Page 114 - Introduction to Naval Architecture

SEAKEEPING 101

any enforced reduction. For many ships their schedule is of great
importance. The concept of ship routeing can be used to avoid the
worst sea conditions and so suffer less in delay, danger and dis-
comfort and saving on fuel. Savings of the order of 10 to 15 hours
have been made in this way on the Atlantic crossing. Computerized
weather routeing systems are now fitted to a number of ships
allowing the master greater control rather than having to rely upon
instructions from shore.

Wetness The bow can dig into the waves throwing water over the
forecastle. At lesser motions spray is driven over the forward part of
the ship. The main factors affecting these phenomena are the
relative motion of the bow and wave surface and the freeboard
forward.

Slamming Sometimes the pressures exerted by the water on the
ship's hull become very large and what is known as slamming occurs.
Slamming is characterized by a sudden change in vertical accelera-
tion followed by a vibration of the ship's girder in its natural
frequencies. The region of the outer bottom between 10 and 25 per
cent of the length from the bow is the most vulnerable area.

SHIP MOTIONS
Fundamental to an understanding of the response of a ship to the
seaway are the natural periods of oscillation in the three degrees of
freedom, chosen to be dealt with in this chapter. These are considered
first.

Undamped motion in still water

Consider a ship floating freely in still water which is suddenly disturbed.
The motion following removal of the disturbing force is that to be
considered.

Rolling
If<p is the inclination to the vertical at any instant, and the ship is stable,
there will be a moment acting on it tending to return it to the upright.
The value of this moment will be: