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How Forward-looking Sonar Technology Works

Other Sounders are History! Some of the most popular vertical sounders show depth with a delay of up to 16 seconds*. At just 5 knots, this means the indicated depth is the depth 80 yards behind the boat. (at 10 to 15 knots the delay does not bear thinking about!)

At EchoPilot we believed that most boat owners would prefer to see hazardous obstructions in front of them. Several years of intensive research at EchoPilot led to the technical breakthrough which became the F.L.S. - Forward-Looking Sonar.

This achievement brought forward looking capability within the reach of the leisure boat owner for the first time. The first model, the FLS I, was launched in 1992, and there are now thousands of Forward-looking Sonars in use worldwide.

Real-time forward-looking sonar is unique to EchoPilot, who hold patents on the design. "Real time" means that the FLS does not gradually build up the picture like a phased array sonar; instead, the FLS both scans the whole forward view and updates the screen several times a second. An obstruction ahead of the boat will show on the screen and if the boat turns to another heading with no target ahead then the FLS screen will immediately show clear water ahead. Information displayed is not historical, as is the case with all digital sounders and fish finders, instead it is real time. The heart of the FLS is a state-of-the-art micro-processor capable of executing 10 million instructions per second. It is sheer computing power that enables the FLS to process enormous amounts of incoming data and select correct signals from spurious noise. Typically, the FLS performs 100,000 calculations for each screen update. The FLS transducer completes the system; it has no moving parts and is small enough to be easily fitted to any craft.

The FLS displays a two-dimensional profile of the sea bed beneath and ahead of the vessel on a high-resolution and daylight-viewable LCD matrix. Digital depth and shallow alarm setting are also displayed. The addition of an optional log transducer leads to speed and log data being shown on the screen. Some models also act as GPS repeaters and can drive a second station unit.

Mounted close to the helm the FLS can provide all necessary information - forward depth, current depth, speed, distance and GPS position data.

FLS Models - Performance

An FLS can show the seabed three to five times the depth ahead when on a flat, muddy bottom (e.g. river or estuary). This ratio increases to eight or nine times if the seabed shoals upward, and more than this for a large target. Rocky bottoms give especially strong reflections and hard, vertical surfaces like quay walls or coral reefs will often be seen at 150 m or more.

In the vertical plane, the FLS scans through 90 degrees from straight ahead to straight down. A useful analogy is to think of the FLS as an underwater radar but on its side and scanning in the vertical plane rather than the horizontal. In the horizontal plane (port-starboard), the beam is 15 degrees to 3 dB. In reality, the polar diagram is like an elongated balloon.

The FLS is not designed to be a fish finder, it is a collision-avoidance device. In reality, it does pick up larger fish and sometimes shoals of smaller fish. Dolphins show up well and some owners use the FLS to tell the foredeck crew when the anchor is about to break the surface!

There are three different FLS instruments to cover the needs of a range of boat owners. The FLS Silver has a compact case and is ideal for mounting where space is at a premium. It has a forward view of 150 m, digital depth and optional speed/log facility. The FLS Silver, in common with other FLS, has our unique forward alarm. The whole of the screen is "alarmed", so if the FLS is on the 100-m forward range then dangers shallower than the alarm setting up to 100 m ahead will trigger the alarm.

The FLS Gold has an extra-wide screen display of up to 150 m ahead. In addition to the basic features, it is also NMEA-compatible and can show GPS information on screen including latitude/longitude, course and distance to way point, cross track error and way point name.

The FLS II has a large screen and can see up to 200 m ahead. This model may be bracket- or bulkhead-mounted. The increased forward view makes it the first choice for serious sailors, and FLS II's may be found in every ocean helping their owners to meet the most challenging navigational situations.

FLS Transducers

Professional or Standard?

The professional (Grey) transducer has a narrower beam, which gives a more accurate representation of the seabed. However, the Standard (Red) transducer can appear better simply because it is collecting data from a wider seabed area.

A thick hull or one with a steep dead rise will usually require the Professional transducer as the bronze skin fitting has 75 mm (3") of useable thread whereas the Glass Filled Polypropylene skin fitting for the Standard transducer has 25 mm (1") of useable thread.

Transducer Cable Extension

The standard cable length is 12 metres, but this can be extended in multiples of 8 metres.

