Weighing a boat with bathroom scales

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Weighing a boat with bathroom scales

Post by PeteThePen1 » Sun Oct 30, 2016 5:59 pm

Hi Folks

I know it is a bit early for the Festive Season puzzle, but here is something for you. I recall reading about how one can use one's bathroom scale to weigh all sort of large objects using ingenuity and a few bits of timber. The text was American in origin and could have sworn it was in the handbook for my trailer. However, careful examination of that has proved me wrong.

Anyway, I am looking to try to weigh Frances Ann which is currently in our garage sitting on a trolly with six big castors. The name of the game is to see what capacity my planned new narrow trailer will need to have. The idea is to get a trailer that is small enough to reverse onto our frontage and will fit in the garage too. The present one is way too big and does neither of those things.

Regards

Pete
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Re: Weighing a boat with bathroom scales

Post by TahoeSteam » Sun Oct 30, 2016 7:09 pm

If you're friendly with five of your neighbors you can borrow their scales and use your scale, putting one under each caster. Adding up all the weights should get your net weight.
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Re: Weighing a boat with bathroom scales

Post by Lopez Mike » Sun Oct 30, 2016 7:43 pm

I've done this with a loaded trailer. Here's what worked for me. A modification of this setup should work for your castors. Just more fooling around. With any more than three points of contact the leveling is much more critical.

You need a bathroom scale or a scale that isn't too thick. A big old platform scale would work but the problem of getting everything on the same level gets worse.

A reasonably stiff plank. You might get by with a 2" x 8" but thicker is better. Thickness is more important than width. Eight feet long is probably enough.

A tape measure and three round hunks of wood maybe a foot or so long each.

Some stacks of wood scraps to level things up. A jack to lift your boat and trailer up a bit one side at a time.

The round stuff is so that there is a reasonably accurate way of measuring the distances from the various contact points.

Start by jacking up one side. Place one round piece on the ground about a foot inside the tire lying parallel to the trailer. Put one end of the plank on that round and the other on another round that is on the scale. Place the remaining round on the plank under the tire and lower the trailer down on to it.

If nothing has cracked, now level the trailer up, right to left and fore and aft. Probably you'll need some hunks of wood under the other tire and will have to adjust the tongue jack to get things right.

Measure the distances from the round on the ground to the round under the tire. Then the distance from the round on the scale to the round under the tire. Record the scale reading. Divide the two distances, the smaller into the larger. Multiply this number by the scale reading. Thats the weight of one side.

If you are fairly sure that the right and left side weight the same, fine. If not, do the same drill on the the other side of the trailer.

Now remove all of this stuff and weigh the tongue weight. If it is more than you scale can handle then you will need to do the whole plank routine there as well.

Things that can go wrong:

The trailer can roll in some direction. Level ground helps as well as some wheel chocks on the other side. The trailer will want to move sideways while you are jacking up the other side from the scale side. Depends on how stable your jack is. Maybe you can block that other side up first by doing some measurements of your setup.

The results will be affected by poor leveling. Not that much but it's worth dealing with. Maybe.

Not to ask you when your mother stopped sucking eggs but you are weighing everything including all the junk you are hauling around in the boat, the water in the boiler and what not. Be aware.

Have your spouse standing by with a phone with the first few digits of your emergency services dialed. Kidding.

I'm sure I've forgotten something.
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Re: Weighing a boat with bathroom scales

Post by Gudmund » Mon Oct 31, 2016 5:19 am

There must be a public weigh bridge near you. 2 visits; one with the boat on the trailer, the other without. Simple arithmetic does the rest.
When thinking of your new trailer, consider 2 wheels are very much easier to manoeuvre than 4
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Re: Weighing a boat with bathroom scales

Post by Lopez Mike » Mon Oct 31, 2016 5:53 am

Well, he doesn't have a trailer yet. The boat is on a cradle with six castors. It really complicates things.

It will be challenging to get the numbers right with six wheels to deal with. All six castor wheels have to be lifted off of the floor by the same distance with probably not much room for error.

That's why a three point measurement is so much easier. Best to weigh the rear two wheels on one side at the same time. And then the other rear two wheels. And then the front two wheels. In other words, use equalizer planks between the pairs of wheels I specified so that leveling won't be so touchy.

