Impeller Logs and Compasses

Sailing and navigation…Measuring Direction and Distance

For Measuring distance at sea, the old type of log that gave us the knot as unit of speed has lengthy when you consider that given manner to greater sophisticated mechanical and digital devices arc length calculator.

Walker logs

One of the oldest is the Walker log. This makes use of a torpedo-formed spinner some inches long towed at the back of the boat on a period of braided line. As it actions thru the water, spiral fins on the torpedo make it spin, twisting the road. The on-board end of the line is addicted to to the back of the log tool, where it turns a shaft related to a reduction equipment container. This in flip movements the hands on a sequence of dials, alternatively like the ones of an old school fuel meter, to provide Direct studying of the distance the spinner has moved through the water.

Advantages of the Walker log are its rugged simplicity and the convenience with which weed or debris can be cleared from the pinner. Its hazards are that its show has to be installed right in the back of the boat; that the log line (normally 30 or 60 toes in duration) must be streamed earlier than the log can be used, and recovered before entering harbour; it has a tendency to below-study at very low speeds; and at speeds over approximately ten knots the spinner is willing to jump out of the water and skitter alongside the surface. There are precise strategies for streaming and recovering a mechanical trailing log, supposed to reduce the risk of the road tangling. To stream the log, first connect the on-board end to the hook at the returned of the show unit. Then, retaining the spinner in hand, feed out all of the line to shape an extended U-fashioned loop astern before losing the spinner overboard, nicely off to one facet of the loop. Some proprietors want to keep on to the road just astern of the show unit for a few seconds, just to soak up the seize because the load comes on to the road.

When recovering the log, velocity is crucial, particularly if the boat is transferring rapid. Unclip the inboard give up from the hook on the returned of the display, and drop it overboard, allowing it to trail out astern whilst you pull in the log line. Then conserving the spinner, acquire inside the line, coiling it as you cross. Trailing the line astern like this permits any kinks to get to the bottom of.

Electrical trailing logs

The electric trailing log is superficially just like a Walker log, inasmuch because it uses a spinner towed astern of the boat on a long line. In this case, but, the spinner is in elements, and the ‘log line’ is an electrical cable. The the front part of the spinner is attached to the cable and most effective the rear component is unfastened to rotate. As it does so, an electronic sensor inside the front part makes and breaks an electrical circuit, so the on-board display unit gets a short pulse of strength whenever the spinner rotates. These pulses are counted electronically and are offered as a digital display of velocity and distance run.

The advantages and downsides of this form of log are much the same as for the mechanical Walker log besides that it’s miles depending on electric electricity from internal dry batteries, which in return allows the show unit to be established nearly everywhere on board, and that because the road itself is not twisting, it’s far as a substitute less complicated to move and get better.

Hull-established impeller logs

On cruising boats, hull-hooked up logs are by a long way the maximum popular kind, although in principle they are a good deal similar to the electrical trailing log: a rotating impeller sends a circulation of electrical impulses to a display unit mounted inside the cockpit or close to the chart table.

The impeller – which can be either a miniature version of the trailing log’s spinner, or a paddle wheel an inch or so in diameter – is set up in a becoming known as a transducer, which either protrudes via the lowest of the boat or hangs down under the transom.

The dangers of this machine are that an impeller so close to the hull may be affected by the water go with the flow around the hull itself, and that it’s miles tough and probably dangerous to withdraw the transducer to clear weed or particles from it at sea. The cause in-hull logs are so famous is in most cases the convenience of not having to stream and recover 30 toes or extra of log line at the start and give up of every passage.

Other logs

At the top of the dimensions of charge and class are several alternative methods of measuring pace through the water:

Electromagnetic logs are based totally at the identical principle as generators and electric motors: that electricity is created if you move a magnetic area past an electrical conductor. In this case the conductor is sea water and the magnetic discipline is created by means of the transducer. As the transducer actions thru the water a small electric cutting-edge is installation, measured by way of sensors on the transducer.

