Why the U.S. Coast Guard thinks the series drogue is better than a para-anchor in storms.
The two conventional drogue configurations are the cone drogue and the parachute drogue/sea anchor. Both types have been used successfully in a variety of applications. A third type of drogue called a series drogue has been developed as part of this investigation. The series drogue is intended to provide near optimum performance under storm conditions and to avoid some of the problems encountered with cone and parachute drogue/sea anchor.
The series drogue offers the following desirable features:
If pre-rigged and coiled down into lazeret, the drogue is simple and safe to deploy under difficult storm conditions. The boat, under bare poles, will be either running off lying ahull. The anchor can be slipped over the stern and the line payed out. The drogue will build up load gradually as it feeds out.
It is almost impossible to foul it or entangle it enough to make the drogue ineffective.
The drogue ride-s beneath the waves and is not affected by the following sea even if a wave should break in the vicinity.
There are cases on record where a cone drogue has been pulled out of the face of a following wave, and even instances where the drogue has been catapulted ahead of the boat. It is difficult to weight a cone or parachute drogue so that it will ride at a sufficient depth to avoid the wave motion. As discussed previously in this report, a weight causes the drogue to collapse when the towline goes slack.
When the boat is in the trough of a large wave, the towline tends to go slack thus permitting the boat to yaw. With the series drogue, the anchor sinks pulling the drogue backwards and taking some of the unwanted slack out of the towline.
When a breaking wave strikes, the drogue must catch the boat quickly to prevent a broach. The series drogue, since some of the cones are near the boat where towline stretch is low, will build up load faster than a conventional cone or chute at the end of the towline/bowline. A computer study shows that two seconds after wave strike, the series drogue will develop 40% more load than an equivalent cone or chute. Similarly, if the breaking wave strikes at an angle to the towline rather than directly astern, the series drogue will build up load much faster than the conventional types.
The series drogue is durable as demonstrated by the testing described in this report. The load on each individual element is low. No single failure can make the drogue ineffective.
The series drogue can double in function as a spare anchor line and can use the boat's regular anchor as a weight. All 90 cones weigh only four pounds.
U.S. Coast Guard Report CG-D-20-87 sec 6-5....
Series Drogues and boat design
“With a series drogue deployed, a well-designed and properly constructed fibreglass boat should be capable of riding through a Fastnet type storm with no structural damage. Model tests indicate that the loads on the hull and rigging in a breaking wave strike should not be excessive.
Many sailors are reluctant to deploy a drogue from the stern because they fear that the boat may suffer structural damage if the breaking wave strikes the flat transom, the cockpit and the companionway doors. The model tests do not show this to be a serious problem. The boat is accelerated up to wave speed and the velocity of the breaking crest is not high relative to the boat. The stern is actually more buoyant than the bow, and will rise with the wave. However, the boat may be swept from the stern. The cockpit may fill and moving water may strike the companionway doors. The structural strength of the transom, the cockpit floor and seat, and the companionway doors should be checked at a loading corresponding to a water jet velocity of approximately 15 ft./sec.
When a boat is riding to a series drogue no action is required of the crew. The cockpit may not be habitable and the crew should remain in the cabin with the companionway closed. In a severe wave strike the linear and angular acceleration of the boat may be high. Safety straps designed for a load of at least 4g should be provided for crew restraint. All heavy objects in the cabin should be firmly secured for negative accelerations and drawers and lockers should be provided with latches or ties which will not open even with significant distortion of the hull structure”.
U.S. Coast Guard Report CG-D-20-87 sec 6-4