To elaborate, with only three points to weigh, moving one point up and down a little bit doesn't affect the weight on the other two points very much. But with more than three points, everything interacts. It's called being 'over determined' in engineering jargon.

The three points have to be at the apexes of a triangle. No crossing over and stuff as you will surely get into with six castors unless you heed my advice above about reducing the measuring points from six to three. And you must have all three equalizer planks in place at the same time and the ones not in use with the scale and all of its stuff must be on single point supports. It will be surprising if it doesn't all fall over with a great crash.

Fun problem. Maybe borrow a trailer long enough to do as Gudmund suggests. Even just a flat bed rental trailer with some cribbing. Certainly much, much easier! Even if you do the measurements at home by following my earlier instructions, having it on a trailer will make things much less fussy. And either way your friend with the loaner trailer will find out how much his trailer weighs.
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Re: Weighing a boat with bathroom scales

Post by fredrosse » Tue Nov 01, 2016 2:22 am

"It's called being 'over determined' in engineering jargon."

I believe this condition is actually called "statically indeterminate", at least that is what they called it in my engineering classes, many decades ago. This is where several solutions can exist, but the actual correct numerical answer cannot be determined from the data known.
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Re: Weighing a boat with bathroom scales

Post by Lopez Mike » Tue Nov 01, 2016 3:04 am

My exposure to the term has been in telescope design. All large telescopes these days are alt/az which means being designed like a surveyor's instrument. They rotate around in a circle and are supported by pads on a smooth track with oil pumped into the pad. No static friction.

All of my life I had designed things with three point supports for stability. The hot shots who design things like the big stuff in Hawaii told me that they support them on four points because it makes the overall structure stiffer. In other words, carry the stiffness of the mountain all the way up through the telescope. An all new idea to me.

They even bore test holes through the ground under the telescope so that they can include the structure of the rock in their calculations.

An amazing data point is that the Keck scopes have a rotating mass of four million kilograms or around 5000 tons. And if they turn off the direct drive motor the scope will drift with the wind. Zero static friction. Hurrah for plain bearings!

Now back to steam launches. This has been a classic thread hijack.
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Re: Weighing a boat with bathroom scales

Post by RGSP » Tue Nov 01, 2016 9:42 am

The statically indeterminate bit doesn't matter as long as the castors are arranged in line each side, and it helps a lot if the trolley is rigid enough to be lifted at is balance point (or thereabouts) on each side.

If the above is so, proceed pretty well as Mike Lopez suggests: i.e.
Select a bit of timber heavy enough to act as a lever arm to lift one side of the trolley
Guess what the weight of one side might be, and arrange the lever ratio to bring that within
the range of your scales
Support the inner end on some round timber under the trolley but on the floor + packing.
The fulcrum point should be the trolley frame somewhere near the side balance point.
The outer end of the lever can be supported on the scales, which will read the one-side
weight divided by the lever ratio.

The two sides ought to be about the same if the boat is symmetric, but do the above for both sides if it isn't.

Everything needs to be fairly level (for safety if nothing else), but a few degrees out will not be any worse than the basic accuracy of the set up + scales.

Personally, I'd borrow a trailer, weigh it at a weighbridge, load the boat onto it, and weigh again.

If you know any agricultural or civil engineering types, then most tele-handlers and many loaders have a readout of the lifted weight, via the hydraulic pressure and lifting geometry, and you could lift the boat either using forks underneath, or a set of lifting straps from above.
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Re: Weighing a boat with bathroom scales

Post by Mike Rometer » Tue Nov 01, 2016 10:59 am

Any weight found by the above scales method will include the weight of the trolley. Such a method could also be used 'end to end' I think? Might be easier in Pete's garage!
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Re: Weighing a boat with bathroom scales

Post by Lopez Mike » Tue Nov 01, 2016 3:39 pm

Another possibility, suggested by Bart, is to modify a small hydraulic jack to add a pressure gauge. Given the area of the ram and the pressure you have a very compact and high capacity scale. Use some combination of the various schemes suggested but without the teeter totter stuff to make a small household scale do the job.

With such as tool you can weigh all sorts of things including your tow rig. And be the envy of your peers for being the sole possessor of a hydraulic load cell. There will some seal friction issues. I don't know how bad any hysteresis issues might be but we are not doing big time research here.

I am eyeing one of my older and someone abused jacks with this in mind. I haven't a clue how to get it apart yet.
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