Page 123 - Introduction to Naval Architecture

110 SEAKEEPING

Such values as the significant motion amplitude in the given sea can be
used to compare the performance of different designs in that sea.
There remains the need to consider more than one sea, depending
upon the areas of the world in which the design is to operate, and to
take into account their probability of occurrence.

LIMITING FACTORS IN SEAKEEPING

A number of factors, apart from its general strength and stability, may
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limit a ship's ability to carry out its intended function . Ideally these
would be definable and quantifiable but generally this is not possible
except in fairly subjective terms. The limits may be imposed by the ship
itself, its equipment or the people on board. The seakeeping criteria
most frequently used as potentially limiting a ship's abilities are speed in
waves, slamming, wetness and human reactions.

Speed in waves
As the waves become more severe die power needed to propel the ship
at a given speed increases. This is because of increased water and air
resistance and the fact that die propulsors are working under adverse
conditions. At some point the main machinery will not be able to
provide the power needed and a speed reduction will be forced upon
the master. The master may choose, additionally, to reduce speed to
protect the ship against the harmful effects of slamming or wetness.

Slamming
Slamming is a high frequency transient vibration in response to the
impact of waves on the hull, occurring at irregular intervals. The most
vulnerable area is the ship's outer bottom between about 10 and 25 per
cent of the length from the bow. The impact may cause physical
damage and can accelerate fatigue failure in this area. For this reason
this area of the outer bottom should be given special attention during
survey. Slamming is relatively local and often in a big ship, those on a
bridge well aft may not be aware of its severity. Because the duration of
the slam is only of the order of $5 of a second, it does not perceptibly
modify the bodily motion of the ship but the ensuing vibration can last
for 30 seconds. A prudent master will reduce speed when slamming
badly. This speed reduction leads to less severe slamming or avoids it
altogether. Often a change of direction helps. Lightly loaded cargo
ships are particularly liable to slam with their relatively full form and
shallow draught forward, and enforced speed reductions may be as
high as 40 per cent. Slamming is less likely in high speed ships because
of their finer form.