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Old 10 February 2019, 14:14   #11
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Here in the USA, there is a simpler way to be more accurate when you want to weigh your rig. I don't know if you have Recycling Centers in the UK, but here in the USA, they have them everywhere. Some of them have scales that you drive on to and they weigh your load. It will weigh your trailer, motor and your boat, with fuel and give you a very accurate weight. Some charge a minimal fee and some do it for free. mathematical calculations are not accurate, as they do not compensate for density of materials.
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Old 10 February 2019, 14:39   #12
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It would be an interesting exercise to pit the theory against empirical evidence. I.e. formula v weighbridge
yes i will visit the weighbridge and update accordingly, i too am intrigued as to the accuracy that can be achieved using this method ,, just one small descrepancy that i hav,nt mentioned is as well as moving the boat forward in round one calculations i also moved the swinging cradle (16 rollers ) same distance and the winch post
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Old 10 February 2019, 15:22   #13
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Originally Posted by DiabloLoco View Post
. mathematical calculations are not accurate, as they do not compensate for density of materials.

Diablo, we do have recycling centres but (at least round here) they donít have weigh bridges but most counties have a public weighbridge which can be used for a very small fee.

Iím intrigued why you think density matters for the calculation. Perhaps it does, but at a glance it seemed to be a ďsimpleĒ set of lever calculations?
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Old 10 February 2019, 15:31   #14
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Diablo, we do have recycling centres but (at least round here)


Maybe he means scrap yards
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Old 10 February 2019, 16:05   #15
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Diablo, we do have recycling centres
Any scrap yard or waste collection outfit (skip hire etc) will have a bridge. There are at least 5 within 15 miles of my keyboard and I'm in the sticks.
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Old 10 February 2019, 16:47   #16
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Didn't somebody once post a theoretical method of calculating the net boat weight by observing the increase in cross sectional area of the trailer tyres and the psi pressure increase in them once the boat was loaded onto said trailer ?

Have to say I also prefer the weighbridge method
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Old 10 February 2019, 19:00   #17
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Diablo, we do have recycling centres but (at least round here) they donít have weigh bridges but most counties have a public weighbridge which can be used for a very small fee.

Iím intrigued why you think density matters for the calculation. Perhaps it does, but at a glance it seemed to be a ďsimpleĒ set of lever calculations?
I do not believe they are as accurate as just weighing the boat. Too many variables, unless you are a mathematician. Or at least incredibly good at math. So, weighing it, if possible is easier and more accurate for the average person. But that's just my opinion.
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Old 11 February 2019, 15:31   #18
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I do not believe they are as accurate as just weighing the boat. Too many variables, unless you are a mathematician. Or at least incredibly good at math. So, weighing it, if possible is easier and more accurate for the average person. But that's just my opinion.
Clearly mathematical literacy varies but refer to post #2 on this thread. Subtract, multiply, divide.

Quality of measurement is much more crucial... This bit may get a bit mathsy...

Digital bathroom scales probably quite accurate these days. Certainly within +/-1kg. So worst case one under-reading and 1 over reading means a 2kg error. But the 2kg error gets multiplied by ~25 so becomes +/- 50kg.

The other measurement is distance. While theoretically you could measure to +/-1mm for these sorts of distances actually repeatedly measuring the same point is probably harder...

In the example it is 4.6/0.2 which is a factor of 23. If that is 4.65 / 0.22 that would be factor of 21. With a 50kg difference in weights a factor difference of 2, makes the difference 100kg. Add both errors, perhaps +/- 150kg accuracy on a 1100kg load. So 10-15% error...

Ideally you'd repeat the measurements and use averages. Hard work moving boat back and forth although could move back 10cm, move back further 10cm. Forward 10cm, back to original.

** This weights the boat not the boat and trailer. A weight bridge weighs the combo. **. To weigh a boat alone on a bridge you need to weigh on trailer, take off trailer and re-weigh.

If you want to check accuracy of your measurements you could add a known weight to the boat like a fuel tank and do it with and without...
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Old 18 February 2019, 10:42   #19
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Ok so managed to get boat and trailer accross weighbridge at the port authority today .
so ribcraft 5.3m ,commercial grade build with yamaha 60 4 stroke , two man jockey console ,a frame etc ,full of fuel (not sure how big tank would be ,maybe 90 litres std or maybe specced more ,lots of electronics ,traile weighs 327kg
TOTAL WEIGHT 1200kg
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Old 18 February 2019, 16:41   #20
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Maybe he means scrap yards
Here we call them Recycling Centers. They recycle plastics, glass and certain metals, like aluminum, copper, etc...Ö...
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