| Sailing and navigation...Measuring Direction and Distance | | | | navigation, traditional bow dividers have the big |
| For Measuring distance at sea, the old type of log that | | | | advantage that they can be opened and closed with |
| gave us the knot as unit of speed has long since given | | | | one hand, by squeezing the bow to open them, and |
| way to more sophisticated mechanical and electronic | | | | squeezing the legs to close them. |
| devices. | | | | Sometimes it is necessary to draw arcs of measured |
| Walker logs | | | | radius on the chart, for which it is useful to have a |
| One of the oldest is the Walker log. This uses a | | | | drawing compass. Again, the type intended for |
| torpedo-shaped spinner a few inches long towed | | | | technical drawing can be used so long as it is big |
| behind the boat on a length of braided line. As it moves | | | | enough, but it is generally better to use the larger and |
| through the water, spiral fins on the torpedo make it | | | | less sophisticated versions intended for marine |
| spin, twisting the line. The on-board end of the line is | | | | navigation. |
| hooked on to the back of the log instrument, where it | | | | Compasses and Measuring direction at sea |
| turns a shaft connected to a reduction gear box. This | | | | Direction at sea is measured using a compass - |
| in turn moves the hands on a series of dials, rather like | | | | essentially an instrument which points north, and goes |
| those of an old fashioned gas meter, to give Direct | | | | on pointing north regardless of the movement of the |
| reading of the distance the spinner has moved through | | | | boat around it. In practice most yachts carry at least |
| the water. | | | | two compasses. One, steering compasses are |
| Advantages of the Walker log are its rugged simplicity | | | | relatively large, fixed to the boat, and used to measure |
| and the ease with which weed or debris can be | | | | heading. The other is usually smaller, portable and is |
| cleared from the pinner. Its disadvantages are that its | | | | used to measure the direction of distant objects, so it |
| display has to be mounted right at the back of the | | | | is called a hand bearing compass. Sometimes one |
| boat; that the log line (usually 30 or 60 feet in length) | | | | compass can do both jobs: on many ships and a few |
| has to be streamed before the log can be used, and | | | | large yachts an attachment called a pelorus allows the |
| recovered before entering harbour; it tends to | | | | steering compass to be used for taking bearings, while |
| under-read at very low speeds; and at speeds over | | | | on very small craft, a hand bearing compass clipped |
| about ten knots the spinner is inclined to jump out of | | | | into a bracket can serve as a steering compass. |
| the water and skitter along the surface. There are | | | | There are many ways of making an instrument that |
| definite techniques for streaming and recovering a | | | | will stay pointing in one Direction, including gyroscopes, |
| mechanical trailing log, intended to reduce the risk of | | | | and what are called 'ring laser gyros', but although |
| the line tangling. To stream the log, first attach the | | | | these have their advantages, they are much too |
| on-board end to the hook on the back of the display | | | | sophisticated, and therefore expensive, to be of |
| unit. Then, keeping the spinner in hand, feed out all the | | | | practical interest for yachts. The Overwhelming |
| line to form a long U-shaped loop astern before | | | | majority of yacht compasses Depend on magnetism, |
| dropping the spinner overboard, well off to one side of | | | | and in that respect can be seen as direct |
| the loop. Some owners like to hold on to the line just | | | | developments from instruments that were probably in |
| astern of the display unit for a few seconds, just to | | | | use several thousand years ago. Compasses make |
| absorb the snatch as the load comes on to the line. | | | | use of the fact that the earth has a magnetic field, |
| When recovering the log, speed is essential, especially | | | | which is very much as though a huge bar magnet |
| if the boat is moving fast. Unclip the inboard end from | | | | were embedded in its core and aligned with its |
| the hook on the back of the display, and drop it | | | | North-South axis. |
| overboard, allowing it to trail out astern while you pull in | | | | Any magnet that is free to swing tends to line itself up |
| the log line. Then holding the spinner, gather in the line, | | | | with the earth's magnetic field. This effect is particularly |
| coiling it as you go. Trailing the line astern like this allows | | | | obvious in the small, flat compasses used for |
| any kinks to unravel. | | | | orienteering and rambling on land, in which a single |
| Electrical trailing logs | | | | straight needle-like magnet gives a direct Indication of |
| The electrical trailing log is superficially similar to a | | | | north. In marine compasses, several such magnets, or |
| Walker log, inasmuch as it uses a spinner towed | | | | a single magnet in the shape of a ring, are mounted |
| astern of the boat on a long line. In this case, however, | | | | underneath a circular 'card', with a scale of degrees or |
| the spinner is in two parts, and the 'log line' is an | | | | compass points marked on it. The whole thing is |
