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Weighing a boat - under 1000 kg

Question: 

Is there an easy way to WEIGH or closely assess the weight of a boat hull, say up to 1000 kg (2200 lbs) ?

                                                                                                                                                Stuart-C:   AU

Answer:   Well, I guess ‘an easy way’ would be to rent a mobile crane with a scale attached, but I’m assuming you want something easy to use at home at the lowest cost …. so how about a system that uses just a bathroom scale !    This is a method I have used myself on many occasions and I’m totally ok with the result it gives … but the set-up must be done with some care and precision for it to be accurate.

First, I would take off all items that can more easily be weighed separately, so that you’re down to a simple hull as bare (and light) as possible.

The following sketch will show you the basics, but there are a few points to underline.

You will ‘set up’ and then measure the weight of each end and then add the results.    Although the location of points C and D are not at all critical, they should be as far apart as is practical, not be changed after you start the measuring, and the hull itself MUST be at the same level horizontally for both readings.  

Setting up a long spirit level on deck blocks and taping in place, is a good way to assure this … and measuring the ‘relative’ heights of A and B will be a backup verification, with the difference in heights A and B always staying the same, regardless of which end is being measured.

The weight at each end could be up to around 500kg (1100 lbs), so the leverage required needs to bring this down to within the range of the bathroom scale.  About 4:1 will give a maximum of say 125kg (275 lbs) on the scale.   For a quick estimate of ‘the range’ of figure you can expect to see on the scale with this set-up, just divide your rough estimate of hull weight by 8.   So if it’s 640kg you are weighing, the scale weight will only be 80kg (176 lbs), right in the range of weighing a human.    

It will be important afterwards, to take your scale to a post office or freight company and get someone of about the same weight (as when under your boat) to stand on it and compare the figure with that given by a regularly calibrated Post Office scale, so that you can calibrate your own.  It may easily be reading 3% low or high, so you will need to factor that in.

The stabilizing lines can be anything, just to be sure the hull does not tip over .. but should hold the hull as vertically over the support blocks as possible, so that there’s virtually no load on this side support … a kilo or two will then not change the results in any measurable way.

Always make sure both the scale and any other supports, jacks etc. are sitting on strong, flat pad for stability …  400mm squares of 19mnm ply are great.

It’s important to measure the lever lengths (X and 3X) as accurately as possible, as 3X is only given as a guide.  You may, as an example, find that your set-up at one end is not exactly the same as the other (not a problem) and you may have an X of say 410mm with a 3X of just 1190mm.    So in such a case your leverage value would be (1190 +410)/410 or 3.902. 

So for that end, you would multiply the Scale load indicated, by 3.902.  It’s perfectly normal that the Leverage Value for the other end will be slightly different.

After adding together the two end weights, you will need to weigh all the other parts (or amas and akas in the case of a trimaran) and make a table to check the complete list and view the end result.

Have fun, and make sure you don’t tip the hull over while levering it up on to the scale or blocks.  It’s a fun project to do with a bunch of friends around to help prevent that.   Take on a few bets and the worst loser pays for the beer ;-)

…… mike 2019

PS:  But how do I get the boat up into that position ?

Typically, one can use strong planks (25-60mm thick depending on the boat weight) and slowly raise the boat, one end a little at a time, using scissor jacks from an automobile (always place anything loaded on to a flat, sturdy piece of plywood to stabilize it).

But I have also weighed a 2000lb 26ft tri by first weighing it on its trailer, using the above system but this time transversely under the aft axle … taking off a wheel, first one side and then the other, to make space for the scale and any needed shims.    In this case, you need to accurately check the level in both the longitudinal direction as well as transversely, before each reading.    You will also need to measure the load under the hitch and finally add all 3 weights.    Then, once the boat is launched, you can weigh the trailer alone to assess what weight to deduct from your total, but if you weighed the boat & trailer minus one wheel, remember to deduct that wheel weight from your trailer weight, or your boat weight will appear a little less than it really is ;-)

With a little imagination, all sorts of heavy weights and configurations can be weighed with a bathroom scale …. but always, keep the item on the same horizontal plane for each measurement or your result will be far from accurate.

 

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