Page 297 - Practical Ship Design

Design of Line.5 255

There are a few ship types, of which icebreakers are one, which should not have
bilge keels, in this case because they are too easily ripped off by ice. On some
vessels used for scientific research, such as fisheries and oceanographic, scientists
tend to argue that bilge keels should be omitted to lessen “noise”, but this should
only be accepted if an effective tank stabilisation system is being fitted.
The shape of the midship section also influences rolling with a small bilge
radius giving a squarer shape which has more inherent resistance to rolling.

8.7.5 Slamming

As slamming occurs when the forebody re-enters the water after having emerged
during pitching, all the measures already suggested as ways of reducing pitching
help to minimise this also.
In addition, a deep still water draft is an important factor in minimising
slamming because it reduces the risk of emergence in a seaway . The advantage to
be gained from a deep draft is particularly relevant to the ballast condition.
If it is impossible to stop the forebody emerging from the water the next best
thing is to ensure that it re-enters with minimum force, which can be done by
shaping the bottom with a pronounced V form in the slamming region so that it acts
like a knife.
Frigates and corvettes, having a small draft and the need to maintain speed in
very rough seas, would tend to slam very badly if their lines were not very carefully
designed with this in mind. They are given a very high rise of floor which both
increases the draft and creates a V bottom, extending the full length of the ship,
which can re-enter the water without much fuss but, although greatly reduced,
some slamming still occurs.
Slamming/pounding or something very like it can still take place even if the
forefoot doesn’t actually emerge. It is essential that there should not be a large flat
area of bottom in the region of maximum relative motion/ acceleration which goes
quite a long way aft.
The bottom of bulbous bows should be angled as shown in Fig. 8.14.

8.7.6 Broaching

Broaching occurs when a ship is travelling down wind andor down the path of the
waves in a seaway. Yachts, which broach much more frequently than ships, do so
when a particularly strong gust of wind generally from a slightly further forward
than the prevailing wind hits their sails. This makes the yacht turn uncontrollably
into the wind bringing the relative direction of this from aft round to the beam.
With the change in the wind direction there is a broadside force on the sails which
causes the yacht to heel very severely often “onto her beam ends”.