Sonic logs use accurate measurements of the velocity of sound among two transducers installed one ahead of the opposite. Each transducer emits a non-stop circulation of clicks, inaudible to the human ear, at the same time as listening for clicks transmitted from the opposite. When the boat is shifting, the movement of the water beyond the hull slows down the clicks journeying ahead at the same time as rushing up the ones travelling aft. The instrument accurately measures the time taken for every click to make the experience, compares them, converts the consequences right into a display of speed thru the water, and from this calculates the distance run.

Another type of sonic log uses sophisticated echo sounder technology to degree the charge at which plankton and debris are moving past its transducer.

The big advantages of all 3 types are that they may be plenty much less liable to fouling than regular in-hull logs and that they are able to move on running at very excessive speeds or in rough sea conditions, whilst turbulence or air bubbles make impeller logs unreliable.

Calibrating logs

No log may be relied upon to be one hundred in step with cent accurate. This is specially proper of hull established logs due to the fact – quite aside from any inherent inaccuracies in the tool itself – the sluggish build-up of fouling as the season progresses means that the boat is dragging an ever-thickening layer of water at the side of it, so the water float beyond the impeller can be slower than the boat pace via the water. Conversely, round a few elements of the hull, which includes along a sailing boat’s keel or near the propellers of a motor boat, the water waft may additionally certainly be extended, making the log over-examine.

Errors can usually be allowed for if you understand about them, and maximum digital logs have a calibration facility that lets in them to be adjusted to take account of these versions. Finding, and if vital, correcting, log blunders is called calibration. In principle it involves measuring the time taken to cover a recognized distance, the use of this to calculate actual speed, and comparing this with the velocity indicated by the log. Any correctly-recognised distance may be used, even though the pleasant are certainly the measured distances’ set up especially for the purpose. They include (or once in a while 3) pairs of transit posts, marking the start and end of a exactly-measured distance, and proven on the perfect chart. The direction to persuade to cover the Measured distance is also shown.

Settle the boat on course and at a steady velocity earlier than crossing the primary transit line; word the time at that you move the begin ine and hold that path and velocity without making any allowance for wind or tide till you cross the finish line, and notice the time taken. Note the actual log studying at durations of, say, 15 seconds so you can training session the average log velocity for the complete run.

As perfectly nevertheless water is rare, it’s far vital to repeat the system within the opposite course. Having found the velocity over the ground in both directions, the velocity thru the water can be calculated by way of taking the common, through including the two speeds collectively and dividing by way of .

A greater accurate end result may be acquired with the aid of making four or six runs, however this can be a very
time-ingesting system, especially as log mistakes are not necessarily the same in any respect speeds, so the calibration runs want to be done at a range of various speeds, and repeated as a double check after the log has been adjusted.

A not unusual mistake is to work out the common time taken and divide the gap via this. The result invariably understates the boat’s velocity, as it need to had been travelling inside the ‘gradual’ course longer than inside the ‘fast’ direction.

Some huge scale charts (harbour plans) have a honestly marked scale of distance – rather just like the one you may discover on a road atlas – usually published someplace near the lowest side. But this isn’t usually the case, and on the smaller scale charts used for coastal and offshore navigation it’d be impractical to provide this type of scale due to the fact the scale of the chart varies slightly from pinnacle to bottom. One sea mile, but, is with the aid of definition one minute of latitude, so the latitude scales on every facet of the chart represent a scale of distance.

The mild distinction between a sea mile and an worldwide nautical mile is so small that for regular navigation it may be left out: what’s vital, on small scale charts, is the distortion caused by the Mercator projection, this means that that distance must be measured on the range at which it is for use. The longitude scale on the top and bottom edges of the chart is useless as a scale of distance.

It is pretty rare to find ourselves faced with the task of measuring distance in an precisely north-south line, so we want some means of moving the space between any two points at the chart to the range scale. Dividers are the device for the task. For study room navigation the form of dividers utilized in technical drawing are perfectly adequate, and their sharp needle factors supply a reassuring feel of precision, but for practical navigation, conventional bow dividers have the large advantage that they may be opened and closed with one hand, through squeezing the bow to open them, and squeezing the legs to close them.

