The ideal characteristics for a POD tow rig are that it must be:

• towable at the same depth irrespective of speed
• able to tow at up to 14km/h
• buoyant so that nothing is lost if it breaks
• seen easily at the surface if lost
• light and have low drag
• deployed without slowing the boat

All the above characteristics can be achieved using a skimmer rig, if a surface tow line is acceptable.

The rig consists of a diamond shaped skimmer board about 60 cm long, 35 cm wide, 4 cm thick is used. It is towed along the surface from one of the sharper corners and the two edges either side of this corner are ‘sharp’, while the two trailing edges are squared off. The POD housing is attached by a thin nylon line about 6 metres long to the back corner. The downward pull of the POD keeps the skimmer board stern-low so that it skims over the waves. It can be made of fibreglass, covering polyurethane foam, with attachment eyes at the front and back.

The POD is housed in a 110 mm plastic ground pipe with a hemispherical insert at the front with a central hole for the tow line, which goes through and is formed into a loop that can be attached to the POD to secure it. The 110 mm pipe is long enough to reach beyond the end of the POD by about 8 cm and the sides of the tube are cut away from the stern end far enough to give the transducer a clear view sideways.

A wooden bar about 2 cm square and 1 m long is clipped to the top of the stern of the tube using two screws in the bar and ‘keyholes’ in the tube. A groove in this bar holds an O-ring that goes around the POD transducer and pulls the POD forwards and the bar back, so that both are secured.

The hemispherical nose carries a hydroplane of stainless steel. This is about 30 cm long and bent at right angles into a strong dihedral. It is attached by two screws and is given a small angle of attack by a 3 mm nut under the leading screw.

In use, the POD has batteries only in the inner battery pack. The switch angle is set to allow it to be on at most angles.

The POD housing and skimmer board can be thrown over the side of the boat at the same time and brought into movement with some hand control of the tow line, which is let out to 100 or 200 metres as required. A light buoyant line is adequate. If the POD hits an obstruction underwater it floats to the surface with the wooden bar standing up vertically, which makes it easy to pick up.

The POD stays below the skimmer board by using the principle of a kite in reverse. Because of the low angle of attack of the hydroplane, it runs with an angle of around 20 degrees down, so that the POD does not feel the skimmer board tracking the surface too strongly.

An alternative would be to dispense with both the skimmer board and the hydroplane on the tow housing and force the towing line down near the boat. If the POD was just buoyant, it would tend to follow at the same level as the tow point behind and below the boat.