Using the hydrostatic curves

After the end drafts have been taken it is necessary to interpolate to find the 'mean draft'. This is the draft immediately below the LCF which may be aft, forward or even at amidships. This draft can be labelled dH.

If dH is taken as being simply the average of the two end drafts then in large full-form vessels (supertankers) and fine-form vessels (container ships) an appreciable error in the displacement can occur. (See Fig. 16.8.)

Displacement in metric tonnes 15000 20000 25000 30000 35000 40000 45000 50000 55000

Displacement in metric tonnes 15000 20000 25000 30000 35000 40000 45000 50000 55000

Fig. 16.7. M.V. 'Tanker'

Let us assume the true mean draft 'dH' is 6 m. The Navel Architect or mate on board ship draws a horizontal line parallel to the SLWL at 6 m on the vertical axis right across all of the hydrostatic curves.

At each intersection with a curve and this 6 m line, he projects downwards and reads off on the appropriate scale on the 'x' axis.

For our hydrostatic curves, at a mean draft of 6 m, for example, we would obtain the following:

TPC = 19.701 DisplaceMT = 10 293 t

These values can then be used to calculate the new end drafts and transverse stability, if weights are added to the ship, discharged from the ship or simply moved longitudinally or transversely within the ship.

LCF$ and LCB$ are distances measured from amidships ($).

Draft Fig. 16.8

Nowadays these values can be put on a spreadsheet in a computer package. When the hydrostatic draft dH is keyed, the hydrostatic values appertaining to this draft are then displayed, ready for use.

A set of hydrostatic values has been calculated for a 135 m General Cargo Ship of about 10 000 tonnes deadweight. These are shown overpage. From those values a set of hydrostatic curves were drawn. These are shown in Fig. 16.9.

Hydrostatic values for Fig. 16.9 (overpage). These are for a 135 m LBP General Cargo Ship

Draft dH (see below) m

tonnes

KB m

Displace MT tonnes

KML m

MCTC tm/cm

KMt m

LCFtt m

LCBa m

9m

20.64

4.80

16 276

146.5

167.4

7.71

2.10 aft

0.45 aft

8m

20.36

4.27

14 253

161.8

162.9

7.54

1.20 aft

7m

20.06

3.74

12 258

181.5

158.1

7.44

0.50 aft

0.45 forward

6m

19.70

3.21

10 293

207.4

152.5

7.46

0.05 forward

0.80 forward

5m

19.27

2.68

8361

243.2

145.9

7.70

0.42 forward

1.05 forward

4m

18.76

2.15

6486

296.0

138.3

8.28

0.70 forward

1.25 forward

3m

18.12

1.61

4674

382.3

129.1

9.56

0.83 forward

1.30 forward

2§m

17.69

1.35

3785

449.0

123.0

10.75

0.85 forward

1.30 forward

Ship Stability Hydrostatic Curves
Fig. 16.9. Hydrostatic curves: based on values tabulated on previous page. These are for a 135 m LBP General Cargo Ship
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Responses

  • elisa li fonti
    Why hydrostatic curves are requirr?
    3 months ago
  • LYLA
    What are hydrostatic curves used for in ship calculations?
    5 days ago

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