Load cells and weighing multihull builds

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

  1. fallguy
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    fallguy Senior Member

    Last edited: Jan 7, 2022
  2. Scuff
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    Scuff Senior Member

    Subscribing to see how you set this up. There are videos on using a lever arm with the scale this should keep the measured weight within the individual cell limits. You would just need to measure the corner weights like a car?
     
  3. Heimfried
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    Heimfried Senior Member

    In June I was weighting my kat with 4 digital bathroom scales, weight capacity 180 kg each (397 lb). I'd chosen the these scales because they are cheap (10 $ each). But I learned by testing them in order to save battery it is a feature to automatically turn off the weight display after a few seconds. Which is fine for the intended use in a bath, but a bit cumbersome for weighing a boat. Most of the time was required to balance the "weight distribution" setup to not exceed at any scale the maximum capacity.

    This is the pair of scales aft. Arrangement: a beam cut off under the 3 stringers of the bridge deck, downwards at each side an adjustable support, an wooden plate to prevent to break the glass cover of the scale, the scale (black), a peace of plywood and some bricks a support.
    001k.jpg


    The same setup at the bow with the "working part": a hydraulic car lifter under a post is lifting the beam with the boat until a gap of some millimeters opens on both sides between the top of the metal suppports to allow the scales set to zero. Then carefully lower the lifter until the post is free. Helper reads the scales all the way. This is be done half a dozend times to ensure a reliable and repeatable result. For safety reasons we made only one measurement at a time (fore or aft).
    002k.jpg

    Edit: I forgot to mention the pre condition. The demi hulls normally sit on 2 cradles each, so the first thing in the setup was to lift the hulls so far that they coudn't touch the cradles any more through the weighing process and all of the boats weight was resting on the beams for and aft.
     
    Last edited: Jan 2, 2022
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  4. fallguy
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    fallguy Senior Member

    I am facing a much higher hurdle to weigh around 10,000#, or 5000kg or 1200kg per stand.

    The stands are sort of four points. So livestock scales might work. The stands themselves are easily 100# each, so they need to be weighed to back them out.

    The stands are on wood blocks to keep the boat level..an added element of fun. The stands in back are 8" off the ground, for example.

    4272A368-EC6A-43BD-9D4D-56F6AB31D89A.jpeg
     
  5. fallguy
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    fallguy Senior Member

    A big airbag with 100psi and a gauge would be easier. But not sure it would be easy to calibrate. The surface area of the bag would matter I believe. Making a bag sized to fit just under the hull would be complicated. Say you had four bags four inches by a foot each. You'd put a gauge on each bag and add 100# of air. Then set the boat down. The bags are 48 square inches. The boat say at 2000 pounds per corner exerts 2000/48 pounds and the gauge would read about 140psi or so. But the bag that is sized right is harder to find... blood pressure cuff?

    Of course, the issue here becomes whether my hull can tolerate the weights. Right now, think I have a 7" by 22"board with foam and carpet, so that is more like 150 square inches for the 2000# or about 13.3 psi. Probably need to stay away from weighing the hulls this way..unless the bag is big as possible.
     
  6. Heimfried
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    Heimfried Senior Member

    Air bags are not the way to go, I believe. Additional to the air pressure it is not the (allover) surface area of the bag you need to know, but the part of the area of the bag which is transferring its load to the ground (contact area). The latter is not easy to determine.
     
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  7. Barry
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    Barry Senior Member

    Might be able to rent 4 industrial scales from an equipment rental company.
     
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  8. Heimfried
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    Heimfried Senior Member

    @fallguy
    Do you have devices to lift your boat? How will you put the scales or load cells under your boat? If I understand the picture right, it could be quite difficult to put one load cell under each cradle. This could generate constrainig forces which possibly falsify the results. It's just a guess, I don't know about the sensitivity of load cells to not perpendicular forces.
     
  9. fallguy
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    fallguy Senior Member

    I can carefully lift one corner at a time of each stand, or I can carefully lift the hull for bottom painting off the stands. I have a bunch of lifting devices that can lift, but no longer move the boat.

    Another option is the boat has beaching keels. I could perhaps weigh on them..maybe like a four point weight at say 3000# per 'point'. The lift would provide a little less info about weight disbursement this way. The stands currently sit under bulkhead one for and bulkhead 5 aft.

    the pictures include the back stands I built from heavy steel before I lined them with 2x lumber and carpets; front ones are more complex, but same idea, stands are about 4'x4' give r take
     

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  10. fallguy
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    fallguy Senior Member

    I figured I'd need four points under each stand or 16 load cells.. :(
     
  11. fallguy
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    fallguy Senior Member

    Gonna call Twin City Scale on Monday and see if they have any ideas or rentals...
     
  12. guzzis3
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    guzzis3 Senior Member

    I used to make transducers. I think I even have a piece of paper somewhere certifying me (a lot of people think I should be certified).

    Load cells, or force transducers, are usually built using strain gauges. Strain gauges don't generate current. You apply power to them and you get an altered result.

    I picked up a 1000kg load cell off ebay cheap. Maybe $130 or something. The readout is built in but I think it has a port for a cable. Runs on 4 AA batteries. I did a really rough calibration and it's within a kg (2 pounds) or 2. It's probably better than that but I don't have the gear to be sure.

    You don't need 4. Prop the boat up at each corner. Use an engine hoist or block to lift each corner just off the supports. Add the 4 weights and that will be near enough.

    piezo materials are used to make transducers but not usually high capacity force transducers. They are better for measuring acceleration and vibration.

    Edit: This is the type I bought. I think I paid more than that so that's a bargain...

    1000KG 1Ton 2200 LBS High Precision Digital Crane Scale Heavy Duty Hanging Scale | eBay
     
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  13. Ad Hoc
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    Ad Hoc Naval Architect

    Go with 4, as this is more accurate, and also you can easily establish the LCG too.

    Also, your loads cells wont be properly calibrated for the price. So, get a large known weight, and measure it.
    Measure it several times and see the result..that then becomes your 'adjustment'.
     
  14. fallguy
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    fallguy Senior Member

    How would you place four under stands like I showed?
     

  15. fallguy
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    fallguy Senior Member

    gonna be pretty hard to lift it and use a crane scale..boat is pretty high
     
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