| Marine electronic navigation equipment | | | | identified by switching to a deeper operating scale |
| Echo sounders, for measuring depth,are now virtually | | | | which will indicate the true depth. |
| standard equipment on all but the smallest and | | | | Recording paper sounders. |
| simplest of boats. It works by transmitting pulses, or | | | | Although they look very different and are much |
| clicks, of ultrasonic sound from a transducer mounted | | | | more expensive, recording paper echo sounders use |
| on board, down to the seabed, and then receiving | | | | much the same timing system as rotating neons, |
| the returning echoes. Although the speed of sound in | | | | except that instead of a flashing light the timing |
| water varies slightly, it is always in the order of 1400 | | | | display is a stylus or electric pen'. This is mechanically |
| metres per second, so the time taken for each pulse | | | | swept across a moving roll of special paper - similar |
| to complete a down and back trip depends on the | | | | to that used in fax machines - producing a mark each |
| depth of water. | | | | time a pulse is transmitted and each time an echo is |
| The most readily-understood timing system is that | | | | received. Like the flashes of a rotating neon sounder, |
| used in the 'rotating neon' type of sounder, in which | | | | the distance between these two marks corresponds |
| the heart of the display unit is a fast-spinning rotor | | | | to the depth. Over a period of time as the recording |
| with a neon lamp or light-emitting diode at its end. | | | | paper unrolls, successive traces build up to produce a |
| Each time the rotor passes the upright position, the | | | | continuous permanent record. Although they have |
| light flashes and the transducer is triggered to | | | | their uses for some commercial operations and for |
| transmit its pulse. When the returning echo is | | | | surveying, recording paper sounders have no |
| detected by the transducer, the light flashes again, | | | | particular merit for pleasure craft, especially as the |
| but by this time the rotor has moved on. How far it | | | | need to keep them supplied with recording paper is |
| has moved depends on the time interval between | | | | an expensive nuisance. |
| transmission and reception, so the depth of water is | | | | Electronic displays |
| indicated by the position of the second flash. It can | | | | Electronics manufacturers are seldom keen on |
| be read directly off a scale marked on the face of | | | | mechanical components, and whatever the merits of |
| the instrument around the window that covers the | | | | rotating neon echo sounders their dependence on |
| rotor. | | | | fast moving mechanical parts makes them potentially |
| For operation in deep water, the rotor speed can be | | | | unreliable and power hungry, while possible variations |
| slowed down, increasing the range of time intervals | | | | in motor speed can make them inaccurate. As |
| that can be measured and increasing the time | | | | technology developed and electronic timing devices |
| between successive pulses, but reducing the | | | | became a practical proposition, most of the more |
| accuracy and precision of the depth measurement. | | | | up-market manufacturers offered display units that |
| With practice the appearance of the returning flash | | | | indicated the depth by means of a moving pointer on |
| gives a clue to the nature of the seabed: a hard | | | | a graduated dial. Some of these units have survived, |
| seabed such as rock produces a crisp echo which | | | | but they have been almost completely superseded |
| appears as a short flash; while a very soft bottom | | | | by all-electronic displays giving either a digital read-out |
| such as mud or weed gives a more drawn-out echo | | | | or a graphical presentation similar to the trace of a |
| and produces a more diffuse or drawn-out flash. | | | | recording paper sounder. |
| Sometimes, however, the echo sounder can be | | | | Added features |
| misleading. | | | | Echo sounders are basically simple instruments |
| Air bubbles are good reflectors of sound waves, so | | | | measuring a single quantity - depth - so there are |
| turbulence caused by the wash of passing ships can | | | | few added features that can usefully be |
| produce a mass of shallow flashes. The swim | | | | incorporated. Most, however, include a shallow water |
| bladders of fish also contain air, so a single large fish | | | | alarm which can be set to sound a bleeper when the |
| can produce a brief flash, while a dense shoal of small | | | | indicated depth is shallower than a chosen limit, and |
| fish produces a more consistent flash at a depth | | | | many have a deep alarm which bleeps or flashes |
| corresponding to the depth of the shoal. Fishermen | | | | when a pre-set depth is exceeded. |
| find this useful and the echo sounder principle has | | | | A carefully-set shallow alarm has an obvious value as |
| been developed into fish finders, but for navigation | | | | a warning function when operating in shoal water, |
| purposes such echoes are simply a nuisance. Luckily, | | | | and a deep alarm can be useful when anchored, as a |
| they are usually easy to identify because they are | | | | reminder to let more cable out to cope with the |
| short-lived and erratic. | | | | rising tide. Used together, they can play a part in |
| Another type of spurious flash can sometimes be | | | | pilotage or in fog navigation, when they can be used |
| seen in shallow waters over a hard bottom, and is | | | | to guide you between two contour lines. |
| caused by the returning echo reflecting back from | | | | Installation and calibration. |
| the sea surface to make a second trip down to the | | | | For echo sounders to work, the transducer has to be |
| seabed and back. If this second echo is strong | | | | able to send its pulses down to the seabed. Wood is |
| enough to register on the echo sounder, it is called a | | | | a very effective insulator of sound, so in wooden |
| reflection echo and appears as a relatively weak flash | | | | boats a through-hull installation is essential, with the |
| at twice the true depth. | | | | transducer mounted in a watertight housing so that |
| A particularly worrying type of spurious echo can be | | | | its transmitting face is in direct contact with the sea |
| produced by hard bottoms when the water is so | | | | water below. A similar set-up can be used in GRP or |
| deep that the echo does not return until after the | | | | metal boats, but it is not essential because these |
| rotor has completed one full revolution. The returning | | | | materials transmit sound. In-hull mountings can be |
| echo produces a flash on the display which is | | | | used, so long as there is no air gap or bubbles |
| considerably shallower than the true, depth: if, for | | | | between the transducer and the hull skin. The |
| instance, the echo sounder is set to an operating | | | | transducer can be bonded directly to the hull with a |
| range of 0-25 metres and the true depth is 30 | | | | layer of epoxy glue or (better) mounted in a tube |
| metres, the indicated depth will be 5 metres. | | | | bonded to the hull and filled with vegetable oil to |
| Fortunately these second trace echoes can easily be | | | | exclude the air. |