Twisted Rod Rigging - now what?

J

jkalucki

New member
35
4
San Francisco, CA
For several (really stupid) reasons, and a minute of inattention, I managed to put about four turns of twist in the cap shroud of a J/109 and now I'm wondering how bad this is.

Basically I inadvertently backed the pin between the turnbuckle and the rod fitting out of the fitting a bit, noticed my error, and undid the problem in exactly the wrong way. Only after I finished did I realize that my fix had twisted the rod itself, not the pin. (Considerable swearing.) I don't think there was any damage to the threads. All this was done with between say 55 and 60 Loos "units" of tension -- right around the "base" setting for the cap shroud.

While I'm waiting for a professional rigger to come by and inspect things, my concern is that putting twist in the rod might possibly have damaged the rod and/or or the upper fitting. Now ~4 turns (I lost count) of twist over 45' to the top of the mast may not be a lot, or it might be far too much, and perhaps I've weakened something. Or, maybe people twist their rod all the time like this and it's not a problem?

Usually I'm really careful to not let the rod rotate by more than say an 1/8th of a turn, and then immediately reset it back to where it was.

Opinions? Experiences?
 
Go Left

Go Left

Super Anarchist
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Not good. Very not good. Bad.

But let's look at the cost/benefits:
Replacing shroud = $.
Replacing rig and torn sails = $$$$
Insurance adjuster spotting the twisted shroud you decided not to replace = $0 claim approval.

IMHO, I wouldn't let my crew stand to leeward of that rig in more than 6-8 knots of breeze.
 
H

Howler

Animal control officer
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422
I would contact some technical folks at Navtec or wherever the rigging came from, and ask them. I guarantee that there are people there who actually, definitively know the answer to "how much twist is too much?" but I'm not sure if they will let you get through to them. Worth trying.
 
Hitchhiker

Hitchhiker

Hoopy Frood
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Go Left said:
Not good. Very not good. Bad.

But let's look at the cost/benefits:
Replacing shroud = $.
Replacing rig and torn sails = $$$$
Insurance adjuster spotting the twisted shroud you decided not to replace = $0 claim approval.

IMHO, I wouldn't let my crew stand to leeward of that rig in more than 6-8 knots of breeze.
Click to expand...
As it happens, stupidity and negligence on the part of the owner are generally covered.....
 
ExOmo

ExOmo

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Howler said:
I would contact some technical folks at Navtec or wherever the rigging came from, and ask them. I guarantee that there are people there who actually, definitively know the answer to "how much twist is too much?" but I'm not sure if they will let you get through to them. Worth trying.
Click to expand...
so just a public service announcment for the group but Navtec went out of business a few years ago...
 
M

mckenzie.keith

Aspiring Anarchist
1,412
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Santa Cruz
If you unload it and let it be completely free, does it untwist on its own or has it "taken a set"? If it has taken a set that is definitely bad. If it rotates back to straight on its own volition, that is a good sign but I am not the person who can tell you that it is OK. I would definitely talk to a good rigger with strong technical background.
 
neuronz

neuronz

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europe
As long as you have only deformed it elastically it is fine (think torsional spring), plastic deformation will require replacement.

The length of the twisted section will be important in determining whether you have one mode of deformation or the other. Basically there will be a limit of twists per length unit where elastic deformation ends.
 
Remodel

Remodel

Super Anarchist
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You say you put "four turns of twist" in your cap shroud. That's 1,440 degrees of rotation. I can't believe your shroud isn't compromised. I'd replace it before leaving the dock again.
 
H

Howler

Animal control officer
424
422
Remodel said:
You say you put "four turns of twist" in your cap shroud. That's 1,440 degrees of rotation. I can't believe your shroud isn't compromised. I'd replace it before leaving the dock again.
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It depends upon the length of the twisted section.... 1,440 degrees of rotation in a mile of rod is probably fine; 1,440 degrees of rotation in a 5 foot section of rod is probably trashed; inbetween who knows. Certainly my intuition isn't good enough to have any sense of this. This needs expertise.
 