| electrical cable. The front part of the spinner is | | | | suspended in a bowl filled with a mixture of water and |
| attached to the cable and only the rear part is free to | | | | alcohol, which slows Down the movement of the card, |
| rotate. As it does so, an electronic sensor in the front | | | | to reduce the swinging that would otherwise be |
| part makes and breaks an electrical circuit, so the | | | | caused by the pitching and rolling of the boat. |
| on-board display unit receives a short pulse of | | | | Compasses intended for fast motor boats are much |
| electricity each time the spinner rotates. These pulses | | | | more heavily damped than those intended for sailing |
| are counted electronically and are presented as a | | | | craft; the rapid slamming of a planing boat can be |
| digital display of speed and distance run. | | | | enough to make the card of a sailboat compass |
| The advantages and disadvantages of this type of log | | | | rotate continuously. |
| are much the same as for the mechanical Walker log | | | | Steering compasses |
| except that it is dependent on electrical power from | | | | On a steering compass the fore-and-aft line of the |
| internal dry batteries, which in return allows the display | | | | boat is marked by a line or pointer on the compass |
| unit to be mounted almost anywhere on board, and | | | | bowl, called the lubber line, against which the boat's |
| that because the line itself is not twisting, it is rather | | | | current heading can be read from the card, so it is |
| easier to stream and recover. | | | | obviously important for the compass to be installed so |
| Hull-mounted impeller logs | | | | that the lubber line is accurately aligned with, or parallel |
| On cruising boats, hull-mounted logs are by far the | | | | to, the centre line of the boat. Many compasses have |
| most popular type, though in principle they are much | | | | supplementary lubber lines offset by 45° and |
| the same as the electrical trailing log: a rotating impeller | | | | 90° on each side, intended mainly for use in |
| sends a stream of electrical impulses to a display unit | | | | situations such as tiller-steered boats where the |
| mounted in the cockpit or near the chart table. | | | | helmsman is likely to be looking at the compass from |
| The impeller - which can be either a miniature version | | | | one side or the other. |
| of the trailing log's spinner, or a paddle wheel an inch or | | | | Of course, there are variations intended to suit |
| so in diameter - is mounted in a fitting called a | | | | particular applications. On many small and medium |
| transducer, which either protrudes through the bottom | | | | sized sailing yachts, where cockpit space is at a |
| of the boat or hangs down below the transom. | | | | premium, the compass is set into the aft bulkhead of |
| The disadvantages of this system are that an impeller | | | | the superstructure, so that the rear edge of the card is |
| so close to the hull can be affected by the water flow | | | | visible, rather than its upper surface. A compass |
| around the hull itself, and that it is difficult and potentially | | | | intended for this type of mounting has an aft lubber line |
| dangerous to withdraw the transducer to clear weed | | | | and a scale of degrees marked on the down-turned |
| or debris from it at sea. The reason in-hull logs are so | | | | rim of the card. An even more extreme variation is |
| popular is primarily the convenience of not having to | | | | occasionally found in compasses intended for steel |
| stream and recover 30 feet or more of log line at the | | | | craft, whose structure effectively masks the compass |
| beginning and end of each passage. | | | | from the earth's magnetic field. This problem can be |
| Other logs | | | | reduced by mounting the compass as high above the |
| At the top of the scale of price and sophistication are | | | | hull as possible, so compasses have been produced |
| several alternative methods of measuring speed | | | | that can be mounted on the wheelhouse roof, with |
| through the water: | | | | mirrors or prisms arranged so that the helmsman |
| Electromagnetic logs are based on the same principle | | | | effectively looks upwards at the bottom of the |
| as generators and electric motors: that electricity is | | | | compass card. |
| created if you move a magnetic field past an electrical | | | | Grid compasses |
| conductor. In this case the conductor is sea water and | | | | Grid compasses, intended primarily for aircraft |
| the magnetic field is created by the transducer. As the | | | | navigation, enjoyed a surge of popularity after the |
| transducer moves through the water a small electric | | | | Second World War, when many boats were fitted out |
| current is set up, measured by sensors on the | | | | from Army surplus stores! The claim that they were |
| transducer. | | | | easier to steer by maintained their popularity for at |
| Sonic logs use accurate measurements of the speed | | | | least 20 years and several marinized versions were |
| of sound between two transducers mounted one | | | | produced. A grid compass has a card with a |
| ahead of the other. Each transducer emits a | | | | particularly prominent north set in a flat-topped bowl. |
