Jordan series Drogue Attachment

[Wallaby]

One way might be to size your attachment points for the breaking load of the line to the drogue. The loading on the attachment will depend on how it is set up.

For example on my 36K lb boat, the line is 1" double braid nylon (once used to tether a hot-air balloon)--30K lbs min tensile strength. When I finally get around to completing the install, I will use two trapezoidal 3/8" stainless steel plates, 12" long by 9" high x 3", attached to the hull so the narrow end will project past the transom about 2" (in order to fit shackles) Each plate has 3/8" bolts, to backing plates. The load is in the plane of the attachment plate, so the bolt numbers have to be adequate to resist the shear force. see table 6 http://www.ssina.com/download_a_file/fasteners.pdf
Assuming a 15k lb load, I require a minimum of one square inch of bolt cross section.

[ImaginaryNumber]

Originally Posted by estarzinger

so, something that is perhaps not so commonly understood about series drogues . . . is that there are two distinct load cases....
-------------------------------------
www.bethandevans.com

Evans,
Sorry to drift off-topic, but is your interesting and useful website permanently off-line?

[kmacdonald]

No one has mentioned an appropriate factor of safety in the design. Generally speaking a FS of 3 would be a minimum and generally when human life is at stake in the case of failure a FS of 10 or more is used. Now determine what is needed in terms of line and attachment points. You will quickly find it's not feasible to design for worst case scenario.
Fatigue should also be considered.

[thinwater]

Originally Posted by estarzinger

so, something that is perhaps not so commonly understood about series drogues . . . is that there are two distinct load cases.

The first load case would occur in a semi-typical strong gale/storm - where you have strong winds, large waves with decent breaking crests. The boat speed will accelerate a bit down the waves and when hit by the crests, but generally not be way above hull speed. In this case, the series drogue loads are actually quite manageable - on Hawk (47' mono with 150 cones) we measured peaks of 3500lbs (which is consistent with the USCG findings for this situation). We might have been doing 7ish knots and perhaps accelerated up to a max of 11. Note: as a point of reference - this is quite similar to our max #3 jib sheet loading.

A stern cleat (and deck) should (on the size boats we are talking about here) actually be able to hold 3500lbs. We actually used both cleats and our primary winches (at different times) for holding the bridle lines. We never had any problems. (our cleats were 14" aluminum welded to deck with doublers, so probably unusually strong, but our winches where just pretty typical and typically mounted).

Cruisers would consider this a quite serious storm. You can make a standard tropical circumnavigation, and summer higher latitude cruising and never encounter these conditions, but you might.

The 2nd load case is rather more difficult to deal with. It is if you encounter an enormous breaking wave. In this case the boat can possibly be accelerated to breaking crest speed (quite quickly), or understood a different way - the boat falling down a 45-foot near vertical breaking wave. The physics of this are pretty impressive. Commercial vessel experience confirms this order of magnitude loading . . . . these unusual waves can (worst case) exert a pressure of about one hundred tons per square meter and other testing has found somewhat 'common' (in extreme conditions) wave loading of about 100,000-170,000 lbs/sq meter for 7 seconds on static structures. The 1,000-foot cruise ship Norwegian Dawn encountered a 70-foot wave that broke windows designed for 5 ton/sq meter loads up to the 10th floor.

Now these sorts of conditions are (fortunately) extremely rare. We never encountered this sort of worst case in two circumnavigations, including long distances in the southern ocean and above the arctic circle. However, they do happen. In yachting terms the sydney to Hobart storm was such a situation - now, it should be noted that no cruiser should have been caught in that if they were paying attention to the weather. But if you do get caught in these sorts of worst case scenarios it is difficult to tell what will break. In the sydney to hobart storm, there were decks crushed and windows broken in. I'm guessing a typical series drogue will be shredded (no drogues were deployed in that storm). The engineering calculations for the series drogue have suggested that you need to be able to hold between 50-100% of vessel displacement (large range and take with grain of salt, but that is the best rough estimate). Essentially this means that (if you want to engineer for this worst case) you want to be able to lift the boat from the drogue attachment points.

We had drogue tangs engineered into hawk which could take this loading. However we never used them. Honestly for a typical cruising voyage, with prudent weather navigation, I would suggest engineering for case #1 is sufficient. And if you want to engineer for case 2 you have to (to be consistent) think about what else will break in such possibly crushing loading.

The limiting force, however, is not the water hitting the boat, but rather the maximum speed the boat will attain while pulling the JSD. JSD dragt is the intentional the weak point. That is the a part of the beauty of the design, because if it did not give, the boat would have to. Yes, they are related, from a point of view. In "normal storm conditions," 11 knots sounds reasonable and 3800 pounds is what I measured during at that speed (few er cones, adjusted). That squares very well with the above figure. However, a large breaking crest could hit 25 knots and that force would increase over 4 times to 20,000 pounds. But would the JSD allow the boat to accelerate to that speed? No. But the actual speed and load, somewhere in between, as Starzinger points out, is impossible to accurately gauge.

