Calculation for Chainplate for Series Drogue

[evillen]

I've had two chain plates made to take a series storm drogue. The drogue is deployed out of the back of the yacht on a bridle attached to two points on the stern. The displacement weight of my Hallberg Rassey 42 is 12500Kg.

The designer of the series drogue reckons that the maximum static load on each leg of the bridle will be 4166 Kg and the snatch load will be 5833 Kg (if the yacht yaws badly).

The chain plate is made from 60 mm wide * 6mm thick 316 stainless steel. There is a 32mm hole drilled 30mm from the end, which only leaves 13mm of steel to take the load of the shackle. (it was supposed to be a 30 mm hole, but they didn't have that size of drill...) I've attached the original drawing.

1. How do I calculate if the chainplate will take the load safely?

2. If it's not strong enough how do I modify it to take the load?

[philSweet]

Can you show us a detail of how it is to be mounted on the deck or hull or transom? There are several possible failure modes to consider.

[Tad]

They seem to be taking the tensile strength of the pennant as the working load....I don't think that's a great idea......I would far rather the rope breaks and the whole works drifts away than having the chainplate rip out of the boat.......As a working load I would multiply the breaking strength of the rope by 2-3 minimum.....

[evillen]

Thanks for the swift replies. I've already mounted the chain plates and attached a photo of the installation. The bulwark/toe rail is about 5 inches thick and solid, so I'm not particularly worried about it being ripped out of the deck.

I'm thinking that the weakest point is right at the end of the chain plate where the hole for the shackle is.

[Jim_Hbar]

evillen:

Both Tad and Phil have very valid points. I would add that a good design tends to look balanced and reasonable..

If you check out 5/8" shackles that are rated for load carrying applications, such as those from the Crosby Group, a 5/8" G-209A Crosby® Alloy Screw Pin Shackle is rated for a 5t working load. It has a 3/4" pin, and a distance "A" (gap between the ears) of 1.06"... So that particular shackle is designed to fit over a maximum thickness of 1", and 7/8" might be a reasonable design thickness for the lug, where the shackle fits..

And please note that that shackle does not meet the design snatch load.

Added comments-

The question of reasonableness also enters into it - the snatch loads given are approximately 1/2 the displacement of the boat... Ask yourself if you would trust those brackets (as installed) to hold your boat nose down suspended by a crane. If one bracket could do that, then you would have a service factor of 2.

Hope that helps,

Jim

BTW - I'm a professional engineer, and have designed and rated machine rigging and hoisting apparatuses (apparatii?) in the past.

[Tad]

Typically the pin hole will be offset from the plate end to provide more metal beyond the pin (in line with the load). I don't think there's near enough metal left and would suggest a doubler plate. To calculate the load a given section can handle multiply the sectional area by the material yield stress.......

An easy way to increase the strength of attachment hole is to make the plate thicker.......add another plate on top (with all the same size holes in it) that carries forward at least beyond the third caprail bolt.....that may require new longer bolts.......

[evillen]

Thanks for these replies. I particularly like Jim's comment:

Ask yourself if you would trust those brackets (as installed) to hold your boat nose down suspended by a crane

Errr, not really It was only when I mounted them that I started to think that it wasn't enough - it just didn't seem right.

So the consensus seems to be that I need to make additional plate that is 1/2" thick and move the hole so that there is more material at the end. I guess that I should buy some 3/4" shackles and then work out the dimensions.

A couple more questions:

1. Is it going to be stronger if I get the two plates welded together?

2. Tad says "To calculate the load a given section can handle, multiply the sectional area by the material yield stress....... " What is the Material yield stress of 316 stainless and can you give me an example calculation?

[upchurchmr]

evillen,

First thing you should do is not taper the plate like the one you show. Make the full width all the way out to the hole end, drill the smallest practical hole for the proper size shackle pin.
I don't have a suggestion for the thickness, the conditions are not something I understand, but go BIG or stay at home. Yes welding the additional plate to the original will be better, but better still is just replacing what you have, it will be much better.
Have you talked to anybody who has actually used this system? Checking their attachment sizes and understanding the conditions might help you understand what is good enough.

[jehardiman]

Originally Posted by Tad

Typically the pin hole will be offset from the plate end to provide more metal beyond the pin (in line with the load). I don't think there's near enough metal left and would suggest a doubler plate. To calculate the load a given section can handle multiply the sectional area by the material yield stress.......

An easy way to increase the strength of attachment hole is to make the plate thicker.......add another plate on top (with all the same size holes in it) that carries forward at least beyond the third caprail bolt.....that may require new longer bolts.......

