Page 70 - Practical Ship Design

Setting Design Requirements 41

heels to an angle at which it starts to lose stability are, in certain circumstances,
permitted to operate at reduced freeboards. The requirements should therefore
state what type of freeboard is to be used.
Most hopper suction dredgers have diesel-electric machinery, which is usually
sized to meet the high power demand of the dredge pumps. This provides the
possibility of a relatively high sea speed, although most dredgers do have quite full
lines and a high appendage resistance from the overside dredging gear even when
this is in the stowed position. It is probably wise therefore to specify the minimum
speed required for operational reasons whilst making it clear that advantage is to be
taken of the installed power to maximise the actual speed.
The type of dredging gear to be fitted should usually be specified in the
statement of requirements, although this may be left to the discretion of specialised
dredger designers provided they are given a clear description of the tasks that the
ship has to accomplish.
Where the spoil to be moved from the sea bottom is handleable by a suction pipe
and there is room for the dredger to move under power, the trailing suction dredger
is almost invariably the most suitable type.
Where the spoil is suitable for suction dredging but in some part of the area to be
dredged there is no room for the ship to move, the addition of a bow suction pipe
extending forward can provide the answer.
Where the bottom is so compacted or stony that suction pipes are inadequate it is
necessary to use a bucket dredger.
As well as their use in deepening or maintaining depth in channels, hopper
suction dredges are used to collect cargoes of sand and/or gravel from offshore
deposits and bring these back to land for civil engineering developments. These
vessels are usually fitted with special self-unloading features unless they go to a
berth which has shore-based special unloading equipment. The unloading method
must be specified.

2.5.3 Icebreakers
The leading requirement for an ice breaker is the thickness of ice through which it
is required to clear a passage, the ice in this case being almost invariably multi-year
ice (see $2.3).
The next requirement is the breadth of the passage needed by the ships that will
follow it through the ice.
The required bollard pull must be stated and should be adequate to provide
effective assistance to the vessels for which the icebreaker is making a passage,
should any of these become trapped in the ice.
Fuel and stores must be provided for a lengthy endurance, and the accommod-
ation must be to a high standard and for a large crew.