5003 Lectures - Faculty of Engineering and Applied Science

E5003 - Ship Structures I 37
© C.G. Daley
Note that the values of a and b are related, so that the average is W . This gives
b a
= 1. 5 − .
W 2W
To move the position of the center of weight (the lcg) the fore and aft ends of the
load diagram are adjusted by equal (and opposite) amounts.
7
54
2
∆ lcg = x ⋅ L or,
lcg
x 2
L
∆
=
54
7
~~~~~~
If the weight distribution is unknown and we have a vessel without a parallel
middle body (i.e. most sail yachts), we need a smoother distribution. The method
below uses a parabolic distribution on top of a uniform distribution. The two parts
each have half the weight.
The equation for the weight is;
W 3 2x
2
W = + W ( 1−
( −1)
)
2 4 L
To shift the total center of weight by ‘x’ we shift the c.g. of the parabola by 2x. This
is done by ‘shearing’ the curve, so that the top center, ‘D’, shifts by 5x. All other
points shift proportionally.