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  #1  
Old 07-10-2007
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Rigging size

Is there a reasonable way to estimate what size (or strength) of standing rigging would be appropriate for a boat?

I have an older Hallberg Rassy 35 (yep, thats the new challenge) - and currently the rigging appears to be 5/16" wire (that's what the local rigger measured, it seemed slightly smaller to me but he probably does a better job at measurment).
The original spec. says the wire was 1/4". Personally, I am not a big fan of oversizing components for no reason - if original designer considered 1/4" to be a proper size, I trust them (and other components are likely sized to the same standard, so bigger rigging may very well be just adding weight aloft).

That said, I am now wondering which way to go. This is complicated by the fact that original spec does not say whether it was a 304 or 316 SS, that would make a difference.

Advice very much welcome.
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Brak-

It could be metric wire, 7mm is a bit smaller than 5/16" but larger than 1/4". 304 is stronger but less corrosion resistant than 316... so if the original spec was for 304, going up to 5/16" for the 316 might make sense.
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Old 07-10-2007
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The rigging on the boat is not original, it was replaced at some point. The original spec says 1/4" on the Hallberg-Rassy european site, so I don't think it was metric.
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Umm... If it was replaced, it could have been replaced with metric wire—since anything could have happened when it was replaced...
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If the information I have for the boat is correct and she weighs between 12,000 and 13,000 pounds and has a righting moment of 27,000 foot pounds at 30 degrees of heel and the beam at the chainplates is 9’ and the mast has one set of spreader with two lower shrouds then the strain on the upper shroud is 4050 pounds and the strain on each lower is 5850 pounds at 30 degrees of heel. I like to calculate the strain at knockdown for my own projects but I don’t have enough information on this boat.

If you use type 316 stainless 7/32” wire is only good for 5736 pounds which is less then what you need for the lowers and 1/4” wire is good for 7481 pounds so I would start by looking at 1/4 “ wire for the boat. By convention all the wires will be the same size so 1/4” is maybe what you are looking for. Did you say 1/4” was standard for the boat?

This was calculated by “the short method” and uses a righting moment from a table based on similar hulls. With more information you can get more exact numbers but you will be in the ballpark this way. As usual you get a disclaimer with this and that is simply that this is not intended to substitute for the service of a qualified professional who would calculate the righting moment from the plans or perform an inclining experiment to determine the actual righting moment and use the proper dimensions from either the plans or measurements of the boat.

If you make a change to the rig be careful about pin sizes. If they went up a size and drilled the chainplates or tangs you may have a weaker rig with the larger wire then you had before with the 1/4” wire. A standard in the industry is that a clevis pin diameter for a marine eye is twice the wire diameter. What do you have now?
All the best,
Robert Gainer
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Old 07-10-2007
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Robert,

You answer is good, as always You are right on the specs too (at least that's what I read).
However, Defender catalog lists 1/4" 316 wire to be rated 6900lbs. Still enough but does not leave a huge safety margin. 9/32" is rated 8700lbs - that should be plenty. If what I have currently is indeed 5/16" - thats an overkill (and also a lot more expensive).

There is additional issue with chainplates - the holes on chainplates are much larger than current pins and holes on turnbuckle jaws. From inspection it appears that these holes are original (or if not - drilled very professionally, but for what reason I do not know). Interestingly, the holes on chainplates (and also on the eye fittings on the wires) are about 1/2" (and pins are only 1/4" or some 3/8" or some such on both sides). Holes on headstay fitting and backstay fitting are smaller (3/8"?). So I've got a mix of things going on there.

So, part of this excercise of replacing rigging is to get properly sized fittings (may be not turnbuckles themselves, but certainly eyes and pins that would match the chainplate hole size).
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Brak-

You shouldn't be using pins smaller than the holes in the chainplates, since causes point loading of the forces on the chain plate and is likely to lead to early chainplate failure.
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The Dog is right and you want the hole and pin to match. The only way to transfer the load from one to the other without exceeding the strength of the parts is for the entire pin to touch the entire hole. Otherwise you just crush the small part that is touching if the pin is too small and you will shear the pin.

Defender is using the breaking strength from Loos & Company for their numbers. Nothing wrong with that but I am using a different table then they are. The factor of safety is already included in my numbers so you want to match load to breaking strength, not safe working load, without “adding” something else. Of course not everybody agrees on the factor of safety so if you want to add more to the load or increase the wire size feel free but keep in mind that rigging is a system where all the parts must match in strength or else you are wasting money and adding needless weight up high where you don’t want it. If you need something that is good for 1000 pounds then get the nearest thing that is equal to or over 1000 but not two sizes over a 1000. The chainplates and tangs are what they are so being much stronger then the other parts of the system is just a waste.

You say the chainplates are drilled for a 1/2” pin. That is the standard size pin for 1/4” wire and tends to confirm that the original wire was 1/4". If you have a 3/8” pin for the headstay that would mean a 3/16” wire and I very much doubt that is standard on the boat. It’s just too small unless you have twin headstays (not cutter but side by side) and even then it’s undersize. But that’s just my opinion and only worth what I am charging you as a consultant.
All the best,
Robert Gainer

Last edited by Tartan34C; 07-10-2007 at 09:13 PM.
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What strikes me as very unusual is that he has heavier than 1/4" wire, but the clevis pins are seriously undersized for the heavier wire. That is very worrying, since the swages on the wire would normally be sized for standard pins, and it sounds like the ones he has are sized for very small clevis pins.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.

—Cpt. Mal Reynolds, Serenity (edited)

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Old 07-10-2007
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Quote:
Originally Posted by sailingdog
What strikes me as very unusual is that he has heavier than 1/4" wire, but the clevis pins are seriously undersized for the heavier wire. That is very worrying, since the swages on the wire would normally be sized for standard pins, and it sounds like the ones he has are sized for very small clevis pins.
A MILL Spec number 20668 style aircraft eye for 3/8” wire uses a 1/2 inch pin. The same fitting in the marine style will use a 5/8” pin. By the way, an aircraft eye for 1/4” wire will use a 3/8” pin. That would be an odd choice but it matches what he has for a pin in the headstay and lets you use 1/4” wire. Would not be my choice but I can see them doing something like this.
All the best,
Robert Gainer

Last edited by Tartan34C; 07-10-2007 at 09:33 PM.
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