Page 345 - Practical Ship Design

302 Chapter 10

10.6.2 Avoiding or minimising vibration
One of the best ways of avoiding or minimising vibration is to eliminate cantilever
construction if this can reasonably be done.
If for any reason cantilever structure is desirable it should be stiffened to take its
natural frequency well clear of that of any possible exciting force. Structures most
likely to need consideration in this way include bridge wings, masts and derrick
posts.
Another most desirable way of minimising vibration and indeed of avoiding
stress concentrations is by ensuring that the ends of superstructure deckhouses
land on steel casings or bulkheads.
It is also desirable to have a number of transverse casings in the superstructure
lining up with bulkheads in the main hull, whilst deckhouse sides should if
possible line up with longitudinal bulkheads, girders or longitudinals supporting
the upper deck.
Openings should be avoided near the ends of deckhouse sides.
Modem “tower” like superstructures tend to have low natural frequencies and
these should be evaluated including their interaction with the main hull girder,
whilst the recommendations for the provision of good support to superstructures
are particularly important.

10.63 Reducing corrosion and facilitating maintenance and repairs
Reducing corrosion is best achieved by eliminating any confined pockets within
which water can lie.
The measures which should be taken to facilitate maintenance and repairs are in
general also measures which will ease construction, although the ability to turn
units upside down during construction may mean that something which is quite
easy to build is by no means easy to repair. The sheer size of large tankers and bulk
carriers makes inspection by surveyors a matter of great difficulty, and it looks as
though provisions to ease this problem are going to be essential in the new
generation of double hull tankers.

10.6.4 Detailed structural design

The importance of good detailed design throughout but particularly in the primary
structure can hardly be over-emphasised. In terms of potential ship loss, the cost of
repairs and incidence of fatigue cracks detail design has been shown by accident
statistics to be more important than hull girder strength per se.
Although cost considerations have in the past made shipbuilders strive to
eliminate redundancy, recent casualties suggest that a measure of redundancy