Sometimes it is important to draw arcs of measured radius at the chart, for which it’s miles useful to have a drawing compass. Again, the type intended for technical drawing can be used as long as it is large sufficient, but it’s far normally better to use the larger and much less state-of-the-art variations supposed for marine navigation.

Compasses and Measuring direction at sea

Direction at sea is measured the usage of a compass – basically an instrument which factors north, and is going on pointing north no matter the movement of the boat round it. In practice maximum yachts carry as a minimum two compasses. One, steerage compasses are exceptionally massive, fixed to the boat, and used to measure heading. The other is normally smaller, portable and is used to measure the course of distant gadgets, so it’s far referred to as a hand bearing compass. Sometimes one compass can do both jobs: on many ships and some huge yachts an attachment called a pelorus permits the steering compass to be used for taking bearings, whilst on very small craft, a hand bearing compass clipped right into a bracket can serve as a guidance compass.

There are many methods of creating an tool as a way to stay pointing in a single Direction, together with gyroscopes, and what are known as ‘ring laser gyros’, but despite the fact that these have their benefits, they are a great deal too sophisticated, and consequently steeply-priced, to be of sensible interest for yachts. The Overwhelming majority of yacht compasses Depend on magnetism, and in that appreciate can be seen as direct developments from devices that have been likely in use numerous thousand years in the past. Compasses employ the truth that the earth has a magnetic discipline, which could be very tons as though a large bar magnet have been embedded in its core and aligned with its North-South axis.

Any magnet this is unfastened to swing has a tendency to line itself up with the earth’s magnetic discipline. This effect is especially obvious within the small, flat compasses used for orienteering and rambling on land, in which a single directly needle-like magnet gives a direct Indication of north. In marine compasses, numerous such magnets, or a unmarried magnet within the form of a ring, are installed underneath a circular ‘card’, with a scale of levels or compass factors marked on it. The entire issue is suspended in a bowl filled with a aggregate of water and alcohol, which slows Down the motion of the card, to reduce the swinging that might in any other case be because of the pitching and rolling of the boat.

Compasses supposed for immediate motor boats are much more closely damped than those meant for crusing craft; the fast slamming of a planing boat can be enough to make the cardboard of a sailboat compass rotate continuously.

Steering compasses

On a guidance compass the fore-and-aft line of the boat is marked by a line or pointer at the compass bowl, called the lubber line, towards which the boat’s contemporary heading may be read from the cardboard, so it’s far manifestly essential for the compass to be installed in order that the lubber line is appropriately aligned with, or parallel to, the centre line of the boat. Many compasses have supplementary lubber traces offset by means of forty five┬░ and ninety┬░ on each aspect, supposed especially to be used in situations along with tiller-suggested boats where the helmsman is probably to be searching on the compass from one aspect or the other.

Of course, there are versions supposed to healthy precise programs. On many small and medium sized sailing yachts, in which cockpit space is at a top rate, the compass is about into the aft bulkhead of the superstructure, so that the rear edge of the card is seen, rather than its upper surface. A compass meant for this form of mounting has an aft lubber line and a scale of levels marked on the down-grew to become rim of the cardboard. An even greater severe version is sometimes determined in compasses meant for metallic craft, whose structure correctly mask the compass from the earth’s magnetic subject. This hassle may be decreased with the aid of mounting the compass as excessive above the hull as possible, so compasses have been produced that may be installed on the wheelhouse roof, with mirrors or prisms arranged so that the helmsman efficaciously appears upwards at the bottom of the compass card.

Grid compasses

Grid compasses, supposed in general for plane navigation, loved a surge of popularity after the Second World War, whilst many boats had been fitted out from Army surplus stores! The declare that they had been less difficult to steer by means of maintained their recognition for at least twenty years and numerous marinized versions have been produced. A grid compass has a card with a particularly distinguished north set in a flat-crowned bowl. On pinnacle of the bowl is a transparent cover, marked with a grid of parallel strains and with a scale of ranges es round its edge. The required course is ready through rotating the quilt, and the helmsman then steers on the way to maintain the –. Mark on the card covered up with the grid.

