# Thread: Technique to weigh small boats

1. ## Technique to weigh small boats

Hi, I can use some advise on a good method for weight a boat in the shop. I do have a way to hang the boat. Scales at each end? Any suggestions for technique and the best scale for the job?
Thanks

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## Re: Technique to weigh small boats

Hi Clinton,

First. what do you think it weighs? I weigh small boat hulls by placing them on an 8x8 block on a bathroom scale that goes up to 300 lbs, then subtract the weight of the block. I center it as well as possible then get someone to balance the hull while I skooch down under with a mirror to read the scale. Can you do that?
.
There are, of course, hanging dial scales.

4. ## Re: Technique to weigh small boats

Was it Michael that had a cool method of moving the boat on the trailer and getting a couple of measurements then figuring it out with some math? There's a thread here somewhere.

Steven

5. ## Re: Technique to weigh small boats

Plop her in the water, notice how much the sea rises, convert to cubic feet and multiply by 64 . . .

But there are more practical ways.

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## Re: Technique to weigh small boats

We have a trash hauler locally who is happy to oblige. Drive on their scales with truck boat and trailer, launch boat, return and weigh truck and trailer. Subtract the smaller number from the larger one and Bob's yer uncle. No charge either.

7. ## Re: Technique to weigh small boats

I couldn't find the thread but I found a thread talking about the thread. Tom Lathrop has a write up of the procedure here: http://www.egyptian.net/~raymacke/Weigh.html

8. ## Re: Technique to weigh small boats

Originally Posted by StevenBauer
I couldn't find the thread but I found a thread talking about the thread. Tom Lathrop has a write up of the procedure here: http://www.egyptian.net/~raymacke/Weigh.html
The method as I learned it was on the Windmill web site.

Clint, weighing with a single scale can be done with a simple sling and a third leg tied forward or aft for stability. Done it many times for one designs.

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## Re: Technique to weigh small boats

Support the boat under the keel at two locations - one near the bow and one near the stern - exact location does not matter. Put a scale under the bow support and then go to the stern and put the scale under the stern support. The weight of the boat it the sum of the two measurements. Simple statics. If you want to be a purist, subtract the weight of the supports.

You can find the CG with this information by summing moments about any fixed point by knowing the weight measurement and the horizontal measurement from your fixed points.

10. ## Re: Technique to weigh small boats

Gib, good question...this would probably work. I like the mirror idea.

Something like the hanging scale canoe yawl shows is nice but most to of my boats are below 20'.

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## Re: Technique to weigh small boats

I bet someone can figure out a slick way using a torque wrench. I've been meaning to measure the tongue weight of my loaded trailer for reference purposes.

12. ## Re: Technique to weigh small boats

You could probably just set it on a couple of bathroom scales...

13. ## Re: Technique to weigh small boats

The go-fast guys use (expensive) scales, one under each tire to balance their cars. You could do the same thing with three bathroom scales under a trailer, just make sure they are rated for the weight first.

14. ## Re: Technique to weigh small boats

Thank you for remembering me, Steven. Yes, the 'Lathrop Method' is the weighing procedure I posted some years ago. I have used it to weigh boats up to 7000 lbs - the thing with heavier boats is to find a scale that will handle the tongue weight, which in the instance of the 7000 lb boat was around 400 lbs. For a boat of around 1000 - 1500 lbs, an ordinary bathroom scale should suffice. To reiterate the "Lathrop Method':

Weighing a boat on a trailer

Shift the boat as far forward on the trailer as it will go.

Place a scale under the tongue support leg. Record the weight.

Measure the distance from the tongue support leg to the tip of the boat bow. Record this.

Shift the boat aft on the trailer a couple of feet, but not so far as to allow the trailer to tip backwards.

Place a scale under the tongue support leg. Record the weight.

Measure the distance from the tongue support leg to the tip of the boat bow. Record this.

Measure the distance from the tongue support leg to the centre of the trailer axle (if a twin-axle trailer, measure to the point exactly half-way between the axles). Record this.

Calculate the boat weight using the following formula:

W = C(W1-W2)/X

Where:
W = weight of boat
C = distance between tongue support leg and centre of trailer axle(s)
W1 = first (heavier) scale reading
W2 = second (lighter) scale reading
` X = distance boat was shifted between scale readings

15. ## Re: Technique to weigh small boats

I had heard about this method of weighing boats much heavier than the available scale but was not able to find it when needed so in the 1990's I did derive the formula shown on Ray Macke's site. I use it several times during construction of a new boat with blocks under the chines so the final weight can be projected fairly accurately. A by product is also a good CG estimate. I find this method to be accurate up to at least 3,000 pounds which is as high as I've used it.. Problem with larger boats is mainly that they will normally sit on dual axle trailers which the method does not work for.

