Marine Navigation Instruments

Marine Navigation Instruments

Modern electronic marine navigation instruments are easy to use, accurate and reliable and it is easy to become dependent on them but as such they are not infallible. The most important computer on board is a skilled human navigator, and that person should be suitably skilled when all the instruments on board fail.

All modern marine navigation instruments depend on electricity, and are susceptible to shorting of current by seawater or lightning strikes. Even with the elaborate backup systems and rigorous insulation, there is a risk of water shorting out the electrical system. Back up the navigation capability with the chart, magnetic compass, pencil and ruler as there is little that affects their function.

The sextant was considered essential on sailing long passages but has now relegated to standby status. This precision instrument took centuries to develop bit is now displaced by the GPS (Global Positioning System) receiver which is more accurate and easier to use.

Chart Table

A secure place to work on navigation is to have a permanent chart table with a horizontal surface large enough for a chart to be laid out at one fold. A chart table should either have a lifting top with chart storage space underneath, or a drawer allowing the charts to be stored flat.

Some navigators prefer to stand at a chart table arranged fore and aft, but commonly chart tables are place athwartships with a seat. The chart table should be situated near the companionway, where boat motion is at its least and the seat facing forwards. This arrangement provides communication with the helmsman and access to the cockpit.

A shelf space for reference books and bulkhead space for marine navigation instruments and communication equipment completes the design. The navigator can keep an eye on the course being steered if a small compass is mounted with the lubber line fore and aft.

If the is no room for a permanent chart table, a flat board large enough to take a folded chart measuring at least 60 x 75 cm (24 x 30 in) but able to be stowed away should be used. The chart is clipped to the board and used on the knees down below or when conditions are suitable, in the cockpit.

The chart table is lit in a fashion that allows the navigator to work on charts without disturbing sleeping crew or affecting the helmsman's night vision. A flexible stalk-light is ideal for lighting the chart and fitting it with a low-power red bulb which minimizes loss of night vision should be considered.

Parallel Rulers

The parallel ruler is used by the navigator to transfer a bearing from the chart's compass rose to the section of the chart which is being worked on. The ruler is lined up on a bearing on the magnetic compass rose. Depending on the design, the ruler is rolled or "walked" across the chart to the selected area where the replicated bearing is drawn indicating the course or the bearing of an object.

When choosing a parallel ruler, try out a range of both roller and walking designs at sea. Roller types tend to be impractical on a yacht and walking types can be unsuited to small chart tables.

Logbook

For recording position, course, distance run, and other crucial information, a logbook is used. There is a requirement to keep one by maritime law, because it in the event of an incident at sea, it may have to be produced.

There is no necessity to buy a printed logbook as there is no standardized format. Experienced navigators produce their own by ruling a few columns in a notebook or loose-leaf sheets with the column headings that suit individual needs. Entries are kept precise and orderly as there is usually a need to refer back to them.

Record key data at half-hourly or hourly when close to shore and less often when offshore. Information recorded in a deck log by thecrew on watch can be transferred at regular intervals to the main logbook.

Plotters

A plotter achieves the same result as a parallel ruler and easier to use on a small boat. Plotters are used in conjunction with the latitude and longitude grid lines marked on the chart, rather than using the compass rose.

The plotter has a compass rose and grid of lines engraved on which are lined up on the latitude or longitude lines on the chart orientating the plotter with true north. The plotter's straight edge is then lined up with the bearing to be measured, and is read off from its compass rose. Some plotters allow variation and deviation to be set on their compass rose where magnetic bearings are read or plotted directly.

A plotter has no need to be used in combination with a compass rose, and is not moved across the chart, making it easier to use and more accurate than parallel rulers. A plotter such as the Hurst has a moveable arm, the Breton plotter, a rotating compass rose and a simpler one being the Douglas plotter with no moving parts.

Dividers

Dividers are usually made of brass with steel tips and used to measure distances on the chart. Purchase a pair of dividers at least 15cm (6in) long as this gives a useful span. The single-handed type with the bowed top is more manageable than the straight type, which requires two hands to open and close it.

Span the appropriate area with the dividers and use the chart's latitude scale to read the distance. If the divider span is not large enough, set it to a suitable width using the latitude scale, then step the dividers across the area. A simple pair of compasses are carried for plotting curved lines of position.

