Page 111 - Practical Ship Design

78 Chapter 3

3.5 APPENDAGE DISPLACEMENT (1 + S)

3.5.1 A first approximation
In order to obtain the full displacement at the desired draft, it is necessary to add a
correction to the moulded displacement to allow for shell and appendages. The
obverse of this is that when the full displacement which will provide the required
deadweight is known, these corrections must be deducted to arrive at the moulded
displacement and block coefficient used to determine the ships dimensions.
Whilst appendages are a comparatively small factor in the displacement
calculation, it can be important where the deadweight is small and margins are
tight to have a good approximation for these items, at least in the later stages when
the design is being refined. If this can be done easily, there seems every reason to
use the same approximations in the preliminary design stage.
For a single screw ship with an all-welded shell, the simplest approximation is
0.5% of the moulded displacement.
If draft is limited, designers should remember to allow for the keel thickness
when setting the draft moulded.
If a more exact estimate of appendage displacement is required, the various
appendages should be considered individually.

3.5.2 Individual items
(i) Shell displacement = - AL) ’’* (3.15)
t
.
(
380
where t = mean shell thickness in mm.
A
(ii) Stern displacement = [(T/H)” - 13 .- (3.16)
1000

where
X = from 2.5 for “fine” sterns to 3.5 for “full” stems, and
H = height of counter.

(iii) Twin screw bossing displacement = Kb(43 (3.17)
where
d = propeller diameter in metres
Kb = 0.2 for stub bossings, open shafts and “A” brackets, and
from 0.7 for fine bossings
to 1.4 for full bossings.