Designers notes 3

 

 

 

    Donald J. Jordan

    Consulting Engineer

    113 Evergreen Lane

    Glastonbury, Connecticut

 

Reproduced with the kind permission of Donald Jordan


 

Sea Anchor Problems

Sailing couple Mike and Joyce Creasy reported that while riding to a parachute sea anchor, they waltzed through 40 degrees each side of the wind in a moderate storm. Heavy loads damaged the attachments of the rode, and the rudder quadrant was broken. Several letters commenting on this event appeared in the December issue. It is apparent from this literature that the authors were not familiar with the series drogue, often called the Jordan series drogue.

I developed this drogue in conjunction with the Coast Guard. It is the first and only such device to be specifically designed for a 'worst case' breaking wave strike. Such a strike is described in Miles Smeeton's book 'Once Is Enough'. Modern engineering tools such as scale model testing in flow channels and breaking wave tanks, computer dynamic simulation, and laboratory testing for structural strength and durability were used in the development, as well as full scale testing using a 42-foot Coast Guard powerboat. The final design was tested in large breaking waves at the Coast Guard National Motor Lifeboat School in Ilwaco, Washington. This work is described in U.S. Coast Guard Report C.G.-D-20- 87, Investigation of the Use of Drogues to Improve the Safety of Sailing Yachts., U. S Dept. of Commerce Nat. Info. Service, Springfield, VA 22191 ($22).

The series drogue has now been at sea for over 12 years. At least 500 - and possibly over 1,000 - are in use all over the world. Many skippers have made their own, a tedious but not difficult job. The drogue has been deployed in many storms, including at least two of hurricane strength. The record has been flawless. No boat has suffered any damage, no crewman has been injured, and the drogue has been retrieved in the as launched condition. Every skipper has been satisfied with the performance. This conclusively puts to rest the old fear of being pooped when held stem to the waves. There are simple and sound engineering reasons for this most remarkable performance.

There is a growing recognition among those using the drogue that 'storm tactics' are no longer required. When the weather deteriorates to the point where useful progress is impossible or even uncomfortable, they deploy the drogue and retire to the cabin with the knowledge that they are protected from anything the sea can bring on. The boat rides easily with less than 10° of yaw, and with a drift rate of 1.5 knots. The drogue loads are low, about 15% of the design load. The design load is only approached in the rare event of a 'worst case' breaking wave strike capable of catapulting the boat ahead of the wave. In this event the drogue is designed to align the semi-airborne boat with the wave, decelerate the boat, and pull it through the breaking crest without exceeding the allowable load on the drogue or boat.

Books such as Cole's 'Heavy Weather Sailing' - a favorite of mine for many years - are actually no longer pertinent. Understandably, this thought is bitterly contested by a few experienced sailors who regret the loss of the need for sea lore, judgment, and skill in handling their vessels in bad conditions. I am an aeronautical engineer and view the drogue as similar in function to the ejection seat on a fighter aircraft - you pull the handle and sit back until it is all over.

In the course of this program, I have studied the history of sea anchors and drogues. A sea anchor is intuitively attractive. It brings to mind anchoring in a harbor, safe and secure. Sea anchors have been carried on some sailing yachts over a long period of time. I have not been able to find a single instance where they provided protection in a major storm, and many instances when they contributed to the loss of the vessel. We now know that the sea anchors used were much too small to pull the bow into the wind.

When mulithulls began to go to sea in the 1960s, a number were capsized in conditions where a monohull would be expected to survive. This led to experimentation with sea anchors. I believe that the Casanovas were one of the first to try the large aircraft surplus parachute. They found that the chute would hold the boat into the wind in moderate storm conditions with little yaw and would prevent capsize. The cyclic loads on the rode were very high but a solution was found by providing a long and stretchy rode to compensate for the relative motion between the immovable chute and the boat. A number of multihull skippers have successfully used the chute in moderate storms.

This led to attempts to use the chute on monohulls. However the situation here is very different. A monohull is directionally unstable when moving backward because the center of pressure of the underwater surface is behind the center of gravity. As any skipper knows, it is possible to run off before a storm - but it is not possible to run off backwards as the boat will yaw. There are two additional sources of instability. The center of pressure of the air forces on the topsides and rigging is ahead of the center of gravity. There is also a third and more complex dynamic instability. The last two instabilities result in the behavior observed when a monohull is anchored from the bow in protected water during a hurricane. "It is particularly unnerving to watch a yacht tacking back and forth on a mooring under bare poles and knocking flat at the end of each tack," reported one who watched a monohull during hurricane Bertha. If the boat had been anchored from the stern, it would ride with little yaw.

When riding to a parachute sea anchor, a monohull will yaw wildly. As the storm increased in severity, it would develop load sufficient to break the rode. A sea anchor does not provide safety in a survival storm.

Although a multihull is also unstable when moving backwards, it is less unstable than a monohull because it has less underwater surface aft. This moderate instability is overcome by the stabilizing effect of the wide bridle - 20 feet or more - and the combination is stable. Thus the boat will ride well in a moderate storm. However, the large chute is essentially immovable, and in a survival storm will develop loads sufficient to break the rode. At six knots, an 18 foot chute would develop a load of 30,000 lbs - if it didn't fail first. The series drogue would only have a load of 900 lbs. The series drogue will protect both the monohull and the multihull in a survival storm.

I would be glad to answer questions via email and to provide supporting documents where feasible.

Donald J. Jordan, Consulting Engineer
Glastonbury, CT



 

 

 

 

 

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