Pasha Bulker 243 - Independent investigation into the grounding of ...

the angle increases, the holding power reduces. A rule of thumb is that a pull at 5º
above the horizontal reduces the holding power by 25 per cent and a pull at 15º
reduces the power by 50 per cent 23 .
Figure 21:
An illustration of a ship at anchor
Therefore, to maximise an anchor’s holding power, the scope 24 of cable should be
sufficient to ensure that, in fair weather, a length of cable will lie along the sea-bed
and thus pull horizontally at the anchor shackle. When this occurs, the cable rises
gently in a curve to the hawse-pipe. The curve, or catenary, absorbs any shocks
when forces on the ship due to wind, tide and current increase the pull on the cable.
A catenary, therefore, is necessary to ensure that the cable exerts a horizontal pull
on the anchor shackle.
According to the IACS requirements for HHP anchors, a scope of cable of ten is
considered normal while a scope of not less than six is acceptable. Most large ships
are fitted with about 12 shackles, approximately 330 m, of cable for each anchor.
Consequently, in water depths exceeding about 45 m, the scope of cable achievable
is less than six.
With regard to the scope of cable, Danton 25 states:
The Admiralty recommends the following lengths, which should be regarded as
the minimum for calm weather and a 5-knot stream.
For special-steel cable, lay out 39 x √D of cable [metres].
(Where D is the depth of water in metres.)
While this formula allows for the relatively lighter weight of the higher strength of
a special-steel cable and is a useful guide, it indicates the minimum length of cable
to deploy in calm weather. Newcastle anchorage, on the other hand, is particularly
susceptible to adverse weather. Furthermore, water depths in the anchorage exceed
30 m and make it impossible to achieve a scope of ten and difficult to achieve one
of six or more.
Pasha Bulker’s master had initially deployed nine shackles of cable. In the 35 m
deep water, this provided the Admiralty recommended minimum length for specialsteel
cable in calm weather. The scope of cable was slightly less than six but when
the weather deteriorated, the additional cable veered on the master’s orders made
the scope nearly seven.
23 Danton’s, The Theory and Practice of Seamanship, 11 th Edition, 1996.
24 The length of the cable deployed, from the hawse pipe to the anchor, divided by the vertical
distance between the hawse pipe and the sea-bed.
25 Danton’s, The Theory and Practice of Seamanship, 11 th Edition, 1996.
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