Page 267 - Practical Ship Design

Pofivring !I 225

the original one. If 60 months’ protection is required, painting to a thickness of 360
pm will provide this. Not only is little or no fouling allowance required as a ship
can now be expected to operate with only minor deterioration in performance
between dry dockings but the intervals between these are being progressively
lengthened, with three or even five years becoming common.
When repainting becomes necessary a wash with high-pressure fresh water is all
that is required and gives a smooth surface for repainting. This contrasts with the
high cost in both money and time of the scraping required by conventional paints and
with the build-up of roughness from old leached out paint even when this is done.

7.7.4 Weather allowance

With fouling now very nearly eliminated, attention is being increasingly directed
to items which can influence the effect which the weather can have on a ship:
(i) the ship size, small ships being much more affected than large ones;
(ii) wind resistance forces on the hull and superstructure;
(iii) added displacement and windage due to ice accretion;
(iv) additional resistance due to the application of rudder angle to maintain a
course in spite of wind and wave forces;
(v) additional distance travelled due to course instability;
(vi) increase in ship resistance due to ship motions;
(vii) decrease in propeller efficiency due to ship motions.
The design corollaries of these weather effects are that steps should be taken to:
minimise the windage area of superstructures.
arrange the superstructure, if possible, in a manner that minimises the
amount of weather or lee helm (this is almost certainly a counsel of
perfection as in most ships the extent and disposition of the superstructure
will be dictated by other considerations);
pay attention in the design of the lines to the provision of a reasonable area
of deadwood aft to help course stability;
provide good, but not excessive, flare forward together with good free-
board to reduce pitching motions and keep the decks forward reasonably
free of water;
provide sufficient water ballast capacity to ensure by a combination of
sinkage and trim that there is good propeller immersion in the ballast
condition;
provide modem navigational aids,such as the auto-pilot which was claimed
to provide substantial fuel savings by reducing helm movements to a
minimum, thereby reducing rudder resistance, by allowing the ship to yaw
and then correct itself as it will often do without the application of rudder.