Load cells and weighing multihull builds

Discussion in 'Fiberglass and Composite Boat Building' started by fallguy, Jan 2, 2022.

  1. Barry
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    Barry Senior Member

    The beaching keel is running along the bottom of the fiberglass keel so the loading through the beaching keel will not put the 250 psi on the fiberglass. I had suggested running a longitudinal 6 x 6 maybe 6 feet long to distribute the weight
    But I am not aware what load/stress the fiberglass above the beaching keel can take or the interface strength between the beaching keel and the fiberglass
     
  2. fallguy
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    fallguy Senior Member

    Oh, the long way...that is much more reasonable...there is a fairly flat section maybe 8', so 96"•3.5" is 336sqin. 5000/336 is 15 psi only..the keel can certainly take the hull weight

    I just can't teeter totter the boat at the beaching keel
     
  3. Barry
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    Barry Senior Member

    My verbalization probably led you astray
    So the 6 foot 6 x 6 could run longitudinally to spread the load on the cross 12 foot 6 x6 running between the two keels. Ie just to reduce the higher loading if the additional 6 foot 6 x6 was not there. The boat would still have to teeter but the contact area would be larger. ie the entire weight of the boat would be carried by the 12 foot 6 x6
     
  4. fallguy
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    fallguy Senior Member

    So, I have already got all the equipment. My son arrived last nite and was going to help me a bit, but I made him Covid test because my father is older and not well and I was going to visit him today, well, he is positive, so we sent him into the east wing which has a separate air handling system. Sorry, I am going somewhere.


    So, if I use the load cells, can I lift the boat off two of the load cells and use the Vg corrections?

    Then could I lift the back of the boat and get the weight of the front and vice versa? And could I get the weight of the port side and star sides same way?

    And from those values, can I make some calculate each corner? And from the corners, can I use the wheel scale to see how far off they are if I want?

    I think @Heimfried was talking about this in a prior post I sort of read past a bit.

    OR

    Can I only use the load cells to weigh the full boat because the deflections trying to read the load cells is going to cause too many errors more than simple corrections?

    AND

    why can't I just kill the feed from a single load cell and get the inputs from the other three? Do the algorithms used require all four inputs?

    Also,

    couldn't you trick the load cell device into reading a zero on one cell and get the other three and of course, et cetera to reading each corner?

    Also,

    I can't really hook up the load cells to the stands and then calibrate; so do I need to build a calibration table, too?

    Also,

    How does the cold affect the scale? It suggests some cold related issues. I am guessing the metal in the cells moves at a different rate. But does the scale read lower or higher and by how much? I still have to read the datasheet..

    Also,

    @Ad Hoc drew a yellow arrow on my stands where to install the load cell. Is there any way to do it differently, so I don't have to remove the stands? I suppose a beam, but the feet are like 2" or so only and the beam would not even stand up until weight was applied, and the beam has to be able to bear all the load. The stands are back breaking work for two men and I am alone. They weigh about 200# each and get taken apart and tipped out from under the boat..it is a lot of physical labor, tipping the vertical section down and back up

    I can't do any work on this today because my work area is in the hot zone. But it'll help me understand it all better to discuss.
     
  5. Heimfried
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    Heimfried Senior Member

    What load will the wheel scale take? Did you get also your live stock scale? If so, what is its weight capacity in total and what for a single load cell?

    In my opinion a lot of your following questions can only be answered by "it depends", because the given circumstances (many of them are not known by now) determine if the answer is yes or no.

    What means Vg? Same as VCG?

    Yes, but there are some requirements and limitations to this.

    Who knows, think of the intended use for weighing live stock and e.g. the surrounding at a sheep shed. It could be possible that a line breaks unpercieved. If the display unit would count this cell as zero, the weight displayed would be wrong but that would be unnoticed. So maybe it will show some kind of warning in this case. You will find it out by a simple test upset on your work bench without your boat.

    May be, try it out.

    Definitely temperature is an issue so you should read the manual/data sheet thoroughly. Especially the differences in temperature are critical. So you will probably find that the scale should be stored for some time before in the place the weighing process will take place and preferably also have the same temperature as the things to weigh. The Display and its electronics will warm up if connected to voltage and you will have to allow for some time that it may need to come to a steady state.
     
  6. fallguy
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    fallguy Senior Member

    Yeah, sorry vcg was written as vg by me. I abbreviate too much sometimes.

    load cells are 5000 each total is 20,000 pounds

    can't find the datasheet on the wheel scale, but the succesor version is 30,000 pounds

    we are not going to the garage today because of Covid, but you are right; I can test the system in the calibration efforts

    I am still wondering about just using a dummy scale and the wheel scale only because so much simpler to do..I could literally weigh each corner today.

    If I am lifting the corners only off the ground say 1/4"; the error for vcg moving approaches zero and the deflection in the system would be enough so the 'table effect' is negated.. no?
     
  7. Heimfried
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    Heimfried Senior Member

    As the total weight of your boat at now will be about 5,000 lb (correct?) it means only the measurement error will be greater as it could be.

