Author Topic: Checking rig tensioning based on frequency  (Read 1791 times)

Viviane2000

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Checking rig tensioning based on frequency
« on: September 14 2024, 11:07 »
So in a couple of days we will have our rigging checked and the tension will be adjusted to the correct amount. I've read that you can check the tension of your timing belt based on frequency. Now I'm wondering if I can do the same for my rig?

When the rigger is done and my rig has the correct tension, can I use a mobile phone app to find the frequency of my rig. I'll make notes of the frequency of different parts of the rig.

In a year time I can check the frequency of my rig then vs the frequency in my notes - that should give the correct rig tension? What are your thoughts? Am I missing something? I'll also write the temperature while noting the frequency, I'm guessing temperature will impact frequency?

Yngmar

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Re: Checking rig tensioning based on frequency
« Reply #1 on: September 14 2024, 11:38 »
I tried this. You can't. The resonant frequency of our shrouds at full tension was too low to detect with a phone microphone. No surprise, it's a much longer span than the average contrabass  ;D

You'd have to find some other way to measure the frequency, or use the stretch or deflection methods. Some riggers use a deflection gauge to measure tension, Selden documents the wire stretch method in their "Tips and Advice" book on how to tension it yourself. Most riggers go by feel, some actually go sail the boat.
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Viviane2000

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Re: Checking rig tensioning based on frequency
« Reply #2 on: September 14 2024, 12:52 »
Ah, shit. When things sound too good to be true..

Jake

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Re: Checking rig tensioning based on frequency
« Reply #3 on: September 14 2024, 15:46 »
A proper rigging tension gauge is not that expensive.  Loos makes one for wire rope and another for rod rigging.  They are the best method of setting rig tension.  Racers use them all the time for repeatable settings, depending on the wind strength.

Jake
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Jake Brodersen
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UP

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Re: Checking rig tensioning based on frequency
« Reply #4 on: October 28 2024, 12:59 »
@jake as a side note, unfortunately the cheap rigging tension gauge market ends at 10 mm. Larger yachts (44ft +) with 12 mm rigging or above have only the ruler method available. Second side note: the accuracy of the ruler method highly depends on the thermal stability of the ruler. Metal and plastic are bad, wood is good, however suspect to moisture.

As for acoustic analysis of the rig - it is a very sound idea and I have tuned my chainplate tie rods with a smartphone app (mast off). However these rods are very short, rather tight, and they create a relatively high pitch which also is amplified by the ships hull. Above deck, "string" lengths are way longer, as mentioned above.

If you're serious about measuring the rig tension acoustically, then a piezoceramic transducer clamped physically to the wire will have the resolution to measure the frequency. Quoting from Seldens tuning manual, a 19/20 rig cap shrouds should be tuned to 25% of the breaking load. For a 12 mm wire this is is 0.25*126 kN = 31.5 kN. Say for a 5.5 m distance on a 18m mast, to the first shrouds, the first natural resonant frequency is around 35 Hz. You will need an amplifier for amplifying the weak low frequency signal. You could then record it with Audacity and run a Fast Fourier transform on it.

Sounds pretty straightforward, but in reality probably will be way more complicated than the wooden ruler method :D Noise will be your likely enemy, plus the X factor.