Page 303 - Practical Ship Design

Design qf’Lines 26 1

sections provide. This will improve intact stability in almost all cases but in the
often more critical case of damage stability will only do so to the extent of the gain
in breadth at bulkhead deck level. Flared sections will add weight and cost and will
raise the VCG. Clearances to cranes, dock walls and tugs must also be considered.
Very infrequently used nowadays although once quite common is the reverse of
flare namely “tumblehome”. The purpose of tumblehome was cost saving and a
lowering of the VCG thereby improving static stability. In practice it was found
quite helpful in keeping the ship topside, even if the ship was slightly heeled, clear
of dockside cranes.

8.8.2 Deck lines

The lines plan shows all decks that extend to the ship’s side: the upper deck,
forecastle, bridge (in merchant parlance a side to side erection amidships), poop -
together with all lower decks.
These decks are, of course, shown in plan view, elevation and sections.
In the past all decks exposed to weather generally had both sheer and camber.
Lower decks might follow the same pattern or could have camber and no sheer, or
neither.
Sheer was traditionally arranged parabolic on the Upper deck at ship’s side.
Standard sheers forward and aft are specified in the loadline rules, the aft sheer
being half of the forward sheer. The standard sheers were intended to keep the
decks reasonably clear of water, but in practice designers often thought it wise to
exceed these values. In metric units standard sheers are:
forward = 0.0166 L + 0.508
aft = 0.00833 L + 0.254
Camber was traditionally also parabolic and again a standard was set by the
loadline rules as breadtW50.
Modern practice is to eliminate sheer over most of the ship and if any sheer is
required to give the height of bow needed, either to meet the rule requirement or
thought necessary as a result of seakeeping tests or calculations, to have a straight
line sheer forward of an appropriately positioned knuckle.
Straight decks with no sheer have several advantages: their steelwork is cheaper
to construct, as is any joinerwork fitted to them; the stowage of containers or
modules is simplified; in association with level keel trim they keep the deck in the
same relationship with port facilities such as passenger gangways, cranes, coal
chutes etc. throughout the ship’s length. Unfortunately they lack the aesthetic
appeal of the old parabolic sheer.
Camber is now generally also arranged on a straight line basis with a level area
extending from the centreline to knuckle points P&S outboard of which the deck is
sloped to provide drainage.