Crash

Crash

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^^^Lacking a rigger's specific knowledge, I do know that shafts of Navy ships twist. The shafts of a Nimitz Class Carrier, which are almost 2 ft in diameter, with a 4 inch wall thickness, and are about 184ft long, at full speed, have one full twist in them...

So some twist, over some length is likely perfectly acceptable...if its the cap shroud of a J/109, then its going to be a bit longer than the I dimension of 46.5 ft. That seems to me to likely be long enough not to have permanently damaged it? But it depends on the metallurgy of the shaft.

Again, the above is speculation based on a butt ton of assumption. Need to get an expert's opinion from a rigger who's experienced in Navtec rod on a J/109. As there's a lot of J/109s around still, that ought not be too hard.
 
J

JoeO

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I'm not sure how much a "rigger" is going to understand the effects of rotational plastic or inelastic deformation on yield strength, or the degree of work haardening that may have occurred, and how that could affect yield strength. For a definitive (?) answer, best to consult a metallurgical engineer or mechanical engineer with strength of materials experience. Otherwise, I expcet most riggers to simply say - "That sure seems bad, I'd replace that shroud". And they probably wouldn't be wrong.
 
JohnMB

JohnMB

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JoeO said:
I'm not sure how much a "rigger" is going to understand the effects of rotational plastic or inelastic deformation on yield strength, or the degree of work haardening that may have occurred, and how that could affect yield strength. For a definitive (?) answer, best to consult a metallurgical engineer or mechanical engineer with strength of materials experience. Otherwise, I expcet most riggers to simply say - "That sure seems bad, I'd replace that shroud". And they probably wouldn't be wrong.
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The shear strain in the rod will be ~2xpixrxn/L (r = radius, n= number of turns, L is length of rod)
The material appears to be cold drawn, which basically means that they stretch the crap out of it to get it up to its current yield strength.

There are some stress strain curves around, and you can probably find more data on the exact degree of cold drawing used by navtec.

If you understand all of this you can probably make a judgement on what damage if any you have done. I would note that your best test would be the one suggested earlier in that if the rod remained fully elastic you have not caused any plastic deformation.
 
D

Dacron

Member
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Inside 3 boatlengths
No qualified individual or organization is going to say its ok and sign off on it in writing. Why would anyone take on that liability? And a conversation and opinion that it was ok isn’t going to be worth shit if it does actually fail and come down in the future.

So if you were going to try and get away without replacing it, keeping quiet was the best answer. Posting details under a name and giving boat type details that an insurer will find in the event you ever have a rig failure in the next 20yrs was your second mistake. You signed the check to replace the shroud when you pressed “post” on this thread.

On the positive side, asking the question does suggest that you aren’t an arse but really are someone who cares and will replace the shroud rather than risk damage and injury. But replacement the only responsible outcome here.

It sucks balls because you don’t really want different bits of rod thats all different ages. Especially not side to side. Keep good records: replacing one shroud is a recipe for the other side staying in service long after it should have been replaced.
 
J

jkalucki

New member
35
4
San Francisco, CA
JoeO said:
I'm not sure how much a "rigger" is going to understand the effects of rotational plastic or inelastic deformation on yield strength, or the degree of work haardening that may have occurred, and how that could affect yield strength. For a definitive (?) answer, best to consult a metallurgical engineer or mechanical engineer with strength of materials experience. Otherwise, I expcet most riggers to simply say - "That sure seems bad, I'd replace that shroud". And they probably wouldn't be wrong.
Click to expand...

I'd guess that consultation would cost more than the labor and materials for a single-shroud replacement. I'm guessing replacing the whole shroud and fittings is in the ballpark of $2k, plus boat downtime. All the shrouds are just 2 years old.
 
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J

jkalucki

New member
35
4
San Francisco, CA
mgs said:
Depending on how the spreader bend is fixed at the upper spreader, the length of twist could be limited.
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It's not fixed at the spreader and popped there, likely releasing the torsion all the way up. That's was the precise moment of realization. Swearing began momentarily.
 
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