Load cells and weighing multihull builds

Yeah, # is medical shorthand for pounds.

It will be an adventure to attempt.

Anyone know if I can extend the leads on each load cell if they are short to the jbox?

 

Tie a rope around the bow mooring cleat on one hull, under both hulls to roughly half way back, then back under both hulls to the aft mooring cleat. Hook the scales into the loop and lift until it leaves the cradles on the scale sides. Move the loop fore and aft as required to lift off both cradles on the scale side at the same time. Repeat on the other side if the weight is assymetric.

If the cradles on the unweighed side rotate around an edge which is not under the hull, support them so they do or lift the hulls slightly in the cradles. Better, sit the boat on the keels.

 

Without much doubt you will need to do that (judging after the pics in the offer). If I had to do it, I would select a cable with much larger diameter leads to not much increase the resistance of the extensions, extend all of the load cells with the same type and lenght of the cable and ensure to create no contact resistance. To check the impact of the extension, it could be wise to set up the load cells arangement without any extension first (e.g. mounting it to a stiff pallet) and do some sample weighings. This way you could also check the impacts of side forces or the lack of beeing installed horizontally. Then install the extensions and repeat the sample weighings.

Did you plan for the mounting of the load cells? As I found in some pics of some different offers, the sensors are made to detect bending forces. So it is not done with simply place them under the load. The examples (live stock) show some kind of steel frame or box or stiff pallet. Additional to this there will be another obstacle. If I understand it right you are interested not only in the weight but also in its distribution to determine the CoG. As the pics show, there is only one cable running from the junction box to the display unit, so this suggests the addition of the signals from the sensors are added up in this box. Which is fine for live stock but not so for your purposes. To find out the load of a single load cell you will have to cut (switch off) the connection of the other loads cells with the box (if that will be possible) or release the loads from the other cells. There may come some other obstacles, sorry to say that.

 

I realized this morning that in order to install the load cell on these; I have
To life and demount the stands..screw and bolt the load cells and then put them back. The load cells would then get set onto something like a bottle jack. No idea how I'd calibrate them then.. unless I have to make a temp mount to calibrate..that is probably it..

 

So, I bought this today which would weigh each point and the stands easier.

Does anyone have any idea how much the other three points would affect the scale?

They'd be fulcrums..

 

This could be a possible arrangement if I do understand the several pics in different advertisements right.

 

Yes @Heimfried , but I am going away from this and going to use the wheel scale because I can get a weight of each corner that way and weigh the stand so much easier and no stand teardown to drill for the loadcells.

I can also add say 6 bags of salt at 50# each and determine if the scale appreciates all 300 pounds or if it reads only say 270 which means 10% (unlikely) of the weight is being taken by the other points.

In other reading; they suggest adding the same height to all four points, so let's say I add 3/4" lift to get the cradles off the ground; then the other 3 cradles need 3/4" lifts as well.

 

Ok, new approach, not easy also. I didn't find data for this scale, the manufacturers website did not recognise "MH300", so it seems out of production.
I would suggest to make 3 dummies (better 4), rigid wood blocks or something similar, with the same thickness as the scale. So it would be possible to interchange the scale with a dummy and vice versa by only lifting the respective "corner point" slightly without disturbing the general arragement.

That's correct and corresponds with what I said above. It would be good, if you made sure, that the ground under the 4 stands is hard and even and the waterline of the boat is horizontally adjusted again.

 

Heinfreds quote 33 If I understand it right you are interested not only in the weight but also in its distribution to determine the CoG. As the pics show, there is only one cable running from the junction box to the display unit, so this suggests the addition of the signals from the sensors are added up in this box. Which is fine for live stock but not so for your purposes. To find out the load of a single load cell you will have to cut (switch off) the connection of the other loads cells with the box (if that will be possible) or release the loads from the other cells. There may come some other obstacles, sorry to say that.[/QUOTE]


I am of the belief that the 4 load cell method will not allow a person to determine the CG of the unit. As the amount of vertical movement is extremely small to get the load cell to provide a signal to the display, the plane at which all the 4 load sensors are at must be within thousandths of an inch to be able to determine the weight of hulls at each corner. Ie if one of the load cells say is 1/4 inch below the others, then depending on where the CoG is, this load cell may
not even give an indication of any load as the boat hulls can be supported quite adequately by any 3 points which are coplanar. Keeping in mind that any three points create a plane. So the fourth MUST actually be precisely at the same height
to produce a representative load.


Certainly, the total readings will give the total weight of the boat but the load carried by any one cell can be changed by minute mounting height

 

@Barry
You are right in general. But I think, there is a way. If you look at the pics in my post #3, you will see at each weighing point a support with a threaded rod in the middle. I used these supports to adjust the rods until the loads on starboard and portside did show the same reading. So if I presume the cog is located in the boats center plane (the buid was a nearly symmetrical plywood construction at this state) I should be able to determine the longitudinal CG. Do I think wrong?

 

Adjusting the load to the same weight would work and provides the solution to locating the LCG. Well thought out

EDIT
I am making the assumption that you mean the rear load cells have one load, say 2000 units and the front load cells are a different load value say 3000 pounds. If so, then the hulls will effectively act as single beam and the LCG can be calculated

 

We have used 4 load cells on all our boats, from small 9m mono's to 80m catamarans very successfully.
Everything is about procedure and understanding the results.

 

I agree. The point is to only balance each pair of scales to display the same load (one pair near the bow and the other one near the stern).

 

@fallguy
To use the dummies I mentioned in my post #38 may not be sufficient as you are not able to control the variation of the load at the 3 dummy points caused by putting the scale under one point.

 

Have you considered renting scales?
Scales for Rent | Bench Scales, Floor Scales, Dynometers & Crane Scales https://twincityscale.com/rentals