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Old 06-12-2013, 21:55   #31
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Re: Octaplait vs Doublebraid For Jordan Series Drogue

Toddedger The mooring bollards are normally on the deck, bolts are in shear and the backing plate stops the bolts pulling through.
1 tonne is no big deal for the strong points on a larger boat. For example a single 13/8 rigging wire has breaking load over 7 tons.
I imagine 40ft boat loading on a drogue won't be much over 2 tons, 1t each side.
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Old 07-12-2013, 07:36   #32
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Re: Octaplait vs Doublebraid For Jordan Series Drogue

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The 13000 lbs (6t) is the breaking strength presumably with a high FOS for wear & tear. Assuming a bridle then actual loading might be lucky to get to 1ton each side. In 1 inch plywood; a couple of 1/2 inch bolts with a good backing plate.
Actually, "The design load is the ultimate, once in a lifetime, peak transient load that would be imposed on the drogue in a “worst case” breaking wave strike. The working load during a severe storm is about 10 % of this value." Since the break is very likely to be slightly off axis, "the transom attachments should be designed to take 70% of the design load....
Tow line diameters are "based on 65-75% of the minimum breaking strength of nylon...."

So, in the case of a boat with a 13,000# design load, the attachment design breaking strength is 9100#. In an example in the report, Jordan stated that a 7500# displacement boat would need fittings desinged for 5300#. You need to be able to lift the boat by these fittings. His suggestion, restated several times, is that a long fitting, like a sail track or chain plate could be designed to distribute the load. Would a few bolts and a plywood backing plate do this? That depends on the topsides lay-up of the boat, but probably not.

Chafe is also a real concern. It doesn't take much to cut a line that is near full load.

---

I also do not believe Jordon was aware of rode heating. However, since his rode design is conservative, if the line is truly cycling at only 10% of breaking strength, heating may be unimportant. Additionally, the cases I have read of were a little lighter than this design basis AND they were parachute anchors, which impose considerably greater strain. They were simply undersized from Jordan's viewpoint. The correct size is pretty beefy.
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Old 07-12-2013, 08:07   #33
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Re: Octaplait vs Doublebraid For Jordan Series Drogue

The difference between the loads drogue see in a "normal severe storm" vs in "the worst case" is quite large (about a factor of 5).

I have measured the loads on ours, in severe conditions with breaking waves and the peak loads are around 4,000lbs. That's very close the calculations that were done on the initial wave crest impact on the Winston Churchill (estimated at 5,000 lbs by cfd). This is the sort of loading a cruiser would typically see in his "extremely severe" storm.

This is crest impact, with the drogue holding the boat aligned thru the crest.

Hiwever, In the (very rare) "once in a lifetime" (and I actually think the frequency is less than that for the typical world cruising routes, the Winston Churchill case was in bass straits) worst case, after the crest impact, if the crest was big enough, the entire boat might/could possibly be in moving crest water and accelerated up to around 30kts. The Jordan design is suppose to decelerate the boat before it hits the trough. And the loads in that deceleration are calculated (I do not think anyone has ever measured this) at about half (or a bit more) of displacement. For hawk they are estimated at 15,000-20,000lbs (15 ton boat), or 4 to 5 times what you see in the 'usual severe conditions'.

I normally take my bridle to my winches, which are well able to handle the 4,000lbs (split by the bridle) sort of "typical peak loads". I do not know how they would handle the "once in a lifetime worst case" sort of loads. . . I do have tangs welded into the stern corners rated at full hull displacement. I have not used they (yet).

There is a theory that a light or easily driven hull (which the Winston Churchill was not) might be carried in front of the crest and experience lower peak loads. But all this is a bit theoretical. These sorts of waves are very rare and difficult to simulate accurately in rank tests or computer models (there is little data to validate the models).

I note in the Winston Churchill write up they suggest a three part tapered rode, as I mentioned in a post above.
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Old 07-12-2013, 08:59   #34
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Re: Octaplait vs Doublebraid For Jordan Series Drogue

It never ceases to amaze me how half the posters insist on posting opinions that they know nothing about. This thread is an example of a few intelligent people sharing info on a subject they have put some time and thought into researching. An engineering problem with some published load results. A series drogue that the creator, an engineer published a lot of detailed research on. Yet the other half of CF poster continue to waist our time to post what they imagine.