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## Re: Technique to weigh small boats

Oldad already mentioned scales at the land fill. There are also scales at some of the cement plants. At ours we can also buy sand, etc. so they just weigh the pickups before and after filling.

17. ## Re: Technique to weigh small boats

Originally Posted by Tom Lathrop
I had heard about this method of weighing boats much heavier than the available scale but was not able to find it when needed so in the 1990's I did derive the formula shown on Ray Macke's site. I use it several times during construction of a new boat with blocks under the chines so the final weight can be projected fairly accurately. A by product is also a good CG estimate. I find this method to be accurate up to at least 3,000 pounds which is as high as I've used it.. Problem with larger boats is mainly that they will normally sit on dual axle trailers which the method does not work for.
You could support the trailer on a pair of jack stands between or adjacent to the rear axles to eliminate the load shifting between the axles and throwing off the effective length of the lever arm.

Originally Posted by Ian McColgin
Plop her in the water, notice how much the sea rises, convert to cubic feet and multiply by 64 . . .
But there are more practical ways.
EUREKA!
But Archimedes was in a bath tub where it was somewhat easier to see the change in water level. Clint, if this works, please avoid any urge to run naked through Syracuse.

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## Re: Technique to weigh small boats

malibusunsetter's method works for me, as does the weigh it on a commercial scale on the trailer and then come back with it off the trailer, then subtract.
How accurate does it have to be? This isn't a chem. quant. annalysis problem is it?

19. ## Re: Technique to weigh small boats

Ever since I spoke with a heavy scale mechanic and learned that a.) vehicle scales can be off by 500 lbs or more when weighing loads less than 10% of their rated capacity (a loaded tandem dump truck weighs around 50,000 lbs, so if the scales are rated for that, your 1500-lb boat isn't gonna get weighed very accurately), and b.) weigh scales are only as good as their last calibration and scales are often not calibrated for months or even years, I don't put much faith in the critters. YMMV...

20. ## Re: Technique to weigh small boats

Originally Posted by MN Dave
You could support the trailer on a pair of jack stands between or adjacent to the rear axles to eliminate the load shifting between the axles and throwing off the effective length of the lever arm.
Yes that is possible (at least theoretically), but can be difficult on most trailers because of all the springs, shackles, mounts, etc, etc, that is in the way of points where the jacks need to go. Tongue weight of all boats that need tandem axles is well above the range of bathroom scales. Then there is the issue of shifting the boat supported in this fashion. I looked at this possibility and determined that it is not a very practical way to go.

I was interested in a simple method that a home builder of my boats could use with normal equipment they might have. Also, I consider the most important data is the weight of the completed boat (dry, minus engine) just before it goes on the trailer and that is most easily done in the building shop for most builders. Like many tasks in building boats (think scarfing), this one is much easier than it might appear.

Michael is certainly correct about accuracy of large semi truck scales at low levels. Drive on scales at recycle sites are probably accurate enough since we often take 1tems of less than 100# in cars there. Still, I consider the formula easier to use and the only one that is practical during construction for boats too heavy for two point weighing of small boats with bathroom scales.
Last edited by Tom Lathrop; 03-17-2016 at 03:05 PM.

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## Re: Technique to weigh small boats

Hi Clint. Two methods to suggest. Since you create such finely crafted kits, I would think that you might be able to weigh everything at the start, then throw all the cutoffs, scraps and cans over in the corner. When you are done, weigh the waste and subtract from starting weight.
On the other hand if you have the boat in hand you might try using purchase power. Set up a sling at CG position. Hook on four part handy billy to rafter above. Run line clear of boat and down. Weigh five gallon Jerry jug and attach to line at waist height. Water weighs 8.33 lbs. per gallon. Times five equals 42.5 plus container, times four equals 170 lbs. Add Jerry jugs and milk jugs as required. Or if you can borrow a 70 or 80 lb. kid you could both have a fun learning experience!!

22. ## Re: Technique to weigh small boats

Originally Posted by mmd
Ever since I spoke with a heavy scale mechanic and learned that a.) vehicle scales can be off by 500 lbs or more when weighing loads less than 10% of their rated capacity (a loaded tandem dump truck weighs around 50,000 lbs, so if the scales are rated for that, your 1500-lb boat isn't gonna get weighed very accurately), and b.) weigh scales are only as good as their last calibration and scales are often not calibrated for months or even years, I don't put much faith in the critters. YMMV...

Good point, however, if it's a comparison (on the trailer weight vs empty trailer weight), then I would think the accuracy you refer to much less of a variable.

23. ## Re: Technique to weigh small boats

Originally Posted by Ian McColgin
Plop her in the water, notice how much the sea rises, convert to cubic feet and multiply by 64 . . .

