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Old 10-02-2015, 03:22   #1
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Drogue bridle legs attachment - plenty strong?

Hi there,

So I built my own Jordan series drogue, and am looking at making some attachment points on the hull atm for the bridle legs. Each bridle leg is calculated to be subjected to 7000lbs in a worst case breaking wave strike in the worst possible conditions (highly unlikely).
Here's what I have laying around and was thinking:

A 1ft section of 3mm SS strap through-bolted to the steel toerail with 3 SS bolts of 10mm. The toerail itself is 3-5mm thick steel (it had some rust that has now been treated), and an integral part of the steel hull of the boat.

Will this be plenty strong in your opinion?
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Old 10-02-2015, 05:07   #2
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Re: Drogue bridle legs attachment - plenty strong?

You don't give a width for the SS strap, or BACKING plate... BUT...Beyond a shadow of a doubt....

Only thing to be concerned with is the integrity of the hull locations to adjoining sections...

One 10mm 316 bolt in tension has a minimum yield strength of 7000 lbs-ish...
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Old 10-02-2015, 06:46   #3
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Re: Drogue bridle legs attachment - plenty strong?

I wasn't really planning on using a proper backing plate as the tension will be in line with the strap (the force applied won't try to pull the strap off the hull in a 90 angle, if that makes sense at all). The idea was to use big washers, one for each bolt.

The integrity of the hull is fine, I did exaggerate a bit... There are a few bad spots along yhe entire toerail, but nothing serious. I also still need to drill the holes, so I can choose the prime spots for those...

Width of the strap is about 2-3 inches
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Old 10-02-2015, 06:53   #4
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Re: Drogue bridle legs attachment - plenty strong?

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Originally Posted by Orchidius View Post
I wasn't really planning on using a proper backing plate as the tension will be in line with the strap (the force applied won't try to pull the strap off the hull in a 90 angle, if that makes sense at all). The idea was to use big washers, one for each bolt.

The integrity of the hull is fine, I did exaggerate a bit... There are a few bad spots along yhe entire toerail, but nothing serious. I also still need to drill the holes, so I can choose the prime spots for those...

Width of the strap is about 2-3 inches
Ahh.... In a shear load... Strap with a hole cut in it for the connection???

Way strong enough for anything you are saying....
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Old 10-02-2015, 07:06   #5
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Re: Drogue bridle legs attachment - plenty strong?

Low cycle strain will likely be significant for you proposal. Static loads and calculations merely provide a starting point for this type of design. Factors of safety only apply to static loads.

I'd suspect the loading vector on your joint to also move around significantly due to vessel motion in a swell relative to your drogue load.

The toe rail at 3mm is a red flag as it will also experience low cycle fatigue possibly in bending. You also mention a single shear joint which is never a good design for dynamic loading.

Spreading the load in the boat structure also requires something more than a few washers.

What about a flexible joint? Spliced nylon or dyneema to a cleat. Is that possible? A tension line self aligns to keep the loading axial.

A picture or napkin sketch is always a good starting point for us trying to give appropriate engineering advice.



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Old 10-02-2015, 07:52   #6
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Re: Drogue bridle legs attachment - plenty strong?

A metal chain plate is the proper attachment for the S D. If you go to their website you will get all the info required as all the engineering has been done a long time ago.
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Old 10-02-2015, 08:12   #7
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Re: Drogue bridle legs attachment - plenty strong?

You get the design load on the website, 7000lbs per bridle leg. They also mention a chainplate is plenty strong, but it has to be backed by a proper backing plate ON A GRP HULL.

There is very little mention of steel hulls, their thickness and the attachment points on those.

Here are two fast paint sketches of what I had in mind. Not sure if they're enough. If need be I'll get drawing again on a piece of paper and take a picture of it.

Here's a sketch of the boat (black) and the two bridle legs (red) coming of the stern.



Here's a sketch of the strap itself. 1ft long, about 3 inches wide and 3mm thick. Three holes (black) for the 10mm bolts, and one hole (red) for attachment of the shackle to the bridle legs.



Attachment to a cleat is not an option. I'll have to make an attachment point myself one way or another, if need be beefed up a little from my original idea.
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Old 10-02-2015, 09:26   #8
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Re: Drogue bridle legs attachment - plenty strong?

Problem with this is that you will conentrate all the cyclic load at the point where the first hole is drilled. you also add a leaver effect between the attachment hole and the first bolt. This is particularly evident if the boat shears away when struck by a wave which is exactly when you will get peak loads. If you don't have a samsom post (or cant fit one) would be more inclined to look at a heavy 'U' bolt, the type uses for inboard rigging chain plates but if possible attached to the hull just below the gunwale. Plan on loading from ahead to 45deg post & starboard and think of impact loads in tons not static loads in lbs. Could easily rip of a section of toe rail or at least bend it. Remember that for stainless a guide is that cyclic loads should be under 10% of break load to avoid fatigue.
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Old 10-02-2015, 09:30   #9
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Re: Drogue bridle legs attachment - plenty strong?

I do have a samsonpost by which, I'm pretty sure, a crane could lift the entire boat... It's a single point though, on the heartline of the ship. The JDS requires two attachment points on either side of the stern.

The U-bolt idea I can see happening, but just to be sure, one of these, yes?

http://www.mailspeedmarine.com/media...el_u_bolts.jpg

And you'd just bolt that through the toerail...? Somehow seems more flimsy than the SS strap, no?
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Old 10-02-2015, 09:46   #10
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Re: Drogue bridle legs attachment - plenty strong?

Spot on for the 'U' bolt. Should be 10mm and yes will be much stronger than a strap because of the fair loading. No I would not just bolt it to the toe rail. Either bolt to the hull below the rail or to the rail but in eathe case fram with knees welded to deck & hull or deck and rail each side to spread the load into the deck plates. If possible the rig the bridal with a backup to the samson post for belt and braces. I lost my parachute because the vibration and shocks shook the knot loose!!! Splice and chain link is better plus use chain anywhere you may get chafe. Then make sure you never need to use any of it!!!!
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Old 10-02-2015, 09:50   #11
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Re: Drogue bridle legs attachment - plenty strong?

I had mine built in the UK. There's a lot of useful information on the manufacturer's site, including the 316 SS plates they recommend, and sell.

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Old 10-02-2015, 10:08   #12
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Re: Drogue bridle legs attachment - plenty strong?

The toerail is an integral part of the hull (it's the same steel plate the hull is made of), but I could attach it below deck. What wouldn't be possible is welding below deck, it's a tiny cramped space!

I'm not worried about knots (everything is spliced) or chafe (there'll be nothing to shafe on).
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Old 10-02-2015, 10:34   #13
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Re: Drogue bridle legs attachment - plenty strong?

Quote:
Originally Posted by Orchidius View Post
Hi there,

So I built my own Jordan series drogue,
Off the topic, but would like to know?
Did you build your JSD from scratch or from the kit you buy?
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Old 10-02-2015, 10:40   #14
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Re: Drogue bridle legs attachment - plenty strong?

Scratch, but off-topic indeed. If you start a new thread I'll respond to it if you send me a link (or just PM)
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Old 10-02-2015, 11:10   #15
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Re: Drogue bridle legs attachment - plenty strong?

Hi -

I recommend reading John Harries' piece on the Jordan series drogue which deals with many issues:

Jordan Type Series Drogue Deployed In Survival Storm Conditions

Also look up Stainless Steel Attachement Plates -you may want to consider buying the chain-plates and backing plates; all you've got to do is install the system.

And you'll get even more info on Roddy Coleman's site: https://www.sv-zanshin.com/manuals/j...structions.pdf
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