Page 295 - Practical Ship Design

Design of Lines 253

8.7 SEAKEEPING AND MANOEUVRABILITY

Although this chapter is devoted to the design of the lines, the subjects dealt with in
this section affect other aspects of design and indeed of ship operation and it seems
sensible to deal with these at one time.

8.7. I Seakeeping
With some exceptions, seakeeping is regrettably low in the design priorities for
most types of ship. The principal exceptions to this are warships, research vessels
and offshore oil production and storage vessels all of which have to spend long
periods at sea and have to provide a platform on which their crews can undertake
demanding tasks whatever the weather conditions. Seakeeping is also recognised
as being important on passenger ships and particularly on cruise liners, although
because these are generally very large ships they can achieve a fairly good sea-
keeping performance by virtue of size without requiring much special attention
being paid to this feature of their design.
Small ships whose seakeeping ability should undoubtedly have more attention
paid to it include fishing vessels, whose broaching problems are mentioned in 38.3.
Deciding what is involved in good, or at any rate acceptable, seakeeping is a
difficult task, varying as it does with ship size and speed and the sea areas in which
the ship is required to operate. The following features certainly enter into it:
(i) shipping water on deck;
(ii) pitching motions;
(iii) rolling motions;
(iv) slamming; and
(v) broaching.

8.7.2 Shipping water on deck
The amount of water shipped on deck is determined primarily by the size and speed
of the ship together with the freeboard at the bow (see Chapter 1 1, 8 1 1.2). Within
these constraints the practical ways of minimising it are well flared forward sections
coupled with well placed knuckles.

8.7.3 Pitching motions

These are mainly determined by ship size and the longitudinal moment of inertia. If
a high proportion of the weight of the ship and its cargo are concentrated near
amidships the pitching period will be relatively small and the accelerations large; if
on the other hand a high proportion of the weight is “winged out” towards the ends