| continuous stream of clicks, inaudible to the human ear, | | | | On top of the bowl is a transparent cover, marked |
| while listening for clicks transmitted from the other. | | | | with a grid of parallel lines and with a scale of degrees |
| When the boat is moving, the movement of the water | | | | es around its edge. The required course is set by |
| past the hull slows down the clicks travelling forward | | | | rotating the cover, and the helmsman then steers so |
| whilst speeding up those travelling aft. The instrument | | | | as to keep the --. mark on the card lined up with the |
| accurately measures the time taken for each click to | | | | grid. |
| make the trip, compares them, converts the results into | | | | Hand bearing compasses |
| a display of speed through the water, and from this | | | | A hand bearing compass is basically a small, portable |
| calculates the distance run. | | | | version of a steering compass, fitted with some form |
| Another type of sonic log uses sophisticated echo | | | | of sighting arrangement that allows it to be accurately |
| sounder technology to measure the rate at which | | | | lined up on a distant object. They can be subdivided |
| plankton and debris are moving past its transducer. | | | | into two groups: those intended to be used at arm's |
| The big advantages of all three types are that they | | | | length, which are usually fitted with a handle; and those |
| are much less susceptible to fouling than ordinary in-hull | | | | intended to be held close to the eye, which are usually |
| logs and that they can go on working at very high | | | | supplied with a neck strap. Which kind is best is very |
| speeds or in rough sea conditions, when turbulence or | | | | much a matter of personal preference, but anyone |
| air bubbles make impeller logs unreliable. | | | | who uses spectacles or a hearing aid is well advised |
| Calibrating logs | | | | to go for an arm's-length compass because even |
| No log can be relied upon to be 100 per cent accurate. | | | | small pieces of ferrous metal such as the hinges of |
| This is particularly true of hull mounted logs because - | | | | spectacles can cause compass errors if they are only |
| quite apart from any inherent inaccuracies in the | | | | inches away. |
| instrument itself - the gradual build-up of fouling as the | | | | Sighting arrangements vary. The classic Sestrel |
| season progresses means that the boat is dragging an | | | | Radiant, for instance, has a prism mounted above the |
| ever-thickening layer of water along with it, so the | | | | bowl, with a V-shaped notch on top. When the |
| water flow past the impeller will be slower than the | | | | compass is held up at arm's length and eye level the |
| boat speed through the water. Conversely, around | | | | lubber line and compass card can be seen in the prism. |
| some parts of the hull, such as alongside a sailing | | | | To take a bearing of a distant object, you line up the |
| boat's keel or near the propellers of a motor boat, the | | | | 'target' with the notch, rotate the compass until the |
| water flow may actually be accelerated, making the | | | | lubber line appears in the prism immediately below the |
| log over-read. | | | | target, and then read off the bearing. Another common |
| Errors can always be allowed for if you know about | | | | arrangement has two sights on top of the bowl, like |
| them, and most electronic logs have a calibration | | | | the fore sight and back sight of a gun, and an |
| facility that allows them to be adjusted to take | | | | edge-reading compass card. Close-to-the-eye |
| account of these variations. Finding, and if necessary, | | | | compasses do not have such obvious sighting |
| correcting, log error is known as calibration. In principle it | | | | arrangements: instead they have a small prism |
| involves measuring the time taken to cover a known | | | | mounted on top, whose optics are arranged in such a |
| distance, using this to calculate true speed, and | | | | way that when you look at a landmark across the top |
| comparing this with the speed indicated by the log. Any | | | | of the compass, its bearing appears in the prism |
| accurately-known distance can be used, though the | | | | immediately below. |
| best are undoubtedly the measured distances' set up | | | | Fluxgate compasses |
| specially for the purpose. They consist of two (or | | | | A new type of compass is rapidly gaining in popularity. |
| sometimes three) pairs of transit posts, marking the | | | | Unlike a conventional 'swinging card' compass, a |
| start and finish of a precisely-measured distance, and | | | | fluxgate compass has no moving parts, but instead |
| shown on the appropriate chart. The course to steer | | | | uses electronics to detect the earth's magnetic field |
| to cover the Measured distance is also shown. | | | | and present that information on some kind of display. |
| Settle the boat on course and at a steady speed | | | | A fluxgate depends on the phenomenon of |
| before crossing the first transit line; note the time at | | | | electromagnetic induction - as used in transformers |
| which you cross the start ine and hold that course and | | | | and the ignition coil of a petrol engine. If you pass an |
| speed without making any allowance for wind or tide | | | | electric current through a coil of wire wound around a |