I shall repeat, I am NOT arguing against JSD straps. These are big forces and a chain plate are the best way to manage that much force in shear. That's why masts are supported by chain plates. I just wanted to add some data about cleats and how strong they can be, if properly mounted. They can be strong enough for other drogue types, which generate 2-3 times less drag and are typically used in less severe conditions. Curiously, such data is very seldom given by manufacturers. Very strange, IMHO.

[sailnautilus]

We attached "drogue towing" stainless steel attachment points when we refit our aluminum Kanter 52'.
These are stainless straps extending off the stern, through bolted to identical backup straps inside the hull about two feet along the side of the hull.
We have practiced with the drogue but never used it in conditions in excess of 60 knots of wind. Worked perfectly in what we experienced.
Additionally, these have proven handy for towing any other disabled boats and even towing our tender.
I will attempt to attach a pic where you can see these on the stern. Horizontal straps midway from waterline to toe rail, chose the place where would get least chafe.

[Boston Blackie]

I also agree with Evans that bespoke plates as specified by Don Jordan's brilliant and detailed design engineering and testing are needed. Ours are 3/8 316L with 3/8 bolts. The balsa coring was routed out and epoxy spoge filled. An oversize 1/4" aluminum backing plate was used. The eye for each 7t WLL bow shackle was waterjet cut oval to allow the shackle clevis to go through the cast SS thimble and swivel with the bridle line to prevent weakening of the shackle by off-axis loading.

Please excuse the photo orientations (but these do simulate Evans' worst case scenario 2).

[SVNeko]

Originally Posted by kmacdonald

The existing cleats are no where near strong enough unless specifically designed for that purpose. There is plenty of information available that will show you how to determine the max load you need to design for. Once you have that get it engineered by some one qualified, not a forum.

Why would that necessarily be the case? My cleats are 13" with 3 12mm bolts through solid fiberglass backed by large SS plates. The pull would be in shear which I believe would be the strongest orientation. I do not think the specifically designed attachments I have seen would be any stronger.

[Lepke]

Coming from a commercial fishing background, attachment points need to hold in the worse case storm. Failure of one part could leave the vessel hanging sideways to the swells. I have seen other boats with cleats (and backers) torn out, anchor winches pulled free, and steel plate peeled up in boats that survived. Others disappeared. Pre Epirb days.

[foojin]

This has me thinking.... quickly googling I did not find one, but is there a make of cleat that might be long, have four or five large through bolts with a large backing plate that would perform like a chain plate?

Of course deck strength would come into play, but would provide an interesting point to begin calculations.

[thinwater]

Perhaps the greatest advantage of JSD plates and chain plates in general is that that they spread the stress over a larger area. Unless the laminate is very purposefully reinforced in critical areas, it isn't intended for these loads. Second, the failure strength of the bolt can be secondary to the laminate failing is shear, and the boat ripping sideways. A 1/2-inch bolt may be monstrous strong, but it consent-rates a lot of force in a small area. A strap, on the other hand, spreads it along. Backing plates protect against pull-out and restrain the bolt heads, but unless they are well-bonded or are built-up glass, they don't do much for shear.

In fact, some of the JSD straps I see are probably too short. They need to be long, like chain plates. Alternatively, because most of the load is shear, I would like to see a large area of built-up glass under the backing plate, adding perhaps 3/16-inch. This is really more important than the thickness of the plate, and can add more strength.

[kmacdonald]

I have never heard of or read about an attachment point failure with a JSD. All failures were chafe or cone failures. This indicates to me that no one has experienced a worst case scenario or at least never lived to tell about it. I don't plan on getting blindsided by a hurricane so don't plan on designing for such an event. I do plan of eliminating chafe as best as possible.
The best and proven way to eliminate chafe is the use of stern mounted chain plates.

[kmacdonald]

Thinwater, the hull just below the deck might be built up some with the deck flange. That is another plus for chain plates in that area.

[Formosa46AK]

No opinion here, though I agree after much reading on the subject that you should use a chainplate.
Attached, at least I think it's attached, is a drawing I made of what I ordered. It is close to scale.
I researched looking for the usual/proper dimensions for my 46 mono at 17 tons. Finally I used the size chainplate for a 49 foot, approx 20 ton boat. It is oversized. I could have contacted a marine engineer for probably $500, maybe $300, but I chose to spend the $3-500 on the oversized chainplate.
The man who made the pieces on the Texas Gulf Coast said he has made many of these. He also said it is oversized (I'm happy with that). He recommended the carriage bolts on the outer plate. He said the rounded heads on the outside look much better than the squared off hex bolt heads. And they didn't 'catch' on other boats or surfaces.
Take this for what its worth.

Attachment 146241

[kmacdonald]

I think to much emphasis is on worst case scenario loads and not enough on chafe. Eliminate the know points of failure. Worst case is not survivable. Ships don't even survive.

[Boston Blackie]

Good points Thinwater. I forgot to mention I added 4 layers of 10oz glass stepping down 1/2" all around on each ply epoxy bonded in a single go to the inner hull layer which was sanded free of the white finish. The aluminum backing plate covers the laminated buildup.

The oval slot feature is also important as shackle WLL decreases substantially with loads which are not orthogonal to the clevis axis according to Crosby.