Designers notes 2

 

 

 

    Donald J. Jordan

    Consulting Engineer

    113 Evergreen Lane

    Glastonbury, Connecticut

 

Reproduced with the kind permission of Donald Jordan


 

Structural Requirements for Series Drogue Attachments

 

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 ¼ 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                         5,000

20,000                         10,000

30,000                         14,000

40,000                         17,500

50,000                         21,000

 

 

Series Drogue

Bridle Design Requirements

 

Displace        Total Load         Bridle Load        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

Donald J. Jordan

donaldjordan@worldnet.att.net
Phone: 1-860-633-1702

 

 

 

 

 

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