You will see many vessels showing their lights at all times, whilst there is nothing wrong with this, in small craft it is not always desirable. Especially on sailing vessels power consumption is a major factor to consider and a good guide as to when to turn the lights on is to ask "will they help to make me more visible?"
One time when navigation lights become very useful, is in poor visibility-especially in harbours or estuaries. In these situations, if a vessel suddenly appears out of the fog, and they do not have any lights on, it is easy to assume that they may be on a mooring, however, the presence of navigation lights immediately indicates that they are under way.
Part (b) says that no other lights should be exhibited. Deck lights or working lights can cause considerable confusion when attempting to identify the class and actions of other vessels. It is in the nature of some vessels work (fishing vessels and dredgers) that they have very bright working lights which make identification very difficult. Obviously, if possible vessels should not show any lights other that those it should show to comply with these rules if at all possible.
Although it is not mentioned in these Rules, a common practice is for tankers to exhibit a single fixed or flashing, all round red light at the after masthead. This replaces the 'B' flag, to indicate they have a dangerous cargo. This light is in addition to the light of this Rule.
If you have difficulty identifying the light on a vessel at night, the best aid a good pair of binoculars. They need to be 7X50, that is 7 times magnification (any more and they shake), with a 50mm objective lens. With these you will find that lights which appear as a blur, will change to points of lights and you will probably be able to see the silhouette of the other craft.
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