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
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
.