Page 75 - Collision Avoidance Rules Guide

visually or by radar. When proceeding at full speed most vessels are
capable of turning through at least 60" in the first 2 minutes if full
helm is applied. Helm action will also cause the speed to be reduced.
Although helm action is usually preferable to engine action as a means
of avoiding collision in the open sea, the officer of the watch should not
hesitate to use the engines if the necessity arises (see page 206). The
engines can normally be used to greater effect for collision avoidance
when proceeding at reduced speed with the engines ready for immediate
manceuvre, in restricted visibility or within port limits.

Taking all way off
If the engines are stopped on a vessel proceeding at high speed, or if
ahead power is substantially reduced, there will be a fairly sharp drop
in speed at first followed by a more gradual decrease, as hull resist-
ance may be considered to be proportional to the square of the speed.
If the engines are put astern shortly afterwards the initial high
decrease of speed will be maintained and headway will fall off even
more rapidly when the astern power becomes fully effective. The
graph shows the fall off of speed against time as observed on three
vessels when the engines were stopped while proceeding at full
speed. The displacements were: vessel A 22,000 tons, vessel B
56,000 tons and vessel C 240,000 tons. The dotted lines indicate the
effect of putting the engines astern at the earliest possible moment.
There will usually be a delay of at least one to two minutes before
the engines can be reversed after moving at full head, depending
upon the type of machinery and other factors. The maximum astern

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