chainplate calculations for a jordan drogue
 

If somebody could verify my calculations for jordan drogue chainplates, it would be really great!

The boat is 30' sloop, 8600 lbs official displacement, but probably around 9,500-10,000 lbs fully loaded.

I'm thinking of the following:

1.5" x 0.25" x 12" with a 1/2" hole and 4-5 bolts. Crosby 7/16 209a shackle - the pin is 1/2". WLL ~= 6000lbs. The chainplate strength should be:

(1.5" - 1/2") * 0.25" * 30,000 lbs/in^2 (tensile strength for 316 without yielding) = 7,500 lbs.

Does it look right?

Many thanks!
Gleb

In a simplistic world yes...
But...
This is pure tension, statically loaded... You start adding dynamic flexural fatigue, take into consideration the stress induction of the pin hole, mounting holes, area reduction for chamfering, etc... maybe 2/3 of the 7.5klbs. Now don't forget your FS, 1.5x Together reduces the load capacity to = 3300 lb...

Of course this is overkill reduction wise but...

Thank you!

I thought the ultimate tensile strength was the worst case scenario, and by using the yield strength, we don't need to do use any further reductions?

70,000 should be the breaking strength of 316, so the breaking strength of 1.5"x0.25"x12 316 chainplate would be ~ 17,500 lbs, in which case, 7,500 lbs gives that >50% safety margin.

The topsides are 1/8" thick, so not sure if going with a bigger chainplate would make a lot of sense. Fiberglass/bolts will probably rip out first before the chainplate breaks.

Has anybody installed jordan drogue chainplates? Have you used a backing plate? Does it have to be steel or could I get away with starboard/aluminum or G10?

Many thanks!
Gleb

Variables
 

Hello,
Let's start with that shackle. The 9,000lb working load is, I believe, one-fifth of the rated break strength. So right away there's a bit of a strength discontinuity with the chainplate numbers you have.
Next, while the yield strength is an important detail, and needs to be considered when determining scantlings for a given material and application, ultimate tensile strength is of primary importance. Imagine a material with an extremely high yield strength. This will break very shortly after it reaches yield, so there is basically no reserve of strength before failure. by contrast a material with a low yield strength (like 316) has a huge reserve, and can be scaled such that the design load never approaches yield.
In your case, the strength of the proposed chainplates would approach 19,000lbs. This is still way short of your shackle strength, so the we need to ask, is the shackle correct, the chainplate correct, or neither? Which of course brings us tot the question we really need to start with: what is the load? This is followed by: What is the appropriate factor of safety? So far we only have vessel displacement and weight, which are related, but not nearly definitive.
Fair leads,
Brion Toss

Hi, Brion,

This is helpful - thank you!

From the drogue design notes, the peak load would be:

Peak line load for both chainplates: 10,000 lbs (displacement) * 80% = 8,000 lbs
Peak load per chainplate would be 8,000 lbs * 70% = 5,600 lbs (that's a breaking wave load on the quarter)
Working load limit in a severe storm = 8,000 lbs * 10% = 800 lbs.

The breaking load on the 1/2" pin 209a Crosby shackle according to the specs should be at least 3xWLL, so the shackle should break around the same load as the 1.5"x0.25"x12" chainplate =~ 18,000 lbs.

Now that I'm looking at these numbers, keeping in mind your note about yield vs ultimate strength, it may be that I don't need dedicated chainplates on the topsides and the cleat and eye pad may be able to handle all but the most severe conditions.



What do you think?

Thank you!
Gleb

Glen,

You are doing it backwards. The first question is what load does the drogue apply, then everything is sized to this. 800lbs may be perfectly sufficient, it may be highly conservative, or it may be incredibly low. Until you know the expected loads nothing else can be worked out.

In this case the peak load is 8,000lbs. The working load for the drogue is ignored, and everything needs to be sized to handle the 8,000lbs worst case scenario. Because of the way Jordan has engineered this however it is reasonable to step down the safety margin however. Where normally a life saving piece of gear should be 10:1 here it looks like in their worst case they have already assumed a substantial safety margin, and something more like 1.5 or 2:1 may be reasonable.

And keep in mind you can't just cut the expected load in half for each chainplate. First because they will be offset which even in a steady state will result in different loads, but for something like a drogue because there is a real likelyhood that one side may bear the brunt of a shock load substantially higher than half. I would like to design around each plate carrying the full load if I could. Giving complete redundancy, but I might be ok with each assuming 75% of worst case value.