Nautical Publications

Pencils The use of soft pencils, such as 2B, on charts prevents permanently marking them. Hexagonal pencils resist rolling off the table when the boat heels. Along with the pencils, is a requirement for a pencil sharpener and a soft eraser.

Nautical Almanac The principal reference book is a current copy of the nautical almanac covering the sailing area which provides tidal information, harbour plans and details.

Pilot Books There may be a need for pilot books that cover the area being cruised. These vary from publications specifically for yachting, to those by hydrographic agencies and intended for all craft.

Tidal Atlas Although tidal information can be gained from nautical almanacs and charts, tidal atlases of the sailing area will show the direction and rate of the tidal stream in a pictorial.

Instruction Manuals Electronic navigation instruments are quite complicated to use so therefore have a full set of instruction booklets. To make full use of the marine navigation instruments, study them carefully.

Lights and Radio Signals If sailing long distances, it is advisable to carry published references for lists of lights and radio signals, especially if a range of large-scale charts for the area is not available.

Marine Binoculars

A pair of marine binoculars suitable for marine use with a 7 x 50 (7 -times magnification, 50 mm object glass) is recommended as a steady view with more magnification than this is hard to get. Also a wide field of view as well as water-resistance along with being rubber armoured. Optionally marine binoculars with a built-in bearing compass are available.

Boat Compasses

The boat compass is the primary navigation instrument on board a yacht and is the means of identifying direction, enabling a course to be steered and to plot a chart position by the means of bearings of navigational marks and shore objects. It is used to check the bearing of other vessels which helps to avoid a collision. Two types of boat compasses, the steering compass, and a hand bearing compass for taking bearings are used in on a yacht for navigation.

Conventional Compass

The conventional boat compass has two or more bar magnets attached to the underside of a circular card and this card has marked in degrees around its edge. This card is mounted on a pivot encased in a glass or plastic bowl filled damping liquid that slows its rotation.

As the boat heels, internal or external gimbals keep the card level. When the compass is turned, the bar magnets align with magnetic north and south. A reference mark, called a lubber line, is marked on the inside of the bowl. The course or bearing is read against the lubber line and a light is fitted allowing the compass to be read at night.

Fluxgate Boat Compass

Fluxgate boat compasses use an electronic circuit to sense the lines of magnetic force and the reading is displayed as a digital readout to the nearest degree. Treat with caution the implied accuracy of the digital readout.

Fluxgate boat compasses must be kept horizontal, or errors can result that are not obvious from the display. Fluxgate boat compasses are used to provide heading information to other electronic marine navigation instruments, such as chart plotters, GPS, and radar sets.

Steering Boat Compass

Fit the boat with a quality steering boat compass, which has a large card, or display, with easy-to-read markings. It is important when the helmsman needs to site the compass that it can be directly seen. Mount the compass with the lubber line on, or parallel to, the boat's fore-and-aft line.

It is for this reason, wheel-steered boats have the steering compass in a binnacle on top of the wheel pedestal. Tiller-steered boats make use of two bulkhead-mounted compasses on the cabin bulkhead, either side of the companionway.

To minimize the effects of deviation, the compass must be at least 2m (6ft) away from the engine. Also it must be far as possible from any large ferrous-metal object, the ship's wiring system and magnetic items.

Hand Bearing Compass

To allow bearings to be taken all around the boat, a portable hand bearing boat compass is used. When using a hand bearing compass, line up the lubber line with the object for which the bearing is being measured and read off the bearing.

The three types of hand bearing boat compasses are the traditional bowl compass held at arm's length, a smaller mini compass held close to the eye and the fluxgate compass, also held at arm's length. The type that is chosen depends on a personal preference so try using a few afloat and decide which suits best.

When using a hand bearing compass, brace the body in a secure comfortable position. There are models that are the 'point and shoot' type which memorizes three bearings

Log

The elements of dead-reckoning are course and distance; the compass tells you the course steered, and a 'log' measures distance sailed.

The common type of log is an electronic instrument that shows speed and distance run, much like the speedometer and milometer in a car. Driven by a tiny impeller attached to the hull, it is reliable unless weed jams the mechanism which requires withdrawing the impeller and cleaning it.

Depth Sounder

Another navigation instrument is a depth sounder which in addition to indicating depth has alarms that are preset to a particular depth, and graphic sounders that display a profile of the bottom. In addition to the safety benefit, a depth sounder is a useful navigation tool which allows a series of soundings to be compared against charted depths.