be necessary.
A bulbous bow may help to reduce pitching but is more likely than a normal
bow to cause slamming.

8.7.4 Rolling motions
Rolling motions are largely a function of the metacentric height. A ship with a high
GM (say 2.0 m or more) will have a short period of roll with uncomfortably high
accelerations; a ship with a low GM (say 0.2 m or less) will be much more
comfortable with low accelerations but the amplitude of roll may be large.
Stabilisers, either of fin or tank type provide the best way of reducing rolling.
The fin type gives the best reduction in roll amplitude when the ship is operating at
speed but if the ship may have to operate at a slow speed, tank stabilisers provide
the best answer.
Whether the ship has stabilisers or not, bilge keels should be fitted if at all
possible and should be carefully sized to their task. On full bodied ships the bilge
keels should extend over most of the length of parallel middle body; on ships with
no parallel middle body, or where the extent of parallel middle body is limited, the
bilge keels must necessarily extend into the entrance and run but should be limited
in length and great care should be taken to ensure that the keels runs along
streamlines. If streamline tests are not available Isherwood’s R.I.N.A. paper
provides a good guide to the line to follow.
On fine lined ships where the length of bilge keels are limited this should be
compensated by the use of deeper bilge keels to maintain the area. The depth of the
keel should be limited so that it does not extend beyond the square of the hull and
the keel should be arranged normal to the shell.
An approximate formula for the length of bilge keel that it is usually practical to
fit is:
length of keel = 0.6 x C,, x L (8.9)

A formula for the depth of bilge keel which offsets the reduced length where the
C,, is small which the author used for ships of up to about 180 m is:
depth of keel = 0.18/(Cb - 0.2) (8.10)

For today’s bigger ships this formula should probably be modified to include some
small scaling with ship’s length.