The purpose of a navigation light is to show presence, aspect and occupation: can you see it—can he see it—do you both know what you are seeing?

Radiation is measured by ‘luminous flux’; the unit of which is the ‘lumen’, but that is not what we need to know. We need ‘luminous intensity’, the amount of light emitted in one particular direction and normally called the candlepower, the unit for which is the ‘candela’. Figure 1 shows how brilliance, in candela, affects range. For the purposes of Imco calculations a transmissivity factor of 0·8 is used; K = 0·74 corresponds to the Iala standard and 0·9 to the old ‘dear dark night’ standard used for oil lanterns. A sailing yacht has also to consider voltage drop and Fig. 2 shows how a lead acid battery while being charged might produce 14 volts, giving a 10 W bulb a brilliance of 22 cd, whereas the same battery not being charged might give as little as 9 cd.

Two devices may be used to enhance the radar echo from a small craft or a buoy. A radar reflector will send back to the radar nearly all the transmitted power it receives, instead of scattering it. It is like a very small heliograph mirror reflecting sunlight, which is as easily seen as a large white sail. A transponder on the other hand is an active electronic device which, when it detects the transmitted radar pulse, immediately transmits its own identifying signal back to the radar. The appearance at the radar display can be seen in Fig. 1 (North is at the top and the range rings are at 1 mile intervals). The transponder was in a small boat at 2·8 miles range on 162°, its response being the Morse letter Y. No. 1 Sea Reach buoy with a reflector on it is visible at 6 miles on 115°.

Some Implications of the New Collision Regulations. In Rule 6 the discussion of what constitutes a ‘safe speed’, paragraph (b) (iv) states that ‘the possibility that small vessels, ice and other floating objects may not be detected by radar at an adequate distance’ is one of the factors which should be taken into account. Leaving aside the question of how the new term ‘safe speed’ differs from the old ‘moderate speed’ this is not a fundamental change, although the provision appears in a different place and with different emphasis in the new rules. It is a provision which ought to be treated more conscientiously by many mariners since, for example, it has been observed that the mean speed of ships through the Dover Strait is about 12 knots and that this does not appear to be significantly reduced when visibility decreases.

In considering the implications for yachtsmen, cruisers as well as racers, of the new Collision Regulations which came into force on 15 July 1977, it is worth recording that Imco-recommended traffic separation schemes were first published in 1967 and that there are now nearly one hundred schemes throughout the world, about ten of which are within the normal cruising grounds of British based yachts. On 1 September 1972 the British government introduced legislation making compliance with the recommendations mandatory for all United Kingdom registered vessels. Ten other countries have followed suit but a significant exception, at least as far as yachting is concerned, has been France. However from 15 July 1977, under Rule 10 of the International Regulations for Preventing Collisions at Sea, 1972, compliance is required of all vessels. The word ‘vessel’ is defined in new Rule 3 as including ‘…every description of watercraft…’.

Safety problems for a single-hander fall into two basic categories: those affecting the single-hander and those that affect other people. The most obvious difference between sailing in, say, R.O.R.C. races and on one's own is that there is no back-up for the single hander when some particular task such as a sail change has to be carried out. Whereas on a modern ocean racer most jobs can, and indeed have to, be swamped with men in order to achieve not necessarily the most efficient result but the fastest result, for the single-hander the job has to be much more carefully thought out so that at no time does he place himself in the position where a piece of gear or equipment can lose itself because he is elsewhere.

In the contribution to the joint Royal Institute of Navigation/Royal Ocean Racing Club meeting on Safety of Navigation under Sail (p. 467), an expression was quoted for the random probability of collision in the case of a large, relatively fast ship passing through a concentration of smaller craft. It was not appropriate to discuss the derivation of the expression in the particular circumstances of the meeting but the methodology may be of some general interest and it is therefore outlined in this note.

The following letter to Nature, dated 24 April 1873, might make an appropriate sequel to ‘The history of inertial navigation’ by W. Wrigley in the January issue of the Journal. As well as containing some interesting speculation about animal navigation, which we seem no nearer resolving a century later, the letter includes an introduction to the idea of inertial navigation that it would be difficult to improve upon even today.

L. G. Reynold's interesting article (Journal30, 108) gives rise to a question. Why, on the new charts is the light range given as the luminous or nominal range, instead of, as formerly, the geographical range (for a height of eye of 15 ft)? This, from the point of view of the practical navigator, seems a retrograde step for we can no longer see from a glance at the chart the approximate range at which we may expect to sight a light. The luminous range is almost always greater than the geographical range and reference must be made to the Geographical Range Table in the Admiralty List of Lights to find out what the latter will be.

The basic premise of compass adjusting (that is compensation aboard ship for magnetic fields caused by the vessel itself and the determination of residual deviation) is that a reference direction must be available during the operation. Normally visual ranges or the azimuth of a celestial body—usually the Sun—are used, but in some areas and climates reduced visibility often does not permit the use of either. Compass adjusters are frequently driven in desperation to use gyro-compass, radar, or radio bearings—with highly speculative results as the errors of these instruments are generally unknown, and fairly large compared to the accuracy required of the magnetic standard compass (± ½°) which is the primary direction standard aboard ship.

Just a century ago, in 1876, Patrick Weir, an officer of a vessel trading between London and Australia, conceived the idea of a diagram that might facilitate finding the Sun's true azimuth for the purpose of checking the magnetic compass. Some thirteen, years later Captain Weir's Diagram was the subject of a paper communicated by Sir William Thomson (later Lord Kelvin) to the Royal Society of Edinburgh. In his paper Weir outlined the train of reasoning by which he succeeded in constructing a novel diagram which was described by Professor P. G. Tait as ‘a singularly elegant construction which, not only puts in a new and attractive light one of the most awkward of the problems of spherical trigonometry, but it practically gives in a single-page diagram the whole content of the two volumes of Burdwood's Azimuth Tables’. Tait also remarked that the method supplied an interesting graphical plane construction of a function of three independent variables.