Designers notes

 

 

 

Reproduced with the kind permission of Donald Jordan


 

 

By Don Jordan, inventor, retired aeronautical engineer

The series drogue was developed to perform two separate functions:

1. To prevent the capsize of monohull and multihull sailboats in the event of a large breaking wave strike.

2. To improve the motion of the boat in storm waves and to reduce drift.

Most storms do not generate dangerous breaking waves. A vessel may go through a lifetime of cruising without being struck by a breaking wave even though hurricane winds have been encountered.

 

Although storm waves move at speeds up to 30 knots, the water in the wave moves at a much lower speed. A boat lying ahull is not subjected to high forces. Experience and testing have shown that a well found monohull with positive stability at 90 degrees roll angle has little risk of being damaged by non-breaking storm waves. A multihull, however may be capsized.

 

A dangerous breaking wave is formed by the interaction of two or more storm waves. This type of wave has a large mass of water at its crest moving at wave speed (20-30 knots). When a vessel lying ahull is struck by this moving mass of water, a very large force is developed. In a typical event the boat has been successfully riding out the storm for many hours, then, 10 seconds later it lies dismasted and damaged. It is the function of the drogue to turn the stern into this moving mass of water and pull it safely through.

The risk of breaking wave capsize is dependent on the weight of the vessel, with small light boats being at high risk. Above a length of 40' the risk is diminished, and above 60' few, if any breaking wave capsizes are on record.

On a conventional monohull sailing yacht the underwater lateral surface is located aft of the center of the boat while the topside area is greater towards the bow. When struck by a moving mass of water the bow of such a vessel is driven down by a powerful turning moment. Therefore it is necessary to use a drogue from the stern rather than a sea anchor from the bow to align the boat with the moving water and pull the boat through.

A multihull is relatively symmetrical fore and aft, although thre is still some tendency for the bow to be driven down by the wave. Testing indicates that either a drogue or sea anchor, if properly designed, can be effective in preventing breaking wave capsize. However, the force required of the drogue is less than that required of a sea anchor.

Some sailors have expressed reluctance to use a drogue for rear of being "pooped". Testing has shown that a conventional monohull or multihull will perform in a safe manner when riding stern to the sea. Actually, the stern generally has more local buoyancy than the bow and will rise quickly to a steep sea. However, storm waves will have whitecaps containing some moving water and this may splash aboard.

In a dangerous breaking wave strike, moving water may sweep the cockpit and strike the companionway doors. This is unavoidable, and is a necessary corollary to saving the vessel.

Adding a light line in parallel with the drogue to permit the drogue to be pulled in backwards is definitely not recommended since it complicates the gear and may lead to fouling under critical conditions.

 

donaldjordan@worldnet.att.net
Phone: 1-860-633-1702
Donald Jordan
113 Evergreen Lane
Glastonbury, CT 06033

 

 

 

 

 

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