Drogue

This article is about a device used in the marine environment. For the type of parachute used in aeronautics, see drogue parachute.
A drogue

A drogue (also known as a storm drogue) is a device external to a boat, attached to the stern and used to slow the boat down in a storm and to keep the hull perpendicular to the waves. The boat will not speed excessively down the slope of a wave and crash into the next one nor will it broach. By slowing the vessel in heavy weather, the drogue can make it easier to control. A drogue is usually constructed to provide substantial resistance when dragged through the water, and is trailed behind the vessel on a long line. A properly functioning drogue should also prevent pitchpoling.

Use

Most drogues are best deployed out of sync with the boat by one-half of the length of the prevailing waves; thus the drogue climbs a wave when the boat slides down a wave. Nylon rope is widely used for hauling drogues since it best absorbs the shock loading by stretching. However, new research indicates that using rope with less stretch accompanied by chain weight helps to maintain constant force on the deployment rode rendering storm drogue use more effective.[1]

Weights such as chain are also employed to keep the drogue from breaching the surface of the water and skimming across the top. In addition, experienced boaters add a floating trip buoy so that the drogue can be deflated before recovery. The trip buoy line is a floating buoy attached to the top of the parachute cone which collapses the cone when pulled. In the case of series drogue lines, they are attached to the end of the line. Trip lines are especially helpful on series drogues because of their difficult recovery. Although the trip line concept is a derivative of the parachute sea anchor, evidence demonstrates that such a setup is not effective with the storm drogue.[1]

While similar in design, the sea anchor is quite different in application from a drogue. The sea anchor is usually much larger, is intended to slow the vessel to a near complete stop, and is usually deployed off the bow (front) of the boat so that end is presented to the oncoming waves.

Speed-limiting variety

Speed-limiting drogues are single-element devices. They come in several varieties of canopy shapes that are circular in shape like a big round basket. Some are built with solid fabric while others are open in design to permit water to flow through them more readily. Holes or strips are usually cut in the drogue for stability, to reduce loads on the material, or both. Currently, the speed-limiting drogue is the most commonly used storm drogue with many designs available in the market place. See also drogue parachute.

Series drogue

Retired aeronautical engineer Don Jordan tested what is now known as the series drogue, originally conceived and patented by E.J. Pagan and later patented by Sidelnikov in 1975;[2][3] however, before his tests, numerous mariners had experimented with pulling several large drogues in series. Like Sidelnikov, Jordan expanded upon this idea, and affixed a large number of small parachute drogues to a nylon rope with a weight at the end. The large number of smaller drogues results in there always being a drag force on the line; it does not have to be adjusted to be in phase with the waves as the drag is spread out over many waves. Because the drogue line is prevented from becoming slack there is no jerking or snapping of high loads on the line. This prevention reduces damage to deck fittings and reduces the chance of breakage. The number of small parachutes, the length and thickness of the line, and the size of the end weight are all matched to the displacement of the boat. Another key design feature is the V-bridle. The two attachments should be made at the outer corners of the transom with the lengths of the two bridle lines being 2.5 times the width between the attachment points. According to Jordon, special reinforcement is required for the bridle attachment since Jordon projects that forces of 7,000 lb. to 27,000 lb. and even higher can occur with a breaking wave strike.[4] With this deployment no steering of any kind is needed.

The series drogue does not have to be adjusted during a storm. Neither do other storm drogues if they are fully deployed and they adhere to the constant rode tension theory.[1] As sea conditions requiring a drogue are usually hazardous to be on deck, it’s usually smart to fully deploy all of the rode associated with a storm drogue. Also, the series drogue can be deployed safely with one hand from the cockpit as can any other storm drogue. Recovering a series drogue before the storm abates takes effort, but the process is safe and straightforward. It can be winched in on sheet winches if the cones are small enough to travel around the winch drum without jamming. The series drogue is currently made by three manufacturers, one in Australia, one in the United States and one in the United Kingdom. Any sailmaker can make one and you can make one yourself, though it is a tedious job.

Jury-rigged drogues

Studies undertaken by the U.S. Coast Guard have indicated that drogues made of old tires, long lengths of chain, etc. are not effective in slowing most vessels. Old tires may skim along the surface at storm speeds. Extremely long lengths of chain are required for any appreciable drag effect from chain alone. Nevertheless, these drogues continue to be used.

In fiction

In Admiral Hornblower in the West Indies by C. S. Forester, a drogue is secretly made up at night by Hornblower's crew and covertly attached to the rudder of a slave ship in order to slow it down after it leaves its safe harbor the following morning. This is to allow Hornblower's ship to overtake the otherwise faster slaver and free its captives. This particular drogue is made of sail canvas and weighted by an anchor.[5]

References

Wikimedia Commons has media related to Sea anchors.
  1. 1 2 3 Smith, Zack (2011). Shark Drogue Manual, 2nd edition. ISBN 978-0-9830080-0-2
  2. ↑ Shewmon, Daniel (1998).The Sea Anchor and Drogue Handbook. Published by Daniel Shewmon
  3. ↑ Taylor, Roger D. (2010). Mingming & the Art of Minimal Ocean Sailing. Troubador Publishing Ltd. p. 325. ISBN 978-0-9558035-1-2. Retrieved 28 October 2010.
  4. ↑ Jordon, Donald (1987). Investigation of The Use of Drogues To Improve The Safety of Sailing Yachts.
  5. ↑ Forester, C. S. (1958). Admiral Hornblower in the West Indies. Little, Brown and Company. pp. 75–78.
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