Computation of weights and centres of mass

All prediction methods should be calibrated using data from comparable ships. This allows the selection of appropriate procedures for a certain ship type (and shipyard) and improves accuracy.

The prediction of weights and centres of mass is an essential part of ship design. A first, reasonably accurate estimate is necessary for quoting prices. A global price calculation is only acceptable for follow-up ships in a series, otherwise the costs are itemized according to a list of weight groups. In many cases, it is still customary to calculate not only the material costs, but also the labour costs based on the weight of the material.

The largest single item of the ship's weight is the steel weight. Here, first the installed steel weight (net weight) is estimated. Then 10-20% are added to account for scrap produced, for example, in cutting parts. Modern shipyard with accurate production technologies and sophisticated nesting procedures may use lower margins.

The displacement A of the ship is decomposed as

The symbols denote:

Al weight of ship without payload (light ship) WStR weight of steel hull

WStAD weight of steel superstructure and deckhouses Wo weight of equipment and outfit WM weight of engine (propulsion plant) WR weight margin

Wdw total deadweight including payload, ballast water, provisions, fuel, lubricants, water, persons and personal affects

The exact definitions of the individual weight contributions will be discussed in subsequent sections. All weights will be given in [t], all lengths in [m], areas in [m2], volumes in [m3].

For cargo ships, the displacement may be globally estimated using the ratio C = Wdw/A and the specified deadweight Wdw. C depends on ship type, Froude number and ship size. This procedure is less appropriate for ships where the size is determined by deck area, cargo hold volume or engine power,

150 Ship Design for Efficiency and Economy e.g. ferries, passenger ships, tugs and icebreakers. For cargo ships C ^ 0.66

For tankers C ^ 0.78 + 0.05 • max(1.5, Wdw/100 000)

The height of the centre of mass can be similarly estimated in relation to the depth D or a modified depth DA:

VA is the superstructure volume and VDH the volume of the deckhouses. DA is depth corrected to include the superstructure, i.e. the normal depth D increased by an amount equal to the superstructure volume divided by the deck area. Values in the literature give the following margins for CKG:

passenger ships large cargo ships small cargo ships bulk carrier tankers

Table 5.1a Percentage of various weight groups relative to light ship weight

dw/A [%] Wst/Ai [%] W0/Al [%] Wm/Al [%]

cargo ship

5000-15 000 tdw

60-

80

55-

64

19

-33

11

-22

coastal cargo ship

499-999 GT

70-

75

57-

62

30

-33

9-

12

bulker

20000-50000tdw

74-

80

68-

79

10

17

12

-16

bulker

50000-150000tdw

80-

87

78-

85

6-

13

8-

14

tanker

25 000-120 000 tdw

65-

83

73-

83

5-

12

11

-16

>200 000 tdw

83-

88

75-

83

9-

13

9-

16

containership

10000-15000tdw

60-

76

58-

71

15

-20

9-

22

20000-50000tdw

60-

70

62-

72

14

-20

15

-18

ro-ro ship

<16 000 tdw

50-

60

65-

78

12

-19

10

-20

reefer

300 000-600 000 cu ft

45-

55

51-

62

21

-28

15

-26

ferry

16-

33

56-

66

23

-28

11

-18

trawler

44-82 m

30-

58

42-

46

36

-40

15

-20

tug

500-3000 kW

20-

40

42-

56

17

-21

38

-43

Table 5.1b Height of centres of mass above keel [% height of top-side deck above keel]

Table 5.1b Height of centres of mass above keel [% height of top-side deck above keel]

for WSt for Wo for WM light ship

cargo ship

>5000 tdw

60-

68

110-

120

45-

60

70-

80

coastal cargo ship

>499 GT

65-

75

120-

140

60-

70

75-

87

bulker

>20 000 tdw

50-

55

94-

105

50-

60

55-

68

tanker

>25 000 tdw

60-

65

80-

120

45-

55

60-

65

containership

>10 000 tdw

55-

63

86-

105

29-

53

60-

70

ro-ro ship

>80 m

57-

62

80-

107

33-

38

60-

65

reefer

>300 000 cu ft

58-

65

85-

92

45-

55

62-

74

ferry

65-

75

80-

100

45-

50

68-

72

trawler

>44 m

60-

65

80-

100

45-

55

65-

75

tug

>500 kW

70-

80

100-

140

60-

70

70-

tugs 0.65-0.75

Table 5.1 compiles the percentage of various weight groups and the centres of mass.

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