Hand bearing compasses

A hand bearing compass is largely a small, transportable model of a steering compass, equipped with some shape of sighting arrangement that lets in it to be accurately coated up on a far off item. They can be subdivided into groups: the ones intended for use at arm’s length, which might be normally geared up with a take care of; and those supposed to be held close to the attention, that are usually provided with a neck strap. Which kind is high-quality is very much a count of personal choice, but each person who uses spectacles or a listening to useful resource is properly suggested to head for an arm’s-period compass due to the fact even small pieces of ferrous metal consisting of the hinges of spectacles can motive compass errors if they’re only inches away.

Sighting arrangements range. The conventional Sestrel Radiant, for instance, has a prism set up above the bowl, with a V-fashioned notch on pinnacle. When the compass is held up at arm’s duration and eye degree the lubber line and compass card can be seen within the prism. To take a bearing of a distant item, you line up the ‘goal’ with the notch, rotate the compass until the lubber line seems within the prism immediately underneath the goal, and then read off the bearing. Another commonplace association has attractions on pinnacle of the bowl, like the fore sight and returned sight of a gun, and an facet-reading compass card. Close-to-the-eye compasses do no longer have such apparent sighting preparations: instead they have a small prism installed on top, whose optics are organized in the sort of way that when you have a look at a landmark across the top of the compass, its bearing seems within the prism without delay below.

Fluxgate compasses

A new kind of compass is swiftly gaining in popularity. Unlike a conventional ‘swinging card’ compass, a fluxgate compass has no moving parts, however rather makes use of electronics to come across the earth’s magnetic discipline and present that records on some sort of show. A fluxgate relies upon at the phenomenon of electromagnetic induction – as utilized in transformers and the ignition coil of a petroleum engine. If you bypass an electric powered current via a coil of cord wound round a suitable metallic center, the core will become a magnet. Which end is the north pole, and which the south, depends on the route of the current flow within the wire, so if you practice an alternating present day to the cord, the north and south poles of the core trade locations each time the modern reverses. If you have got a 2d coil of wire wound round this whole meeting the continuously-reversing magnetic discipline induces an electric current within the secondary winding.

In a fluxgate there are cores facet by using aspect, with their primary windings receiving alternating current from the equal source, but wound in opposite guidelines. This means that during a magnetically ‘smooth’ surroundings (without a external magnetic affects) the induced magnetism inside the cores might be identical and contrary, so that they might cancel every different out and bring no contemporary at all inside the secondary winding that surrounds each of them. The presence of an outside magnetic subject upsets the stability, inflicting a brief surge of power in the secondary winding every time the number one modern-day reverses. This effect is most suggested if the 2 cores are parallel to the outside magnetic field. In a realistic fluxgate compass, numerous fluxgates are arranged in a circle. By comparing the voltages brought about in the various secondary windings it’s miles possible to infer where north is relative to the ring of flux-gates.

At present, the most commonplace use of this generation is to offer heading facts for different digital system along with autopilots or radars, however it is able to also be used to provide a steering display for the helmsman or because the coronary heart of an electronic hand bearing compass. Apart from the benefit with which fluxgate compasses may be related to other navigational electronics, their massive blessings are that they can be fitted with an automated correction facility, and that due to the fact the sensor and display are typically break free every different, the sensor may be set up everywhere on board and well faraway from distorting magnetic Influences. Fluxgate hand bearing compasses also have the facility to ‘save’ headings, to save the navigator having to memorize them.

Their principal disadvantage is that very massive errors can occur if the fluxgate ring isn’t saved flawlessly horizontal. There are digital answers to this problem, but the reality stays that the compass with out transferring components truely requires extra state-of-the-art gimbal preparations than its swinging card counterparts.