some ships in which the tank is higher than wide, the
As indicated by this formula, the free surface effect
opposite may be true. The normal practice of
varies as the cube of the breadth (b) but only as the first
maintaining the fuel oil tanks 95 percent full takes
power of the length (l). Because of this relationship, a
advantage of the fact that pocketing occurs, at very small
single bulkhead that cuts a compartment in half in a
angles of heel, when a compartment is almost full.
fore-and-aft direction will quarter the free surface
effect.
Length and Breadth of Free Surface
The athwartship breadth of a compartment has a
Chart for Calculating Free Surface Effect
great influence on the reduction in GM caused by the
free surface effect. This influence is shown by the
To avoid having to make calculations from the
following formula:
formula given in the previous section, a free surface
effect chart based on this formula is used to find the
b 31
Rise in G
reduction in GM that occurs as a result of free surface.
12(35W)
Such a chart is shown in figure 12-30.
Where b = athwartship breadth of compartment
To use this chart, draw a straight line from the
1 = fore-and-aft length of compartment
a p p r o p r i a t e p o i n t o n t h e AT H WA RT S H I P
DIMENSION scale (A) to the appropriate point on the
W = displacement of ship
C
D
B
A
CHART FOR
500
100
100
FREE SURFACE EFFECT
BASED ON EQUATION
600
90
90
b3 l
700
RISE IN G =
12 (35w)
800
80
80
b = BREADTH OF COMPARTMENT
900
I = LENGTH OF COMPARTMENT
1,000
70
70
w = DISPLACEMENT OF SHIP
E
60
60
1,500
10.0
2,000
50
50
5.0
4.0
3.0
3,000
2.0
40
40
1.0
4,000
.7
.5
5,000
.4
30
30
.3
6,000
2
.2
7,000
8,000
25
25
.1
9,000
10,000
.05
1
.04
20
.03
20
.02
15,000
.01
20,000
10
10
30,000
40,000
50,000
0
10
DCf1232
Figure 12-30. Chart for calculating free surface effect.
12-17