Come on people, intelligence is knowing what you don't know.
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Old 07-12-2013, 09:43   #35
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Re: Octaplait vs Doublebraid For Jordan Series Drogue

The Coast Guard report says "at a displacement of 7,500 pounds the design load is equal to the displacement, and at 60,000# the design load is 60% the displacement" They were using the 79 Fastnet storm as a worse case scenario.

My little rant was not directed at you Estarzinger. You have brought a lot to the table.
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Old 07-12-2013, 14:40   #36
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Re: Octaplait vs Doublebraid For Jordan Series Drogue

Very interesting Evans. I've now read "Oceanbrake" website as well. I certainly was low on the drag force.

Still think Dyneema is the winner because the low stretch Dyneema would apply the brakes from the beginning of the wave surge with full effect as soon as the drogue straightened out. On the other hand the Nylon would have to stretch quite a way before reaching full braking force by which time the boat would be going faster than it would have with the skids on from the beginning. The load on the anchor points are related to boat speed rather than wave impact force so Dyneema wins.
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Old 08-12-2013, 20:39   #37
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Re: Octaplait vs Doublebraid For Jordan Series Drogue

Rode length?

The Jordan rodes are typically designed around the number of cones, which is designed around the boat displacement. So smaller boats have shorter rodes, and even biggish boats have rodes that are "only" roughly 300' long.

It seems to me the rode length should be dependant on the "worst expected case" breaking crest length and not on boat size.

And our experience in the southern ocean suggests the worst case "once in a lifetime" crest, that the Jordan loads are sized for, might well need 600' of rode to ensure it is not all swept up in the foaming breaking crest.

That's speculation . . . But I can say we have ocasionally found 600' of rode useful when trying to get the single element drogues a wave cycle behind us.
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Old 08-12-2013, 21:53   #38
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Re: Octaplait vs Doublebraid For Jordan Series Drogue

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Rode length?

The Jordan rodes are typically designed around the number of cones, which is designed around the boat displacement. So smaller boats have shorter rodes, and even biggish boats have rodes that are "only" roughly 300' long.

It seems to me the rode length should be dependent on the "worst expected case" breaking crest length and not on boat size.

And our experience in the southern ocean suggests the worst case "once in a lifetime" crest, that the Jordan loads are sized for, might well need 600' of rode to ensure it is not all swept up in the foaming breaking crest.

That's speculation . . . But I can say we have occasionally found 600' of rode useful when trying to get the single element drogues a wave cycle behind us.

And so conservatively sized nylon gets to be quite a bundle. If Amsteel can get you long enough and strong enough with in a practical package, it just about has to be part of the construction.

I never seen a good modeling of wave impacts and energy absorption (this applies both to this discussion and ground anchoring). Is it better to meet the wave, or to try to fight back over the crest and your accumulated momentum? Common sense suggests only a small amount of give, not enough to begin surfing. The optimum would, of course vary with the wave size and steepness.
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Old 08-12-2013, 23:11   #39
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Re: Octaplait vs Doublebraid For Jordan Series Drogue

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I normally take my bridle to my winches, which are well able to handle the 4,000lbs (split by the bridle) sort of "typical peak loads". I do not know how they would handle the "once in a lifetime worst case" sort of loads. . . I do have tangs welded into the stern corners rated at full hull displacement. I have not used they (yet).
I would be cautious about this. Even very large primary winches aren't rated to this type of load. Lewmar 54's for instance are typically found on 50+Foot boats, but are only rated to 3,000lbs. I of course don't know what you have, but my guess is that your primaries may be at or beyond their spec.

One of the things I have considered is to run double legged bridles. So you can bring each leg back to two points.
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Old 09-12-2013, 01:11   #40
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Re: Octaplait vs Doublebraid For Jordan Series Drogue

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Interesting thread. Dyneema looks to be a clear winner.
While I disagree with that conclusion, there have been several viewpoints shown and anyone is free to draw their own conclusion.

I had some extra dyneema line left over and made a long (30-40 feet) dinghy painter from it. That was a huge mistake, as the dyneema absorbed no energy at all and the loads on the boat end and dinghy were enourmous, it would always tighten with a -bang- that could be felt on the whole 20 ton boat. Extrapolating that transfer of forces from a dinghy weighing 300 lbs at best to a storm anchor I would guess that it would be enough to rip out the hard-point for attachment including any backing plates if any amount of slack is suddenly snapped out. Although the design of the Jordan Series drogue is such as to provide constant drag regardless of breaking waves or confused seas, I am sure that at some point in a storm it will happen.
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Old 09-12-2013, 07:20   #41
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Re: Octaplait vs Doublebraid For Jordan Series Drogue

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I would be cautious about this. Even very large primary winches aren't rated to this type of load. Lewmar 54's for instance are typically found on 50+Foot boats, but are only rated to 3,000lbs. I of course don't know what you have, but my guess is that your primaries may be at or beyond their spec.
Well I have done it quite a bit with no ill effect.

