Page 28 - Ship Construction.DJ Eyres 6Ed

Ch03-H8070.fm Page 17 Wednesday, October 18, 2006 6:52 AM
Development of Ship Types 17
became an integral part of the ship structure but retained a lighter structure
than the upper deck structure of other two-deck ships, later referred to as
‘full scantling’ vessels. The ‘shelter deck type’ as this form of vessel became
known, apart from having a lighter upper structure was to have the freeboard
measured from the second deck, and the tween deck space was exempt from
tonnage measurement. This exemption was obtained by the provision of
openings in the shelter deck and tween deck bulkheads complying with
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certain statutory regulations.
At a later date what were known as open/closed shelter deck ships as
open/closed shelter deck ships were developed. These were full scantling
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
ships having the prescribed openings so that the tween deck was exempt
from tonnage measurement when the vessel was operating at a load draft
where the freeboard was measured from the second deck. It was possible to
close permanently these temporary openings and re-assign the freeboard, it
then being measured from the upper deck so that the vessel might load to a
deeper draft, and the tween deck was no longer exempt from tonnage mea-
surement.
Open shelter deck vessels were popular with shipowners for a long period.
However, during that time much consideration was given to their safety and
the undesirable form of temporary openings in the main hull structure.
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Eliminating these openings without substantially altering the tonnage values
was the object of much discussion and deliberation. Finally Tonnage
Regulations introduced in 1966 provided for the assignment of a tonnage
mark, at a stipulated distance below the second deck. A vessel having a
‘modified tonnage’ had tonnage measured to the second deck only, i.e. the
tween deck was exempt, but the tonnage mark was not to be submerged.
Where a vessel was assigned ‘alternative tonnages’ (the equivalent of previous
open/closed shelter deck ship), tonnage was taken as that to the second
deck when the tonnage mark was not submerged. When the tonnage mark
was submerged, tonnage was taken as that to the upper deck, the freeboard
being a minimum measured from the upper deck. The tonnage mark con-
cept effectively dispensed with the undesirable tonnage openings. Further
changes to tonnage requirements in 1969 led to the universal system of
tonnage measurement without the need for tonnage marks although older
ships did retain their original tonnages up until 1994 (see Chapter 30).
Originally the machinery position was amidships with paddle wheel propul-
sion. Also with coal being burnt as the propulsive fuel, bunkers were then
favourably placed amidships for trim purposes. With the use of oil fuel this
problem was more or less overcome, and with screw propulsion there are
definite advantages in having the machinery aft. Taking the machinery right
aft can produce an excessive trim by the stern in the light condition and the
vessel is then provided with deep tanks forward. This may lead to a large
bending moment in the ballast condition, and a compromise is often
reached by placing the machinery three-quarters aft. That is, there are say