Page 76 - Collision Avoidance Rules Guide

power is likely to be less than maximum ahead power. For diesel
engines the proportion will usually be over 80% but for geared steam
turbines it may only be about 40%.
There have been several instances of vessels having their engines
immobilised due to attempting to reverse too rapidly after going at
full ahead. In a paper presented before the Institute of Marine
Engineers in 1957 Mr J. E. Church described the damage which could
result to different types of machinery through a crash stop manmuvre
and suggested that a quicker and safer way to stop a vessel would be
to stop the engines instantly then, after a delay of three minutes or so,
to give slow astern, half astern and full astern, thus avoiding acute
cavitation. More recent evidence from ship trials and model tests
seems to indicate that many vessels, especially those fitted with diesel
engines, could best be stopped by giving ‘full astern’ as soon as
possible, but it can generally be said that even if the engines can be
made to go astern within one minute of the order ‘stop’ the retarding
effect would be small and the risk of damage to the machinery would
be great. The above remarks apply to a vessel moving at high speed.
The engines can be more readily reversed when the speed is low.
The distance that a vessel will cover in a crash stop before being
brought to rest from full speed is likely to be between 5 and 15 ship
lengths according to speed, displacement, type of engine, etc. The time
taken will vary considerably. A cargo vessel of 3,000 tons displace-
ment proceeding at 16 knots has been stopped in less than
3 minutes, but it may take more than 25 minutes to stop a loaded tanker
of over 200,000 tons displacement moving at the same initial speed.

.- Oft 1000 2000 3000 4000 ft L
-.
-.
.-.-.
..-.-, _._._._._._._._.e.-
- - --
-:..-
Om 15k 2m 11 k 4m 8k ’..--.

The diagram shows the path traced out by a vessel of about 23,000
tons displacement when making a crash stop afer going at her full
speed of approximately 1.5 knots.
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