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Old 09 February 2019, 13:00   #1
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Boat and trailer weight ,help please

i was trawling around on here the other day and found a post from an old timer JS i think , it was telling how to calculate boat and trailer weight by weighing hitch weight (with bathroom scales ) and distance ,then moving the boat forward and repeating excercise
measure 1 was hitch weight 92.5 kg ... boat lenth from centre jockey 5.64 metres
measure 2 was hitch weight 140.1kg ...boat (moved forward 200mm ) 5.44m
the other measure i took was centreline jockey to pivot point of wheel 4.6 metres
since reading the post i cant find the post or the formula ,is this water off a ducks back for a structural engineer or similar ? ,any help appreciated
thankyou
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Old 09 February 2019, 13:24   #2
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( Distance to pivot X (difference in weights) ) / movement

4.6 X (140 - 92) / 0.2 = 1100kg

If I remember rightly
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Old 09 February 2019, 13:25   #3
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Boat and trailer weight ,help please

There is a post on here from a very long time ago where I linked to an article in Practical Boat Owner that describes this method... Iíll see if I can unearth it.

Edit: here you go - post 9 in this thread...
Trailer Ponderings
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Old 09 February 2019, 14:14   #4
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Thankyou for that ,super response as usual ,much appreciated
put some meat around the bone .......trailer is 327 kg (i knew that) ......boat is new to me r/c 5.3metre ,twin central jockey , 60hp yam 4 stroke (someone will soon be along to tell me underpowered ) probhably 100 litres of fuel and several electronics etc
amazing really for a boat that is listed bare from factory at 325 KG ,ha ha was originally looking for 4.8m and hoping to get it on unbraked ,think i might of missed the "little lightweight summer runaround " target !! ,had to buy a new trailer before i collected the boat ,lucky i bought 1300kg braked !
hopefully this is a helpfu lesson to all about boat weight !
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Old 09 February 2019, 17:17   #5
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So am i right in thinking if i now move the axle forward 100mm from the 92.5 kg position ,that should deduct another 23.8 kg from 92.5 giving me a hitch weight of 68.7 kg which will suit my discovery nicely ?.
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Old 09 February 2019, 19:53   #6
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So am i right in thinking if i now move the axle forward 100mm from the 92.5 kg position ,that should deduct another 23.8 kg from 92.5 giving me a hitch weight of 68.7 kg which will suit my discovery nicely ?.
Isn't the disco max nose weight 100kg? I find closer to the max nose weight helps keep the trailer steady.
Unless you want to reduce the nose weight for manual handling I'd stay where you are.
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Old 10 February 2019, 02:26   #7
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Isn't the disco max nose weight 100kg? I find closer to the max nose weight helps keep the trailer steady.
Unless you want to reduce the nose weight for manual handling I'd stay where you are.


I thought it was 150kg for the Disco, depending on vintage. I agree, generally, the closer to the max nose weight, the better for towing.
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Old 10 February 2019, 11:48   #8
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Isn't the disco max nose weight 100kg? I find closer to the max nose weight helps keep the trailer steady.
Unless you want to reduce the nose weight for manual handling I'd stay where you are.
Thankyou both , i am enlightened ,as PD says my disco 4 is 150 kg on removable tow bar ,(250 on fixed bar ) ,so like you say i would be better to use a bit more ,however like you say i also need a bit of handling ability as i have to spin the boat on a sixpence before pushing into my back garden , hence i have moved the axle 200mm forward and now getting 99.3 kg which i feel is good compromise , towed it 425 miles fro collection last w/e with 140 kg hitch weight and must admit it towed a dream ,however i usually only tow 2 miles to launch at my local slip
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Old 10 February 2019, 13:14   #9
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It would be an interesting exercise to pit the theory against empirical evidence. I.e. formula v weighbridge
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Old 10 February 2019, 13:32   #10
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It would be an interesting exercise to pit the theory against empirical evidence. I.e. formula v weighbridge


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Old 10 February 2019, 15:14   #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, 15:39   #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, 16:22   #13
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. 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, 16:31   #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, 17:05   #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, 17:47   #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, 20:00   #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, 16:31   #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, 11:42   #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, 17:41   #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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