Chelonia Limited

  Cetacean Monitoring Systems

 

Vertical grapple

Theft of deployed PODs is a problem in some areas. One solution we have developed is the vertical grapple rig, that allows a deployment with no surface marker that can be retrieved without diving.

The mooring is a weight with the POD and a line with a few small pressure-resistant floats, such as those used on the headline of gill nets to hold it up to above around half the water depth at low tide (see diagram below).

Grapple and pressure-resistant floats

Vertical grapple deploymentTo retrieve the mooring a weight (1) is dropped at the position of the mooring using GPS or good sightlines.

The weight (1) has a light line that is wrapped around a buoyant board (3). and this 'unrolls' as the weight sinks and stops unrolling when the weight reaches the sea bed. It then remains at the surface as a marker.

The weight (1) also has a lifting/sweeper line (6) which is a stronger line and carries a length of light chain (4) which is attached at around half the water depth and keeps the line down.

The line (6) has a grapple (2) in the line next to weight (1).

Now the boat makes two circles around the mooring position - or around the marker (3). The rope handler tries to feel the chain (4) just touching the bottom, or tries to keep it above the ground if in a sensitive area - in which case a simple weight would be better than a chain. The boat then goes to a position above the mooring and the lifting line is hauled in. The grapple catches the vertical line with the floats and allows the whole rig to be lifted.

Then you untangle the three lines!

One big advantage of this rig is that, having only small sub-surface floats, it is not pulled by storm waves and is surprisingly quick!