| until you cross the finish line, and note the time taken. | | | | suitable metal core, the core becomes a magnet. |
| Note the actual log reading at intervals of, say, 15 | | | | Which end is the north pole, and which the south, |
| seconds so that you can work out the average log | | | | depends on the direction of the current flow in the |
| speed for the whole run. | | | | wire, so if you apply an alternating current to the wire, |
| As perfectly still water is rare, it is important to repeat | | | | the north and south poles of the core change places |
| the process in the opposite direction. Having found the | | | | each time the current reverses. If you have a second |
| speed over the ground in both directions, the speed | | | | coil of wire wound around this whole assembly the |
| through the water can be calculated by taking the | | | | constantly-reversing magnetic field induces an electric |
| average, by adding the two speeds together and | | | | current in the secondary winding. |
| dividing by two. | | | | In a fluxgate there are two cores side by side, with |
| A more accurate result can be obtained by making | | | | their primary windings receiving alternating current from |
| four or six runs, but this can be a verytime-consuming | | | | the same source, but wound in opposite directions. This |
| process, especially as log errors are not necessarily | | | | means that in a magnetically 'clean' environment (with |
| the same at all speeds, so the calibration runs need to | | | | no external magnetic influences) the induced |
| be carried out at a range of different speeds, and | | | | magnetism in the two cores would be equal and |
| repeated as a double check after the log has been | | | | opposite, so they would cancel each other out and |
| adjusted. | | | | produce no current at all in the secondary winding that |
| A common mistake is to work out the average time | | | | surrounds both of them. The presence of an external |
| taken and divide the distance by this. The result | | | | magnetic field upsets the balance, causing a short |
| invariably understates the boat's speed, because it | | | | surge of electricity in the secondary winding each time |
| must have been travelling in the 'slow' direction longer | | | | the primary current reverses. This effect is most |
| than in the 'fast' direction. | | | | pronounced if the two cores are parallel to the |
| Some large scale charts (harbour plans) have a clearly | | | | external magnetic field. In a practical fluxgate compass, |
| marked scale of distance - rather like the one you | | | | several fluxgates are arranged in a circle. By |
| might find on a road atlas - usually printed somewhere | | | | comparing the voltages induced in the various |
| near the bottom edge. But this is not always the case, | | | | secondary windings it is possible to deduce where |
| and on the smaller scale charts used for coastal and | | | | north is relative to the ring of flux-gates. |
| offshore navigation it would be impractical to provide | | | | At present, the most common use of this technology |
| such a scale because the scale of the chart varies | | | | is to provide heading information for other electronic |
| slightly from top to bottom. One sea mile, however, is | | | | equipment such as autopilots or radars, but it can also |
| by definition one minute of latitude, so the latitude | | | | be used to provide a steering display for the |
| scales on each side of the chart constitute a scale of | | | | helmsman or as the heart of an electronic hand |
| distance. | | | | bearing compass. Apart from the ease with which |
| The slight difference between a sea mile and an | | | | fluxgate compasses can be connected to other |
| international nautical mile is so small that for normal | | | | navigational electronics, their big advantages are that |
| navigation it can be ignored: what is important, on small | | | | they can be fitted with an automatic correction facility, |
| scale charts, is the distortion caused by the Mercator | | | | and that because the sensor and display are usually |
| projection, which means that distance has to be | | | | separate from each other, the sensor can be mounted |
| measured at the latitude at which it is to be used. The | | | | anywhere on board and well away from distorting |
| longitude scale on the top and bottom edges of the | | | | magnetic Influences. Fluxgate hand bearing compasses |
| chart is useless as a scale of distance. | | | | also have the facility to 'store' headings, to save the |
| It is relatively rare to find ourselves faced with the job | | | | navigator having to memorize them. |
| of measuring distance in an exactly north-south line, so | | | | Their main disadvantage is that very large errors can |
| we need some means of transferring the distance | | | | occur if the fluxgate ring is not kept perfectly |
| between any two points on the chart to the latitude | | | | horizontal. There are electronic solutions to this |
| scale. Dividers are the tool for the job. For classroom | | | | problem, but the fact remains that the compass |
| navigation the kind of dividers used in technical drawing | | | | without moving parts actually requires more |
| are perfectly adequate, and their sharp needle points | | | | sophisticated gimbal arrangements than its swinging |
| give a reassuring sense of precision, but for practical | | | | card counterparts. |