So my ideal would be two chainplates each sized for 24,000lbs. If that was considered unreasonable then I would be willing to accept something down to a minimum of around 9,000lbs. Then size the rest of the gear to this. The fact that drogue plates are usually installed outboard and at deck level means that it isn't normal to have a problem with installing them. So oversizing is a pretty easy step to take, and no one has ever complained their gear was too strong.

Of course I am at best an amature, and would be happy to accept correction.

Hi, Stumble,

Sorry if I was unclear in my previous post - these numbers represent forces exerted by the drogue on the chainplates. The maximum force on the boat of this displacement would be 8,000 lbs (on both chainplates). These percentages - 80% and 70% - came from the design notes.

The maximum load on one bridle leg would be 70% of the total maximum load. So the absolute maximum load on one chainplate when a breaking wave hits on the quarter is 5600 lbs for a boat of this size. These calculations have already been done by the drogue designer. Assuming they're correct, the question is what kind of chainplate or attachment can handle this load...

The working load of the drogue, per designer, is 10% of the peak load - 800 lbs for both chainplates, so <560 lbs per bridle leg/chainplate under "normal" operation. In this case, normal means a "severe storm", so I'm assuming force 10-11: <= 64 knots of wind and up to 52 ft waves.

Basically, the question is this: how likely can the existing cleats/padeyes hold the peak load of 5600 lbs? Also, what is the best chainplate/shackle to work with a regular load of ~500 lbs and be able to handle a peak load of 5600 lbs?

Thank you!
Gleb

Clearer...
 

Hi again,
I am enjoying this conversation. Assuming that we need to design the gear around the peak load, what factor of safety shall we use? I am assuming at least 3:1, which would mean a break load for the chainplates of at least 6 tons, and preferably more, as you noted below. Not hard to get.
It is the proof load of that shackle that is about 3 times the work load; the breaking load is about 5 times, and industrial standard.
If your cleats have 4 fasteners of .25", you might get an average break of just under 7,000lbs (see http://www.boatus.org/findings/16/). What sizes are your fasteners? 5/8" single-braid nylon has a break strength of about 10,000lbs. Is that what you hope to use?
Fair leads,
Brion Toss

Brion,

I'm curious what if any weight you ascribe to the maximum peak load as determined by Jordan? I wouldn't quibble with 3:1 but I could also argue that 10;1 would really be proper.

It's also worth noting that current best practice is to use dyneema for a JSD not nylon. The reduced stretch helps keep chaff down, and the minimal stretch actually reduces shock load since the entire drogue picks up loads more evenly with less delay between when the first cones start pulling and when the last ones do. It also prevents the boat from accelerating down waves since load is applied much earlier in the wave cycle.

Of course this also means a smaller, lighter, and easier to handle unit, which is always nice. And on a price/MBL dyneema is actually about the same price as polyester. Of course it does demand splices instead of using a cleat, but that isn't a huge deal.

That's great! Yes, the cleat fasteners are 1/4". Unfortunately, there is no backing plate under the cleats, but the pad-eye has a SS backing plate of about 1/8" in thickness and 2.5" diameter.

From the Crosby specs for 209A shackles:

Maximum Proof Load is 2 times the Working Load Limit (metric tons). Minimum Ultimate Load is 4.5 times the Working Load Limit based on metric tons.

So the limits on the 7/16 209A Crosby shackle (1/2" pin) would be:

WLL: 2.7 tons ~= 6,000 lbs
Proof load: 5.4 tons ~= 12,000 lbs
Breaking load: 12 tons ~= 27,000 lbs

The jordan drogue specs call for 5/8" double braid nylon to be used as a bridle and lead line.

Not sure about the safety margin... Does the 3:1 safety margin mean that each part is designed to handle 3x maximum drogue load on the system? In this case, I doubt that the topsides or deck will hold 18,000 lbs. Is it worth considering a load greater than the boat's displacement or design a system around once in a lifetime occurrence?

Just wondering... To me, it looks like the cleats will hold with a pad-eye as a backup in all, but the most dire events, in which case something else fails before the cleats will. But it's still worth considering chainplates to avoid chafing, which, people say, can be huge in severe storm conditions.

Am I wrong? I'm curious how chainplates and rigging are usually designed.

Thank you!
Gleb