Historically, the durability of drogues and sea anchors, when deployed under severe storm conditions, has been very poor. The equipment either breaks loose or tears apart after a relatively short exposure to heavy seas. Recent tests of model drogues in the circulating water channel at the U.S. Coast Guard Academy, Section 3.3 of this report, investigated the dynamic behaviour of several drogue designs and provided an insight into the probable reason for the early failure of these devices in service. It was found that conventional cone and parachute type drogues alternately fill and collapse, sometimes reversing direction or tumbling. It is this violent motion which can cause structural failure.
A new type of drogue, called a series drogue, was developed as part of this program. A typical series drogue consists of ninety 5-inch diameter sailcloth cones spliced into a 150-foot nylon towline as shown on Figure 16. The end of the line is weighted with an anchor. Model tests, as previously discussed, showed that the series drogue would not foul or turn inside out under simulated storm conditions but the individual sailcloth cones would fill and collapse with the passage of each simulated wave.
The objective of the tests described in this report was to subject the series drogue to the same cyclic loads and motion that would be encountered in a major storm and to investigate the performance and durability.