COMMENT:
(a) Precautions required by good seamanship or special circumstances
Some examples of precautions which may be required by the ordinary practice of
seamen, or by special circumstances, are the following:
1. A vessel under way would be expected to keep clear of a vessel at anchor as
a matter of seamanship. But a vessel under way and stopped must not rely on other
vessels keeping out of her way, unless she is not under command and is displaying the
appropriate signals; she must comply with the Rules.
2. When a vessel anchors she must do so without endangering other vessels which
may be navigating close by. She must not anchor too close to other anchored vessels.
Sufficient cable must be put out according to circumstances and a second anchor
should be used if necessary.
3. In dense fog a vessel without operational radar may not be justified in being under
way at all but should anchor if it is safe and practicable for her to do so.
4. When two vessels are approaching one another at a difficult bend in a tidal river it
has been held to be the duty of the one having the tide against her to wait until the
other has passed (see page 67–68).
5. The effects of shallow water must be taken into account. A vessel moving at fairly
high speed through the water produces pressure fields which become much greater
when the flow of water around the ship is restricted. There is a reduction of pressure
beneath the ship which causes bodily sinkage so that the vessel is said to ‘squat’ in the
water. In addition to an increase in mean draught there will usually be a change of trim,
by the bow or stern according to the circumstances. When the depth of water is less than
about one and a half times the draught this effect is much more pronounced. If there is
shallow water on only one side the pressure fields may cause the ship to sheer away
from the bank which could bring danger of collision if another vessel is passing close
by. Interaction between ships due to the pressure fields will also be greater in shallow
water and the steering qualities are likely to be affected (see pages 71–73).
(b) The term ‘vessels’ includes non-displacement craft, WIG craft and seaplanes by
the definition of Rule 3.
Dangers of navigation and collision
A departure from the Rules may be required due to dangers of navigation or to
dangers of collision. For instance, a power-driven vessel meeting another powerdriven vessel end on may be unable to alter her course to starboard, as directed by
Rule 14, owing to the presence of shallow water close by to starboard or to the fact
that a third vessel is overtaking her on her starboard side.
Special circumstances and immediate danger
This Rule does not give any vessel the right to take action contrary to the Regulations
whenever it is considered to be advantageous to do so. A departure is only permitted
when there are special circumstances and there is immediate danger. Both conditions
must apply. The departure must be of such a nature as to avoid the danger which
threatens.
Squadrons or convoys
The Mariners’ Handbook draws the attention of mariners to the dangers which may be
caused by single vessels attempting to pass ahead of, or through a squadron of warships
or merchant vessels in convoy. Single vessels are advised to take early measures to
keep out of the way, and the vessels in the squadron or convoy are warned to keep
a careful watch and be ready to take such action as will best aid to avert collision.
Mariners are expected to take into account the cautions and recommendations
given in Notices to Mariners and other official publications but if a vessel in
a formation or convoy is approached by a single vessel so as to involve risk of
collision the Steering and Sailing Rules must be complied with.
Action taken in accordance with the advice to avoid a squadron or convoy on the
port bow would not be a departure from the Rules if executed at long range before risk
of collision begins to apply (see pages 26–28).
Duty to depart if necessary
If a departure from the Rules is necessary to avoid immediate danger a vessel would
not only be justified in departing from them but may be expected to do so.
Tasmania-City of Corinth
It is provided by Rule . that, in obeying and construing the Rules, due regard shall be
had to any special circumstances which may render a departure from them necessary
4 A Guide to the Collision Avoidance Rules
in order to avoid immediate danger. As soon then as it was, or ought, to a master of
reasonable skill and prudence, to have been obvious that to keep his course would
involve immediate danger, it was no longer the duty of the master of the Tasmania to
adhere to the . Rule. He was not only justified in departing from it, but bound to do
so, and to exercise his best judgement to avoid the danger which threatened. (Lord
Herschell, 1890)

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