This page is made up of Designer’s notes and a letter to skippers by Don Jordan. We are grateful to him for his permission to reproduce this information.

DESIGNER’S NOTES

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.

Structural Requirements for Series Drogue Attachments. Letter to Skippers.

Dear Skippers,

I am sending this note to all purchasers of the series drogue for whom I have addresses. I want to make sure that you all have a clear understanding of the loads that the drogue may impose at the attachment points.

Several boats have ridden out severe storms including one hurricane with the drogue deployed. The drogue performed as it should. The crews reported that they did not feel threatened and the drogue loads appeared to be moderate. However, none of the boats was struck by a dangerous breaking wave such as capsized the yachts in the Fastnet storm or the recent New Zealand storm. On some of the boats which used a series drogue the bridle was led through a chock and belayed on a sheet winch. This arrangement is suitable for non breaking waves but it may not be adequate for a dangerous breaking wave.

For a boat displacing 30,000 lbs. Model tests and computer simulation predict that the drogue can generate a force approaching 20,000 lbs. When struck by a very powerful (and fortunately extremely rare) breaking wave. The tests also show that when the boat is struck on the quarter, one leg of the bridle will be subjected to 70% or 14,000 lbs Thus the attachment point should have the capability of carrying a once in a lifetime load of this magnitude.

I have no information on the ultimate strength of a typical sheet winch installation and it would be difficult to evaluate each unique mounting. Unfortunately a winch (or a cleat) is not an ideal structure, since the load is applied above the deck line and tends to overturn the winch and tear it out of the deck. The optimum attachment for the drogue is clearly a strap similar to a chain plate, bolted to the hull at the corners of the transom as shown in the sketch. This arrangement feeds the load directly into the hull and imposes no bending or pullout loads on the hull or deck. For a load of 14,000 lbs a strap 1⁄4 x 2.25 x 18 inches attached by six 3/8 inch bolts would provide a conservative design.

Such a strap is relatively inexpensive and should not be difficult to install. You may never need it but it is prudent policy to insure that the full capability of your series drogue can be achieved. Miles Smeeton in his book “Once Is Enough”, which many of you have probably read, presents one of the best descriptions of the power and unpredictability of a breaking wave. It is this extreme case that the series drogue is developed to handle.

Listed below is a table of design loads for a single bridle attachment. These loads are believed to be the worst case loads with some margin. However, since the loads are determined by the size and shape of the worst wave there remains some uncertainty as is the case with many natural phenomena.

Displacement

Single Bridle Load lbs

10,000 lbs

5,000 lbs

20,000 lbs

10,000 lbs

30,000 lbs

14,000 lbs

40,000 lbs

17,500 lbs

50,000 lbs

21,000 lbs

SERIES DROGUE BRIDLE DESIGN REQUIREMENTS

slide1a

Displacement
lbs


Total load lbs


Bridle load lbs


Rope diam.


Shackle size

10,000

8,000

5,000

5/8″

3/8″

20,000

14,000

10,000

5/8″- 3/4″

1/2″

30,000

20,000

14,000

3/4″

5/8″

40,000

25,000

17,500

7/8″

3/4″

50,000

30,000

21,000

7/8″

3/4″

I would welcome any comments or suggestions.

Pleasant sailing

Don 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.

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.

Donald J. Jordan, Consulting Engineer