The transducer is of an active design, that is to say it requires power from the display unit to operate its transmitter and receiver circuitry. This has the advantage of allowing greater cable runs than were possible with the old passive type of FLS transducer.

There is obviously a small but additive loss of performance on any cable length of 20 metres or more. The maximum cable length possible does depend on the area that the vessel will be operating in. Hard seabeds and shallower maximum depths allow for longer cable runs.

Transducer Position

The choice of transducer position will have a major effect on final performance so please consider carefully all factors and, if in any doubt, telephone the factory.

Motor Boats

  • On a planing hull, fit as far aft as reasonably possible. On V-drive and stern-drive boats (inboard /outboard), typically fit just in front of the engine(s). On a shaft-driven boat (not V drive), fit forward of the shaft log or stern gland but behind the gearbox.
  • Keep inboard of the spray rail if possible.
  • If possible, do not fit downstream of any other through-hull fittings (for example, intakes, log, toilet sea cocks, etc.).
  • The transducer should ideally be vertical in both planes at displacement trim. If it leans forward, the seabed appears to creep up and if it rakes aft too much surface clutter may show. Port/starboard heel will affect performance so avoid this if possible.
  • Avoid the temptation to fit the transducer too far forward. The transducer may leave solid water when at speed or when pitching. Also remember the transducer looks down as well as out, so when manoeuvring in confined waters you may wish to see there is water for the propellers and rudders.

Sailing Boats

  • On a fin-keeled boat, alongside the keel (perhaps 60 cm or 2 ft out) is often a well-protected position (but not on a boat with a winged keel or a large bulb). Just forward of the keel is an alternative, but take care when hoisting the boat in a sling!
  • Resist the temptation to fit the transducer too far forward. The transducer will come out of the water when pitching. Remember that the transducer looks out as well as down - you will need to see what is under your boat as well as what is ahead. When manoeuvring in confined spaces, you wish to know what is under the keel or rudder.
  • For long-keeled boats, choose the position with the minimum angle of dead rise (i.e. the flattest area) and fit the transducer with a wedge-shaped chock if necessary to make the transducer as near to vertical as possible with the boat upright.
  • In all cases, ensure the body of the hull does not obscure the forward or side view up to and above the horizontal

Installation

Ultrasonic signals travel well through solids and liquids but are greatly attenuated in air. This means that a stream of turbulence and air bubbles will limit the performance of an FLS. However, if a place can be found on the hull where there is a clear flow of water then good results can be obtained even at speed.

The best place to fit the transducer depends on the boat. In a sailing boat alongside the keel, on a motor boat if it is shaft-driven and twin-screw then a position between the gear box and stern gland is appropriate, and on a stern drive vessel just in front of the engines will give good results. Keep inboard of the lower spray rail if possible and not downstream of other through-hull fitttngs (e.g. intakes, log or toilet sea cocks). Some "heel" port or starboard is acceptable, to a maximum of 5 degrees. In the fore/aft plane, the transducer must be vertical. The transducer protrudes through the hull so that the transmitting face has a clear view forward. It is designed to break off cleanly, flush with the hull if it hits anything. The remains of the transducer are still in the skin fitting and so there is no risk of water ingress.

There are two types of transducer, both available with any FLS model. The larger professional transducer has a narrower beam, which gives a more accurate representation of the seabed, although the standard transducer can appear better simply because it is collecting data from a larger area. A thick hull or one with a steep dead rise will usually require the professional transducer as the bronze skin fitting has 3" of usable thread whereas the plastic standard skin fitting has 1" thread.

Transducers come with 12-metre cable lengths and 8-metre extensions are available.

One final thing - remember you cannot use two sounders or sonar at the same frequency on the same boat (or near another boat sometimes). One will transmit, the other will pick up its echo from the sea bed and both will be confused.

All EchoPilot vertical sounders run at 150 KHz. All EchoPilot Forward-Looking Sonars run at 200 KHz so they work well together. Brookes and Gatehouse use 183 KHz (again compatible) but Raytheon use 200 KHz so they are not.

It is essential to fit a switch to disable any other sounder without an on/off switch working at 200 KHZ. And when using the other sounder, switch the EchoPilot sonar off!