Page 127 - Ship Construction.DJ Eyres 6Ed

Ch12-H8070.fm Page 116 Wednesday, October 18, 2006 6:55 AM
116 Ship Construction
with half ordinates at the ends where a greater change of shape occurs. A
half transverse section only is drawn since the vessel is symmetrical about the
centre line, and forward half sections are drawn to the right of the centre
line with aft half sections to the left. Preliminary body plans are drawn ini-
tially to give the correct displacement, trim, capacity, etc., and must be laid
off in plan and elevation to ensure fairness of the hull form. When the final
faired body plan is available the full lines plan is completed showing also
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the profile or sheer plan of the vessel and the plan of the water-line shapes
at different heights above the base.
A lines plan is illustrated in Figure 12.1. The lines of the lateral sections
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in the sheer plan as indicated are referred to as ‘bow lines’ forward and
‘buttock lines’ aft. Bilge diagonals would be drawn with ‘offsets’ taken along
the bilge diagonal to check fairness.
When the lines plan was completed manually the draughtsmen would
compile a ‘table of offsets’, that is a list of half breadths, heights of decks
and stringer, etc., at each of the drawn stations. These ‘offsets’ and the lines
plan were then passed to loftsmen for full size or 10 to 1 scale fairing, Since
the original lines plan was of necessity to a small scale which varied with the
size of ship, the offsets tabulated from widely spaced stations and the fairing
were not satisfactory for building purposes. The offsets used for building
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the ship would subsequently be lifted by the loftsman from the full size or
10 to 1 scale lines for each frame.

3- DIMENSIONAL REPRESENTATION OF SHELL PLATING When
preparing the layout and arrangement of the shell plating at the drawing
stage it was often difficult to judge the line of seams and plate shapes with a
conventional 2-dimensional drawing. Shipyards used to therefore make use
of a ‘half block model’ which was in effect a scale model of half the ship’s hull
from the centre line outboard, mounted on a base board. The model was
either made up of solid wooden sections with faired wood battens to form
the exterior, or of laminated planes of wood faired as a whole. Finished with
a white lacquer the model was used to draw on the frame lines, plate seams,
and butts, lines of decks, stringers, girders, bulkheads, flats, stem and stern
rabbets, openings in shell, bossings etc.

SHELL EXPANSION The arrangement of the shell plating taken from
a 3-dimensional model may be represented on a 2-dimensional drawing
referred to as a shell expansion plan. All vertical dimensions in this draw-
ing are taken around the girth of the vessel rather than their being a direct
vertical projection. This technique illustrates both the side and bottom
plating as a continuous whole. In Figure 12.2 a typical shell expansion for
a tanker is illustrated. This also shows the numbering of plates, and letter-
ing of plate strakes for reference purposes and illustrates the system