I had a long discussion with harken about it. They said the winch rating was mostly all about the pawl in moving operation. So, if I treated the winches as bollards, and did not load the pawl or spin the winch the effective rating would be way higher (they did not give me a number). So, I use a tug boat hitch with the bridle lines on the winches, which treats the winch as a pure bollard and does not load the pawl.

I do use/spin the winches on recovering the drogue, and the peak loads then are down but I have measured 3000lbs. When recovering there is a strong cycle to the loads - high as you go down a wave and low as you go up the next one.

I also use the winches this way for shore ties. I have never tried to measure those loads, but getting hit by a 60kt williwaw probably applies some significant loads.

Any way, all I can say is a have used them this way quite a bit with no damage to the winches.

If I was ever concerned about the loads, I have several extremely strong stern attachment points . . . . Welded in lifting eyes and tangs and cleats. I use the winches because they give me easy recoverability. I am more likely to use the drogue, if I know it is easy to recover. If I think it is going to be a bear to recover I will put off deploying it, which could then be a serious mistake. My personal feeling is that your multi-part bridle just would make things more complex and me less likely to use it when I should.

I learned this lesson early on with respect to storm sails . . . Set them up so they are immediate/easy to hoist/use, or I (and most people) will put it off until later than I should.
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Old 09-12-2013, 08:22   #42
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Re: Octaplait vs Doublebraid For Jordan Series Drogue

Evans, thanks for pointing this out (using winch like a bollard). I had not considered the point and this eases my mind a bit!

Cheers,

Jim
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Old 09-12-2013, 09:26   #43
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Re: Octaplait vs Doublebraid For Jordan Series Drogue

Zanshin,

I understand what you are saying about the shock loads on a dinghy painter. I have added a length of shock cord to my painter for that reason, even though the painter is polypro. But I don't think it is accurate to compare it to the forces on a drogue. The shock forces on a dinghy painter come from the inertia of the dinghy at the end of the line. Normally your boat yanks on the painter, the dinghy moves forward faster than the boat, creating slack in the line, then the dinghy slows and the boat once again yanks the painter. I don't think that, even in a storm, a JSD could mimic this. As you point out, the loads on the JSD are much more constant, with the strain spread over the length of the rode. Imagine if your dinghy, instead of being a 300lb unit at the end of a 30' rode, was rather a series of 150 2 pound rubber duckies strung out over 300'. Even if wave action were to bunch them all up, the strain when the slack is taken up would not create shock loading.
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Old 09-12-2013, 09:37   #44
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Re: Octaplait vs Doublebraid For Jordan Series Drogue

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Is it better to meet the wave, or to try to fight back over the crest and your accumulated momentum? Common sense suggests only a small amount of give, not enough to begin surfing.
I think this is a good point. Remembering that F=m(v)2, force on the rode increases with the square of velocity. I have only used my drogue once, but the nylon rode allowed the boat to accelerate a bit before all the cones were brought into play. When the drogue finally "bit" the forces were gentle but huge. I felt that we would have been better off with more cones, less stretch in the rode, or both.
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Old 09-12-2013, 09:54   #45
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Re: Octaplait vs Doublebraid For Jordan Series Drogue

^^

The optimal amount of drag to have in the system is a huge debate, with extremely educated and experienced and knowledgeable people supporting both ends of the debate.

I personally think the answer depends on the specific wave, which means for the sailor and what should he carry, it is a probabilistic question - what percentage of the waves look like one type (say short plunging crest) vs another type (say long forward driving break). And unfortunately there is almost no data on that aspect of waves (the shape of the break) so no-one can really answer the question with facts.

It also seems (to me) to depend on the boat design. One that surfs easily might want a different amount of drag than one that does not. Our first boat did not surf at all, and was happier with less drag (relative to displacement) than our current one which surfs pretty easily.

We (a bit unusually) carry four drag devices (edit: actually 5, if you could towing just a warp loop, which we also use occasionally) that span the range from relatively low drag to extremely high drag. We have used them all, but if I had to pick only one, it would be 'medium low drag' because it seems the most frequently useful in 'typical storm conditions'. But I do think it would possibly be less good in that 'once in a lifetime worst case'.
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