Yep, by eye the pin hole was too big for the fitting (as well as the plate being too thin to be cantilivered like that) and a quick calc shows that the fitting as drawn wouldn't support half the load stated. FWIW, the sectional area through the pin hole axis should be about twice the sectional area of the pin (it is a shear issue, not a tension one, see Cozzone, F.P, Melcon, Hoblit. "Analysis of Lugs and Shear Pins Made of Aluminumor Steel Alloys." Product Engineering 21(May 1950): 113-117. and Melcon, M.A., Hoblit. "Developments in the Analysis of Lugs and Shear Pins."Product Engineering 24(May 1953): 160-170.) and it's thickness should be at least 1/2 the pin diameter or it will just twist it up. For a 6 1/2 ton shackle, the pin is 7/8 inch diameter so the fitting should be 1/2" or more thick with a mimimum width of ~ 2" for a 1" through hole.

Note 5833 kg = 5.8 tonne = ~ 5.8 LT = 6.5 ton

[evillen]

Thanks jerhardiman. So how about this - I have the end three inches cut off and a new piece welded on with a 1 inch hole with 1 inch of material at the end. Then I have another 1/4 " piece welded on top going back to the third bolt hole.

The effect would be that the end 12 inches would be 0.5 inches thick * 2.375 inches wide and the hole would be 1 inch, set back from the end by one inch of material.

You also mention "a quick calc shows" - can you tell me what the calculation is?

[Tad]

evillen,

All of us are leery of practicing engineering on the interwebs for a number of reasons, thus we will give you hints and directions to look things up......you can easily google yield stress and ultimate stress of 316 stainless, and you can google chainplate calculations, and we'll give you numbers from lookup tables.......But I won't do the calculations complete and hand them over because, a) I have no control over what you do with them. b) things change and they become obsolete before you understand what you've done, c) This is what I do for a living.......

The welded condition material properties are different than the un-welded......If you hire an engineer to design this chainplate he or she becomes responsible for the result, that's what you are paying for.....and thus that person will use their knowledge and experience to decide on a tensile strength for the material, add an appropriate safety factor, specify the bolts and fastening method, etc.

This isn't a particularly complex problem and I'm not saying you can't figure it out, but you'll have to work at it a bit.......

[Jim_Hbar]

evillen:

Again Tad has hit it right on the head..

It would appear that you are focusing on only one of the possible failure modes.
There are several others that will likely rear their ugly heads before the eye of the bracket fails...

A hint - Consider the load on the rear-most bolt.

BTW - What is wrong with attaching the rope directly to the rear cleat, with a spliced eye on the rope?
After confirming the load capacity of the cleat and it's mounting of course!!

Another hint - When using shackles, their geometry gives good hints as to what the design of the lug they attach to should be like...
If you fill up the available space with the lug, you'll be about right. But it still needs to be checked.
It is not unusual to weld bosses on the bracket to reinforce the area around the hole for the pin, and to get decent bearing length.
The pin hole should have nominal clearance on the pin diameter - perhaps a 15/16" dia. hole for a 7/8" pin..

[evillen]

OK Tad, fair cop. Here's my stab at a calculation:

Yield Strength = 30 KSI; Ultimate Strength = 75 KSI

Existing chain plate

Minimum tensile area = 0.5" * 0.25" = 0.125 sq in
Plate will start to yield at 0.125 * 30000 = 3750 lbs = 1704Kg

Suggested design (ignoring welding factors)

Minimum tensile area = 1" * 0.5" = 0.5 sq in.
Plate will start to yield at 0.5 * 30000 = 15,000 lbs = 6818 Kg

This gives me a safety factor of 60 % on normal loads and 17% on snatch loads. This a bit tight, but I'm not sure what the actual load will be - 66% of the displacement seems very high in my humble opinion. I've asked the drogue manufacturer what they think the loads will be...


OK Jim_Hbar,

Interesting point on the cleats. I did consider just using the cleats, but I don't know how to work out what the maximum load that the cleats could take. Also, I thought that it would be better to have the connection points clear of the stern, so that I don't get any load on the pushpit if the yacht veers around in the storm.

The question in my mind now is - Would I be better off using the cleats? I think nothing of using a front cleat for my main anchor in a 40 knot squall...

I've asked Hallberg Rassey to tell me the load bearing capacity of the rear cleats....

[michael pierzga]

Im curious , why dont you use the Genoa sheet winches as a dead end for the drouge with robust captive fairleads aft ?

Retrieving a long heavy rode hand over hand sounds like torture. Simply hauling in a 30 kg tuna on a hand line wears me out.

How do you set the drogue ? Throw the whole thing over the side and hope it deploys unfouled ?

Also the ability to fine tune the length of your drogue must be beneficial.

[evillen]

Hi Michael,

It's a series of 132 small drogues on a long rope, which you just pay out and go to bed. It's much better than a parachute or conventional drogue and doesn't need to be fiddled with.

The designer's notes are on this web site and he recommends a chain plate like I have fitted:

http://www.oceanbrake.com/jordan'snotes.html

Hauling it back in has to be done using a winch/pulley system. A bit laborious, but I wouldn't mind a bit of work, if I've just survived a major storm...