    The error margin is increasing not only with the actual load but also depending of the load capacity of a scale. As the wheel scale is probably more a quality product (or has been) may be its error is better than the live stock scale. May be it's not, because a scale becomes not better over time and you will not know if it has sufferd from a kind uf accident or misuse. It would be good to compare both scales on known weights. In the best case up to the max load, but will you have got things like this.

    Do it, but understand the results as really questionable until you calibrated the scale. Take pics of the arrangement and post them. So we can better understand the way you work it out.

    Try it, but see above.
     
  8. guzzis3
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    guzzis3 Senior Member

    Oh. Seriously ? How can you people possibly overthink this to this extent.

    If you have that kit with the 4 load cells:

    Get the boat supported by them. For example of you get blocks of timber under the front and back of each keel, or the front and back of each hull. Make sure everything is off the ground. Put a load cell in each stack. Try and load it squarely. Add the numbers. You will be within about 2%.

    If you have only one device do as above. Jack each corner insert device and let that corner down. Repeat each corner. Add the readings. You will be within about 5%.

    If you bought that livestock scale it will be very accurate. Load cells are inherently accurate unless they are stuffed up in manufacture. Your supplier should have done a basic check when they were made. If they passed they will be bang on.

    There can be some error from asymetric loading on an individual cell, but those have small areas so you'd have to get it pretty wrong, to the point where the cell is in danger of popping out sideways for the error to be signifigant.

    I used to work at the national measurement laboratory. We routinely measured stuff to a part in a million, and a part in a billion on request (dearer). To put that in perspective for a 3 ton catamaran we would tell you it's weight to the nearest gram. That requires fiddling. Getting a weight to a few kg isn't hard. Those livestock things will do a great job.
     
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  9. guzzis3
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    guzzis3 Senior Member

    You only need left each corner until you can see daylight. I would use that livestock scale you found, better for your situation, but if that weren't available you just put a sling around each corner. Righ a frame strong enough for one corner, get a hand winch, put the cell between the scale and winch crank till that corner just lifts. Repeat for each corner. Building the frame is the most time consuming part. If you have some steel or timber laying around weld/bold/screw it together, just make sure it's strong and stable enough to be safe.

    But that livestock scale is much better.
     
  10. fallguy
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    fallguy Senior Member

    it isn't as much overthink for me as making it easy

    I want to use the wheel scale cuz so much easier.

    reference Ad Hoc's yellow arrow picture and you'll get an idea of where I want to use the wheel scale
     
    Last edited: Jan 8, 2022
  11. guzzis3
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    guzzis3 Senior Member

    My comment wasn't really directed at you.

    Many years ago an especially pragmatic PhD I worked with was losing her patience with one of her subordinates. She coined the phrase ant f@#$ing, accurate and hilarious. There are situations where disappearing down a hole worrying about small things is appropriate, but overthinking weighing a hull probably isn't one of them. Say you are out by 100kg, that's one big bloke or 2 thin women.

    I am sure whatever you decide will be fine. Just don't get caught up in minutia..
     
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  12. gonzo
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    gonzo Senior Member

    If you are using multiple scales, the easy way is to use only three. That will prevent the titter toting. Also, you can use three points with a single scale, and then add the measurements.
     
  13. fallguy
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    fallguy Senior Member

    BEF0EFA3-1DB8-4CFD-85CF-D52E1D79C768.jpeg 019098C1-1FE1-4060-940E-83DF67769DEB.jpeg 56A0A50F-CDB1-4465-95E6-1DD7E0F7EA72.jpeg Wheel scale doesn't work. It gets a boot up error. Kinda bummer, was gonna use it to calibrate the other ones. Gotta return it.

    Anyone have any ideas on how I can calibrate these? If I install them onto my stands; they won't support the hulls without loads applied because they would be a single point or foot.

    Also, a real challenge to see if I can attach them to the stands without taking the stands fully apart.

    @Ad Hoc

    how would I calibrate them and then connect them to the stands?


    For instance...

    Let's say I calibrate them with a piece of plywood. The calibration is done based on the distance the bridge is unsupported. So, I can't calibrate them away from the stands.

    So, then let's say, I take the stands out from under the boat and drill holes and mount the load cells. I can't zero them with no load, but the stands are not going to be my add load for calibration because they won't balance on a single 2" foot.

    So I have this all setup, but not sure how to calibrate.

    If I could calibrate them off the boat, but on their final mounts and then move the mounts to the stands; that seems like an option..

    Anyhow, I am a but puzzled on strategy here. Might have Covid, be nice.
     
  14. fallguy
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    fallguy Senior Member

    Here are some pics of the stands with the boat.

    Access to the stands is difficult, but there is a minimum of 3" under them at the load point. EF35F78D-C96B-4049-B07C-3AA3D16ABC7C.jpeg E6FEFC92-1EC0-4AD9-B8AD-60705246362C.jpeg 3229B2B2-22C9-4D94-87D1-66A9A257A2C7.jpeg
     

  15. fallguy
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    fallguy Senior Member

    Could I calibrate them on a piece of heavy steel and then move that setup to the stands and mount them? The calibration is to deal with bending off the edge; so as long as the edge bending remains consistent; it should work, no? So, a big old steel plate could literally be bolted to the stand after calibration as long as the bridge bending was unaffected?

    a bit stuck and wanting it to work without 18 attempts
     
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