Page 26 - Collision Avoidance Rules Guide

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
pages 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 circum-
stances. 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 94-97).
6. Rule 10 only applies to traffic separation schemes adopted by
the Organization. Before adoption by IMO a scheme must be
approved by the Maritime Safety Committee. A Government may,
however, in urgent cases implement a new scheme or an amendment
to an adopted scheme before receiving IMO approval. If the scheme
applies to international waters compliance would not be compulsory
for the ships of all nations but it would be good seamanship to
comply with the provisions of Rule 10.

(b) This paragraph is almost identical with Rule 27 of the 1960
Rules. The term ‘vessels’ which has been substituted for ‘craft’
includes non-displacement 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 naviga-
tion or to dangers of collision. For instance, a power-driven vessel meet-
ing another power-driven 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.

7