Drogue
- This article describes a device used in the marine environment. For the type of parachute used in aeronautics, see drogue parachute.
A drogue is a device to slow a boat down in a storm so that it does not speed excessively down the slope of a wave and crash into the next one. 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.
Use
Most drogues, are best deployed out of sync with the boat by one-half of the length of the prevailing waves, so that the drogue climbs a wave as the boat slides down a wave. Nylon rope is widely use for hauling drogues as it absorbs the shock loading best by stretching.
Weights such as chain are usually 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 bouy so that the drogue can be deflated before recovery. The trip buoy line is a floating bouy attatched to the top of the parachute cone which collapses the cone when pulled. In the case of series drogue lines, they are attatched to the end of the line. Trip lines are especially helpful on series drogues because of their difficult recovery.
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.
Parachute Variety
Drogues come in several varieties. Of these the parachute variety is the most common commercially manufactured type. A parachute drogue is generally constructed of heavy flexible material in the shape of a cone. Holes or strips are usually cut in the drogue for stability, to reduce loads on the material, or both. The length of the tow line must be adjusted as the distance between the waves changes.
Series Drogue Lines
Retired Aeronautical Engineer Don Jordan has developed a series of drogues now known as the Jordan Series. This series of drogue lines is essential a large number of small parachute drogues fastened upon a nylon rope. 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 reduces damage to deck fittings and reduces the chance of breakage.
The series drogue is advantagous in that it does not have to be adjusted during a storm. This is an excellent feature as sea conditions requiring a drogue are usually hazardous to be on deck. However, recovering a series drogue is more difficult since the entire length cannot be winched in; the numerous parachutes jam the winch. They are more difficult to inspect and maintain because of the many working surfaces. They are also more expensive since the series drogue is patented and only mass produced by one company.
Homemade Drogues
Studies undertaken by the U.S. Coast Guard have indicated that homemade 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.
References
Chapman Piloting & Seamanship, Elbert S. Maloney 2006