Page 125 - Introduction to Naval Architecture

112 SEAKEEPING

requiring careful alignment of two elements may become impossible
without some mechanical aid. Over and above this the motions, and the
drugs taken to alleviate the symptoms of motion sickness, may adversely
affect a person's mental dexterity.
In broad terms the effects of motion on human behaviour depend
upon the acceleration experienced and its period. The effect is most
marked at frequencies between about 0.15 to 0.2 Hz. The designer can
help by locating important activities in areas of lesser motion, by aligning
the operator position with the ship's principal axes, providing an
external visual frame of reference and providing good air quality free of
odours.

OVERALL SEAKEEPING PERFORMANCE
An overall assessment of seakeeping performance is difficult because of
the many different sea conditions a ship may meet and the different
responses that may limit the ship's ability to carry out its function. A
number of authorities have tried to obtain a single 'figure of merit* but
this is difficult. The approach is to take the ship's typical operating
pattern over a period long enough to cover all significant activities. From
this is deduced:
(1) the probability of meeting various sea conditions, using statistics
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on wave conditions in various areas of the world ;
(2) the ship speed and direction in these seas;
(3) the probability of the ship being in various conditions, deep or
light load;
(4) the ship responses that are likely to be critical for the ship's
operations.
From such considerations the probability of a ship being limited from
any cause can be deduced for each set of sea conditions. These combined
with the probability of each sea condition being encountered can lead to
an overall probability of limitation.The relative merits of different
designs can be 'scored' in a number of ways. Amongst those that have
been suggested are:
(1) the percentage of its time a ship, in a given loading condition, can
perform its intended function, in a given season at a specified
speed;
(2) a generalization of (1) to cover all seasons and/or all speeds;
(3) the time a ship needs to make a given passage in calm water
compared with that expected under typical weather conditions.

It is really a matter for the designer to establish what is important to an
owner and then assess how this might be affected by wind and waves.