Weighing a boat with bathroom scales

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PeteThePen1
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Re: Weighing a boat with bathroom scales

Post by PeteThePen1 » Tue Nov 01, 2016 5:43 pm

Hi Mike and Steamboating colleagues

Thank you so much for the suggestions so far. I have been contemplating Mike's proposal this afternoon (only found that folk had answered the post at lunch time) and wonder if I should add some information and some thoughts.

As the boat is in the garage which has a gravity leveled concrete floor we have a nice level platform on which to work. The trolley is a wooden frame that has three significant cross members in the centre of each is a nylon roller that is proud of the cross member by about 1/4" (Measurement mistake on my part during construction). The boat has two bilge keels that are arranged so that they are at the same level as the keel over part of their length. Thus if placed on a flat surface she would sit horizontally level which is what happens on the trolley. At the rear of the trolley is a V cradle on the cross member which ensures that the boat stays level if the bow is raised thus removing the support of the bilge keels. There is also a V cradle on a 'loose' cross member slotted into the trolley frame at the front. This is the 'string' bit of Belt, Braces and String.

If one had (say) some two inch square section steel tube, it would be possible to slide this under the boat but over the longitudinal frame of the trolley. Taken at a point as far as possible forward where the bilge keels and keel are in line crossways the risk of tipping would be zero. There would be no problem building up supports from my stock of 8 X 2 offcuts and other stuff and that could be topped by a bit of 2" angle iron to give a 'pivot' point from which to measure. As the boat is on one side of the garage the bar could project out into the body of the garage allowing plenty of space for the positioning of the scale. Given that the stern hangs a fair way off the trolley and is or can be lined up with the door into the garden, there is scope for doing a second weighing with the bar at that point. I would imagine that in all cases the boat would not be lifted by more than an inch or so. For additional safety it would be possible to use the engine hoist gantry that sits over the boat to support a safety sling.

Since I have CAD drawings of the cradle and the boat kicking around somewhere I will try bringing them together and show what I was thinking.

Regards

Pete
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Re: Weighing a boat with bathroom scales

Post by Lopez Mike » Tue Nov 01, 2016 5:49 pm

I think you have it under control. Let us know what numbers you come up with. Hard data is in scarce supply.
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Re: Weighing a boat with bathroom scales

Post by PeteThePen1 » Thu Nov 03, 2016 9:41 pm

Hi Folks

Here are a couple of pictures. One gives a reasonable view of the stripped down boat on the cradle along with the engine hoist. The second is a cross section drawing on which I have done a few calcs.
Boat on trolley (500 x 375).jpg
Boat on trolley (500 x 375).jpg (50.95 KiB) Viewed 1839 times
Boat Weighing 2 A4 (500 x 353).jpg
Boat Weighing 2 A4 (500 x 353).jpg (15.41 KiB) Viewed 1839 times
For the calcs I started with the fact that the bathroom scale has a limit of 150kg so I would need to be working on a loading on it of between 75 and 120kg. Given the boat's position, the length of 'short leg' is about 2ft (622mm) as a given. If we then assume that the stripped boat at the forward end has a guessed weight of 1,000lbs (450kg) we can do some calcs. The 1,000lb figure is based on the 1,200 displacement calculation for Rose hulls and the knowledge that Frances Ann floated about on the planned waterline. It was then a case of putting some numbers into the formula. 2511mm was arrived at simply by 'stretching' that bit of the drawing until it was roughly 2,500mm. The weight on the scale at that point would be 111kg (246lbs) which is within the desired bounds. As the 1,000lb guesstimate is probably too much, the scale should be working at a lower level with the real weighing. From the drawing it looks as if we need a bar of about 3.25m in length. So all I have to do is see which friend happens to have any 2 X 2 steel tube in more than 3m length!

Regards

Pete

PS Edited to replace Photobucket rubbish using Mike Rometer's suggestion. Looks like 500 pixels wide is a nice size, at least on my screen.
Last edited by PeteThePen1 on Tue Dec 05, 2017 9:52 pm, edited 4 times in total.
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Re: Weighing a boat with bathroom scales

Post by Lopez Mike » Thu Nov 03, 2016 10:43 pm

Don't forget to subtract the weight of your steel bar and anything else that is resting on the scale and not part of your boat.
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Re: Weighing a boat with bathroom scales

Post by Mike Rometer » Fri Nov 04, 2016 12:17 am

Pete, why not just 2488, a straight 4X?

I think I'd go with 50 x 100mm steel box , on edge, not 50 x 50mm. To keep the flex down.

I had a 4m length of 100 x 100mm, this time last year, but as you know it's now a fence post! :lol:
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Re: Weighing a boat with bathroom scales

Post by Mike Cole » Fri Nov 11, 2016 4:37 pm

Hi Peter
I think I have the sq tube you need, i go and measure it later
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Re: Weighing a boat with bathroom scales

Post by florinmetalworks » Sun Feb 04, 2018 12:43 am

New member, so be gentle.

Is it possible to suspend the boat from the rafters on slings? If so construct a pool underneath using a large new tarpaulin, two adjacent walls & some of that mass of 8 x 2 from above posts. If a pool of 6 x 2 metres is filled with water to a depth of 0.6M ( enough to float te boat?) & the vessel is slowly lowered until the slings are just slack the rise in depth of water will equate to the weight --- given the measurements above a 1000kg boat should produce a rise in level of 84mm. Fails will get you wet feet & not crushed toes/fingers as above suggestions .
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Re: Weighing a boat with bathroom scales

Post by cyberbadger » Sun Feb 04, 2018 11:18 pm

florinmetalworks wrote:New member, so be gentle.

Is it possible to suspend the boat from the rafters on slings?
Well many things are possible that there may be easier options depending.

If there is a pond or lake within an hours drive that might be easier?

Or perhaps for less then $100 a 1000kg/2000lb digital hanging scale...
https://www.ebay.com/i/232561468401?chn=ps

-CB
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Re: Weighing a boat with bathroom scales

Post by Lopez Mike » Sun Feb 04, 2018 11:48 pm

Somehow I don't thing the water level in a pond or lake will change much when you lower a boat into it. Archimedes was, after all, in a bathtub.
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Re: Weighing a boat with bathroom scales

Post by Lopez Mike » Wed Mar 14, 2018 3:34 am

As a followup on this thread, I just weighed my new and partially completed hull. I had just reached a point where I was going to turn it right side up and it was convenient. The is the same Reuel Parker design that Steve Weaver is building. 24' x 6' 7". Except that I am using stitch and glue rather than plywood over frames. It doesn't make that much difference except for leaving out the chine logs and it makes for a very fast build.

I was able to support the hull very near the ends. And such that it was stable and wouldn't fall over and spoil my party.

I made no attempt to predetermine the ratios. I just measured distances once everything was in place. I used pipes and round stock for the contact points. I took readings before taking the loads so that I had a tare number to allow for. The ratios turned out to be in the 3 to 1 range.

The total came in at 678 pounds. Since it was easy, I took it all apart and back together and the total came to 685. This struck me as too consistent to be true but declining energy took over.

Since then I have added much of the deck but not much of the interior and the project is now on hold until late Summer since I have some sailing to do in British Columbia. Barbara has spoken!
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