Strain gauges

[pmagowan]

I have been looking at strain gauges online. They are smaller than a postage stamp, cheap and will accurately read the strain that any components they are attached to are under. I was considering the applications in boating.

Rigging strain
Hull strain
Keel bolt strain
Mast strain
Key component strain such as winches

Is this just more geek madness or is the idea worthwhile?

 

[pvb]

It's more geek madness!

 

[prof pat pending]

I guess that would depend on how you used the data.

 

[pvb]

I guess that would depend on how you used the data.

And also how you'd actually get the data! How would you attach a strain gauge to a keel bolt, for instance?

 

[johnalison]

Brilliant. OP has given himself five more things to worry about while sailing.

 

[savageseadog]

Complete load cells made from 2 or 4 strain gauges are made and sold to measure forestay load, I have one. It's possible to measure lots of other things but it's not necessarily easy.

 

[pmagowan]

And also how you'd actually get the data! How would you attach a strain gauge to a keel bolt, for instance?

You glue it on.

 

[prof pat pending]

Brilliant. OP has given himself five more things to worry about while sailing.

Perhaps you aren't thinking that all the way through

 

[pmagowan]

Perhaps you aren't thinking that all the way through

?

 

[Hydrozoan]

Brilliant. OP has given himself five more things to worry about while sailing.

Are you suggesting that he'll need to measure stress as well as strain?
https://www.bemindfulonline.com/test-your-stress/

More seriously, I had a vague idea that strain-gauge-based anchor alarms were already available - or was it a DIY article in PBO?

 

[pvb]

You glue it on.

And how would you route the wiring?

 

[pmagowan]

And how would you route the wiring?

Any way you like. The bolt only needs to engage with the structure where the washer is. There is plenty of room for 2 tiny wires. You could even make them run integral to the bolt.

 

[prof pat pending]

?

I'm not a sailor, but if I was and into racing, I might be quite interested to know at exactly what point I get the maximum strain on the rigging.

We use load cells in a similar manner

 

[pmagowan]

I'm a sailor, but if I was and into racing, I might be quite interested to know at exactly what point I get the maximum strain on the rigging.

Because I plan to build the hull and in doing so need to test some lamination layup schedules I was thinking I could have some accurate info on tolerances.

With heavy weather it might be nice to know when a shroud is reaching maximum tolerances. The most important use would likely be around grounding and the consequences. Strain gauges could let you know if bolts or nearby structures have exceeded their maximum tolerance and require repair.

 

[prof pat pending]

Look back...no need to make the point to me

 

[JumbleDuck]

I have been looking at strain gauges online. They are smaller than a postage stamp, cheap and will accurately read the strain that any components they are attached to are under.

Hah. I wish.

Strain gauges are useful things, but using them is a black art - it's not just a job of stick 'em on and go. For example, they are very temperature-sensitive, so you have to use them in some sort of bridge set-up. And they are very fragile - attaching and protecting them is non trivial. And the signals they produce are tiny, noisy and have to be conditioned and processed before you can use them. And once you do have the strain measurements, interpreting them is non-trivial, particularly in non-isotropic materials like composites.

What sort of stress factors of safety are you planning to give your design?

 

[pmagowan]

Hah. I wish.

Strain gauges are useful things, but using them is a black art - it's not just a job of stick 'em on and go. For example, they are very temperature-sensitive, so you have to use them in some sort of bridge set-up. And they are very fragile - attaching and protecting them is non trivial. And the signals they produce are tiny, noisy and have to be conditioned and processed before you can use them. And once you do have the strain measurements, interpreting them is non-trivial, particularly in non-isotropic materials like composites.

What sort of stress factors of safety are you planning to give your design?

Buckets! I have looked into the bridging, black art of application etc but it doesn't look too difficult especially when you are only really needing them to read near failure conditions.
I don't expect the design composite will be in any fear of failure but I do intend to 'abuse' the boat and I like the idea of some structural integrity gauges just for the geekiness factor.

 

[JumbleDuck]

Buckets! I have looked into the bridging, black art of application etc but it doesn't look too difficult especially when you are only really needing them to read near failure conditions.

I don't do strain gauges myself, but I have worked with people who do. If your factors of safety are big enough you'll never be near material failure conditions and strain gauges won't help avoid structural failures like column or surface buckling. I think you'd be setting yourself up for continuous worry and false alarms with strain gauges, which simply are not robust or reliable enough for what you want to do with them.

If they did work, what would you do with ten seconds' warning of the mast coming down?

 

[William_H]

I sailed on a boat with a strain gauge and readout on the forestay. The intent was that the running backstays were set for a correct strain on the forestay. However the boat was old by then. Built for 1987 Admirals cup so strain gauge never worked and owner got rid of running backstays anyway.
Used a strain gauge (electronic scales) to measure down force needed to hold my little boat mast horizontal. olewill

 

[rogerthebodger]

I don't do strain gauges myself, but I have worked with people who do. If your factors of safety are big enough you'll never be near material failure conditions and strain gauges won't help avoid structural failures like column or surface buckling. I think you'd be setting yourself up for continuous worry and false alarms with strain gauges, which simply are not robust or reliable enough for what you want to do with them.

If they did work, what would you do with ten seconds' warning of the mast coming down?

I have worked with strain gauges and I agree with JunbleDuck. They are fiddly and are very easy to damage. The black art is knowing what kind of gauges to use and where and how to attach them. The circuits have very low currents in the order of micro amps so subject to interference and temperature drift.

The use of load cells would be an easier proposition but you still need to consider the electronics.

I started a project to used load cells to measure the weight of LPG in my gas cylinder taking the mass of the cylinder into account and displaying the weight of gas left in the cylinder but getting the electronics stable in the boat environment is causing me problems so I've put it on the back burner at the moment.