SUPPORT > TOWING GUIDELINES

Towing the StarFish sonar is relatively straightforward, but there are a few guidelines and precautions you should observe to achieve the best results...

• Let the boat build up speed to 1-2 knots before putting the sonar in the water. This will help avoid getting the towing cable caught in the propeller.
• Keep the boat speed constant, and ideally between 1-4 knots for the best results. The slower the boat travels, the deeper the sonar will tow. At slower speeds more display lines will be acquired for smaller targets (giving a higher resolution image).
• When towing in rivers and tidal channels, remember that the speed of the water current should also be taken into account. For example, if the water is flowing at 5 knots, and the boat is travelling against it at 3 knots, then the water speed against the sonar is 8 knots.
• The faster the water travels, the more force is exerted on the sonar. The recommended maximum towing speed is 8 knots, but better images will be observed at slower speeds.
• Tow the sonar from the stern of your boat, to reduce the possibility of it receiving echoes reflected back from the boat hull.
• Tow the sonar away from the prop-wash (to one side of the boat, for single propeller vessels). The propeller disturbs the water and introduces air into it. If the sonar is towed in this, image quality will suffer.
• Paying out more cable will cause the sonar to dive deeper - the exact depth depends on the amount of cable in the water, the towing speed and the towing position on the sonar. Typically towing depths between 3m and 10m can be achieved, depending on tow speed.
• Tow with enough cable so the sonar is operating below the keel of the boat, but try and keep the sonar at least 3m above the seabed (or higher in known rocky situations).

There two main types of submerged hazard to avoid when using your StarFish sonar are...

Sudden changes in the seabed height (or submerged targets).

The sonar transducer head will only show you the depth of the seabed below it, not from the surface, and not in front of it. So, if you are scanning in close proximity to the seabed, you should be keeping a close eye on the sonar display, and always be ready to shorten the towing cable length to avoid collisions between the sonar transducer and the seabed.

This also applies to submerged objects, such as shipwrecks, that may suddenly appear in front of the sonar. If in doubt of the underwater terrain or possible hazards, always assume the worst case, and give plenty of clearance between the sonar and the seabed.

Mid-water targets

When navigating around objects (such as buoy's, or moored boats) or through busy areas, be aware that the sonar may be up to 20m behind the stern of your boat. Give yourself plenty of room for the sonar to manoeuvre in, and keep a safe distance between you and other vessels.

If passing a buoy, or other object that may be anchored to the seabed, give consideration to the cable, rope or chain extending below this, and don't assume it hangs vertically down beneath it. Water currents may cause the tether to be at an angle, to its mooring on the seabed, and if enough clearance isn't given, this may foul or sever the tow cable.

As the above figures show, you should not rely on the sonar as a navigational aid for your vessel, or for avoiding shallows, submerged or mid-water hazards.

The seabed information it shows is valid only below the sonar, which may be some distance behind the vessel. See the section on interpreting side-scan imagery, for further details on calculating the sonar's towing depth and altitude from the displayed data.