Page 65 - Practical Ship Design

36 Chapter 2

2.2 DIMENSIONAL CONSTRAINTS

Dimensional constraints may impose a limit on length, breadth, draft and air draft,
or two or more of these.
A constraint on length may be set by the dimensions of canal locks or docks. It
may also be set by a need to be able to turn the ship in a narrow waterway. In either
case the necessity of the limit set should be thoroughly questioned if it appears
likely to limit the ship’s length to less than that which would be desirable if there
was no such constraint.
If the limit is set by a dock or canal, question whether the use of these is essential
or so desirable that this limit must be accepted, or whether rerouting could avoid
the canal, or the choice of another port avoid the constraint set by the dock. A limit
set by turning ability can be considerably eased by fitting a high-performance
manoeuvring device such as a bow thruster. The constrained length will usually be
the overall length but in some cases the constraint may apply at the waterline or at
some definite height above the waterline at which the ship will be floating.
A limit on breadth is usually set by canal or dock lock gates, but the breadth of
vehicle ferries is sometimes limited by the dimensions and position of shore ramps
giving vehicles access to bow or stern doors. The outreach of other shore-based
cargo-handling devices such as grain elevators or coal hoists can limit the desirable
distance of the offshore hatch side from the dockside and thereby limit the breadth
of the ship. In general, breadth limits apply to the maximum breadth measured over
fenders (if fitted) and usually must be maintained to at least quay height above the
waterline at low tide. Above this it may be possible for the breadth to be increased
if flaring the ship’s sides is desirable (see Chapter 8) or if overhanging decks,
lifeboats, dredger suction pipes or any similar item are a feature of the design. Any
overhanging features demand a most careful survey of the places where the ship
may berth to ensure that there is no possibility of contact with objects on the quay.
A draft limit is usually set by the depth of water at low spring tides in the ports
(or their approaches) to which the ship is intended to trade.
For very large tankers the depth of the ocean itself must be considered. The
impact of a draft limit can be minimised if the ship’s routing, fuelling and storing
are so arranged that fuel and stores are at a minimum and the ship is on level keel
when it is passing through the shallow water that sets the limit.
The last of the dimensional constraints is that of air draft. This is the vertical
distance from the waterline to the highest point of the ship’s structure and denotes
the ship’s ability to pass under a bridge spanning the seaway which forms part of
the projected route. Where necessary, air drafts can be greatly reduced by equipping
the ship with folding or telescopic masts and funnels. Other measures which can be
taken to reduce the effect of an air draft limit are to arrange that transit under the
bridge takes place at low tide andor to load or ballast the ship to the deepest