Occasionally divers get separated from their boat or drift out from the shore and find themselves alone at sea. There have been several such incidents reported in the international press and there is even a film, Open Water, about one of them. Many more incidents are never reported.

When such an incident does occur it is essential to call for help as soon as possible but the understandable sense of relief felt by those on the scene when the Coastguard arrives is often short-lived.

Even having professional emergency services take over the search is no guarantee that the divers will be found quickly. You may have provided accurate information about where and when the divers were last seen and the Coastguard will have up to date tidal information for the area. Put the two together and you would be forgiven for believing that a computer could predict with reasonable accuracy where the missing divers should be.

In practice this has been found not to be the case and in one incident off Rathlin Island, the most northerly point of Ireland, two missing divers were eventually picked up by a yacht miles away from where they were predicted to be.

While the divers were happy to have been rescued the Coastguard were keen to learn why their estimation of the divers’ position had been so far off. They hypothesised that despite the fact that the divers were submerged apart from their heads, wind must have had some effect too. This would mean that tidal calculations, no matter how accurate, would not be enough to find a body in the water.

Determined to test this hypothesis they set up a series of experiments that they called Orange Harvest.

Stage 1

The first experiment took place in an enclosed inland water area, Lough Neagh, and used weighted dummies. This site was chosen for the lack of current as it was hoped to reduce this variable altogether. However, it had its limitations in that being fresh water buoyancy would be different from that experienced in the sea. Additionally the wave pattern was different and the dummies could not experience any real effect of waves breaking.

Nevertheless the data collected was enough to justify further experimentation in the open sea.

Stage 2

In an attempt to reproduce the effect of a person in the water weighted dummies were once again employed, this time in Belfast Lough. The timing was essential as the only way to eliminate the effect of the tide was to carry out the experiment at slack water. In this case a monofilament net was dumped into the water alongside the dummies to act as a control. It would not be affected to any degree by wind. As the dummies became separated from the net it became apparent that the hypothesis was correct and that leeway did have a measurable effect on the behaviour of objects in the water.

Stage 3

As successful as the experiment was there can be no substitute for the real thing and so the Coastguard decided to repeat their experiment with live subjects in the water. They contacted the British Sub Aqua Club and subsequently a small group of divers were set adrift. Put into the sea at slack water, again hoping to eliminate any effect of tide, the divers were accompanied by more monofilament net. There position was logged every 30 minutes and after four hours they had separated for the net by half a mile. Interestingly it seemed to make little difference as to what kind of equipment the divers wore or whether they remained upright or lay spread-eagled with fins pointing skyward. They were all affected by up to 2% of the prevailing wind speed.

Stage 4

Having satisfied themselves that wind would have to now be taken into account when determining the drift pattern of a person in the water the Coastguard were also faced with the irrefutable fact that while the divers were not where they would have otherwise have been expected to be, neither was the net! As it would not have been affected by the wind, it seemed clear that local tides behaved in a manner that was not completely in sync with admiralty predictions. While these small and isolated differences may not have a significant effect on shipping they did seem to have an effect on bodies in the water.

Stage 5

The Coastguard decided to recreate the incident that had occurred at Rathlin Island. On this occasion boxes of oranges were dumped into the sea along side the divers. Close to being neutrally buoyant they would not be susceptible to any leeway although as luck would have it there was little wind on the day of the experiment. Without the wind the Coastguard could now expect the oranges and divers not to separate. This proved to be correct and after 4 hours and 13 miles later, the divers were throwing oranges at each other to relieve the boredom. What did happen, however, was that both oranges and divers had drifted some distance from where they would have been expected to be and the discrepancy was carefully logged.

The information collected from Orange Harvest has ramifications for every organisation searching for divers who go missing on the surface.

Without a doubt it will ensure that if divers ever again go missing at Rathlin the Coastguard will be looking in the right place. If you ever plan to get separated from the boat, that is the place to do it.