# Load cells and weighing multihull builds

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

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You will need a means of raising and lowering the whole hull.
Then you have the load cell placed in the centre of each cradle/support:

If you're unsure about this, you can make a simple cradle to span the 2 hulls - if you haven't got one already, and then place each load cell directly under the CL of each hull.

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### HeimfriedSenior Member

@fallguy
What is the distance between the hull CLs, and the distance between the bulkheads 1 and 5 (position of the stands)? What it the boats depth (height from keel line to top of cabin roof) and in which height from keel line would you estimate the CoG?

I suppose there is a base plane horizontal and parallel to the design waterline and the deepest point of the keel is located in the base plane (z = 0).

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### fallguySenior Member

the beaching keels would be benath dwl they are quite heavy; the beaching keels, are about 11 feet long; there is a sacrificial timber under them made from laminated ash..they are laminated with 4 and 8 layers of 600/225 and about 4" squarish shaped

Let me know if you want more absolute details a/o the purpose of the inquiry. Perhaps just trying to get a feel for size?

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### fallguySenior Member

That centerpoint is where I had planned to raise the hull from, but perhaps the hull can be lifted with my mobile stands and we could get the load cells under the midpoints... The mobile stands all have four 1000# trailer tongue jacks on them.

I'll give this some thought. The livestock scale was 5000# rated only, too bad because that would be a couple hundred dollars only..

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### fallguySenior Member

Here is one livestock scale. Could calibrate on the fly a bit with salt bags above the bulkheads. 4 bags is 200#. I'd have to get a bit creative to mount it all. Hard to believe that load cell can handle an offset of 7,500#.. the back stands are some 8-10" off the ground, so they'd need to set down on some 3 ton floor jacks and not be too wobbly or leave any part of the main cradle on the ground. 2% error on 10000 is pretty ugly, but like I said, salt bags may help. It didn't have any signs on the 0.02, but the other one below is 0.02% which is much less, so this is probably same..

Last edited: Jan 3, 2022
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### fallguySenior Member

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### HeimfriedSenior Member

I'm interested in your problem and may be able to contribute a bit to solving it. (I was thinking a lot about some different ways weighing my much lighter boat.)
And to be able to give meaningful posts I should know some things about your boat.

Last edited: Jan 3, 2022
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### fallguySenior Member

Beaching keel starboard from forward. Just curious. This adds to my displacement, correct?

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### fallguySenior Member

I ordered this one.

I calculated my plywood internals in the cabin at 300#. I'm a bit mad I didn't make them lighter now. Most of it is 3/4" ply and 1/2" would have saved a hundred pounds...If you look close at the locker on the right; you can see some holes. I can probably hole cut around 20-30 pounds out still..

The benchtop panels could be remade from foam core I suppose. But the spans are about 22" or so. All that work making an ultralight hull erases quickly with plywood. I hate to admit it, but I rationalized it some earlier as offsetting my 200 pound excess engine weights. That was foolish. Never rationalize adding any weight.

Last edited: Jan 3, 2022
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### HeimfriedSenior Member

Yes, cause it has mass (weight) it adds to your displacement mass as every other part of the boat. As it will be immersed it adds displacement volume also. I think your question is: will the boat sit deeper in the water with this beaching keel than without it? The answer depends on the density of this part (wood, glue, glass, coatings, maybe bolts, ...). Would it float in water? If yes, it will give the boat some additional buoyancy. If it (alone) would sink it will contribute some additional weight causing the boat to sit deeper then otherwise.

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### HeimfriedSenior Member

I hope this will turn out right for you. (I don't like offers which contains some contradictions, cause I'm not sure, which of the given data are correct and which are faulty.)

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### ScuffSenior Member

You can use a lever arrangement so that you don't overload the livestock scales
What kind of plywood?

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### fallguySenior Member

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### fallguySenior Member

Marine

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By # this symbol means pounds?

We have done this several ways in the past:

A beam, which we also used for launching the boat, is placed under the hulls.
The load cell can be placed just about anywhere that is 'easy' for your set up. If you place under the hull, then just the cradle it is supported on, needs to be deducted.
If under the beam, inline with the hydraulics - or whatever device you use to lift - then you need to account for all this additional weight, just weight it after the lift.
The closer the 'offset' from the lift to the CL of the hull, the less stiff the beam requirements.
or
You lift the boat under the raft, making sure you place the lifting mechanism under structure that can support the lift and/or sufficient load bearing material to spread the load.

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