Rigging Math - SailNet Community
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Rigging Math

What resources would you recommend if someone wanted to run the numbers themselves for some rigging puzzles?

For example:
You replace your standing rigging with synthetic and figure you could shorten the keel a couple inches and have the same stability. How would you calculate that?

You are comparing two boats and want to calculate the margin of safety that the two builders used based on observed rigging.

You are thinking of replacing your chain-plates with titanium, how much smaller can you make them considering that titanium is several times stronger than steel.

You use a load cell to measure the force required to banana a boat with the for-stay and back stay. How do you calculate is that is the right amount of stiffness or if the boat has lost structural integrity?

But if such a person is not available or you want to check their work how would you make the calculations yourself?

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Old 05-22-2013
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Re: Rigging Math

For #1, you might be able to lower the keel 1/4 of an inch, depending upon the size of the boat. For a C&C 115, making the mast out of carbon and going with somthing other than SS wire, save IIRC from a magazine article. 50 or so lbs up in the air, which translated to 200 less rail meat lbs! for a mid 20 ft boat, the same change in riggin saved as little as 50 lbs.

Reality is, there is some math that is involved, it is a matter of knowing the formula's, plugging in numbers, and seeing what happens!

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Re: Rigging Math

Quote:
Originally Posted by blt2ski View Post
For #1, you might be able to lower the keel 1/4 of an inch, depending upon the size of the boat. For a C&C 115, making the mast out of carbon and going with somthing other than SS wire, save IIRC from a magazine article. 50 or so lbs up in the air, which translated to 200 less rail meat lbs! for a mid 20 ft boat, the same change in riggin saved as little as 50 lbs.

Reality is, there is some math that is involved, it is a matter of knowing the formula's, plugging in numbers, and seeing what happens!

Marty
Thanks for the comments on #1. That is the problem without the math it is not possible to know if any of these good ideas are really good ideas or not.

So I'm looking for resources to find the formula's.

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It is a lesson about the limitations of wax as an adhesive.
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Old 05-22-2013
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Re: Rigging Math

If you get a dregree in engineering you will have - or know how to get and use all the formulas needed. Much quicker to trust the engineering and design (but not necessarily the marketing) of reputable builders and designers.

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Old 05-22-2013
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Re: Rigging Math

I don't get the whole fiber rigging deal as the cheaper stuff is never a direct fit due to the sizes needed to stop creep

On the higher end you start at expensive PBO and work your way up to crazy expensive aero carbon rigging

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Old 05-22-2013
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Re: Rigging Math

The issue with chainplates is not the strength of the chainplate itself. If it were, we could get by with teeny-tiny chainplates, compared to what most boats have. The point is that you have to distribute the load to a large enough surface of the boat's structure. That is why chainplates are large--not because they have to be that big to handle the forces from the rigging.
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Old 05-22-2013
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Re: Rigging Math

+1 Tommy, shrouds should be sized to be 10-20% of breaking load or less to mitigate creep. I'd error on the 10% side. That being said, better materials at better prices are coming out every day. And the reason Dux is so expensive is b/c they're the only company that is making a pre-stretch heat set dyneema SK75. That will change this summer.

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Old 05-22-2013
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Re: Rigging Math

Anyone seriously contemplating such modifications probably ought to use consultancy services like Bob Perry's.. but as an exercise I'd suppose any structural engineer would have the necessary tools.

Ron

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Old 05-22-2013
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Re: Rigging Math

The biggest load on the chainplate is the bearing load where the pin loads the hole in the chainplate. That load determines the size of the chainplate in terms of thickness and cross sectional area. But below the hole the chainplate can be considerably smaller. You can see this on the Valiant 40 chainplates where I doubled the plate thickness at the hole. I think most builders just use a single thicknes because it's easy to do.

Be conservative with chainplates. Having your mast in your lap can spoil a nice day of sailing.

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Old 05-22-2013
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Re: Rigging Math

Quote:
Originally Posted by bobperry View Post
But below the hole the chainplate can be considerably smaller.
Okay, you got me. "Teeny-tiny" was an exaggeration (and how many millions of times a day do I remind myself not to exaggerate? ).

But the above is what I was referring to. If there weren't the issues of spreading the load adequately to secure the chainplate to the hull, then they could be made much, much smaller than they typically are. But those issues do exist, so even if you use titanium, while you might be able to use thinner material, the other dimensions would have to remain essentially the same in order to spread the loads out and adequately secure it to the hull, or through the deck to a bulkhead.
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