But there are more practical ways.
Measure 100m.

Floor the car-trailer-boat combo over this distance, and time the result.
Now do the same without the boat.
The increased time in drag #1 is due to the mass of the boat.

More accurate results will be achieved in a vacuum.

Andy

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## Re: Technique to weigh small boats

Recycling places may have drive-on scales that are meant for smaller loads than semi trailers and have more accuracy.

25. ## Re: Technique to weigh small boats

For cases where the anticipated (or experimental) load is above the range of the scale, you can use a beam to get a measurement by a ratio. For example, if you have one end of the bean on the ground, the load in the center and the scale at the other end, the scale will read half of the actual load. Any ratio will work. The math. with whole, low ratios is easier. You can use different ratios if you're doing a three-point measurement.

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## Re: Technique to weigh small boats

Back when I weighed <90kg myself, I checked the bathroom scale was good for 200kg. I weighed myself first, then picked up the boat and stepped (okay, staggered) back on... That was for a very lightweight rowing dory though.

27. ## Re: Technique to weigh small boats

Originally Posted by mmd
Thank you for remembering me, Steven. Yes, the 'Lathrop Method' is the weighing procedure I posted some years ago. I have used it to weigh boats up to 7000 lbs - the thing with heavier boats is to find a scale that will handle the tongue weight, which in the instance of the 7000 lb boat was around 400 lbs. For a boat of around 1000 - 1500 lbs, an ordinary bathroom scale should suffice. To reiterate the "Lathrop Method':

Weighing a boat on a trailer

Shift the boat as far forward on the trailer as it will go.

Place a scale under the tongue support leg. Record the weight.

Measure the distance from the tongue support leg to the tip of the boat bow. Record this.

Shift the boat aft on the trailer a couple of feet, but not so far as to allow the trailer to tip backwards.

Place a scale under the tongue support leg. Record the weight.

Measure the distance from the tongue support leg to the tip of the boat bow. Record this.

Measure the distance from the tongue support leg to the centre of the trailer axle (if a twin-axle trailer, measure to the point exactly half-way between the axles). Record this.

Calculate the boat weight using the following formula:

W = C(W1-W2)/X

Where:
W = weight of boat
C = distance between tongue support leg and centre of trailer axle(s)
W1 = first (heavier) scale reading
W2 = second (lighter) scale reading
` X = distance boat was shifted between scale readings
Michael,
do you think this method would work for a boat around 350-400 lbs? What is the +/- on this method?

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## Re: Technique to weigh small boats

Clint, You say you do have a way to hang them? There are fish scales ranging from \$10-40 on Amazon (hate them though I do) that would probably work a treat in that weight range.
https://www.amazon.com/Hanging-Hunti...sh+scale&psc=1

29. ## Re: Technique to weigh small boats

I do but I am trying to get help from a couple customers with a Calendar Islands Yawl...need an easy way for them to do it.

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## Re: Technique to weigh small boats

Isn't the answer 175lbs?

for a boat this small I like the bathroom scale idea - maybe three scales and simply sum the answers if one wants to avoid any balancing and moving.

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## Re: Technique to weigh small boats

Originally Posted by Paul Scheuer
For cases where the anticipated (or experimental) load is above the range of the scale, you can use a beam to get a measurement by a ratio. For example, if you have one end of the bean on the ground, the load in the center and the scale at the other end, the scale will read half of the actual load. Any ratio will work. The math. with whole, low ratios is easier. You can use different ratios if you're doing a three-point measurement.
You could fairly easily construct a balance scale.

A sawhorse, a longish beam of consistent weight-per-length, a tape measure and an object of known weight that will rest on the beam or can be suspended from it.

32. ## Re: Technique to weigh small boats

Clint, the "Lathrop method" posted above I think I learned on the Windmill website. Works accurately enough so racing classes recommend it. If you go the hanging method, sling to the chainplates or some oarlock sockets, then use a third leg that can be adjusted to level the boat.

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## Re: Technique to weigh small boats

The scales in post #3 with two slings that may have to be secured by a line around the stem and one round the transom.Alternatively use a 6X2 suspended by it's middle with a sling at each end.

34. ## Re: Technique to weigh small boats

Originally Posted by Clinton B Chase
Michael,
do you think this method would work for a boat around 350-400 lbs? What is the +/- on this method?
For a small boat, I'd think a chain fall with a load cell would be the simplest thing - a couple of straps and a central beam would do for support. Know the strap and beam capacity and weights. You'll want a factor of safety of at least two for the lift and the weight of the tackle so you can subtract it from the total on the load cell.

35. ## Re: Technique to weigh small boats

Town dump has scales for bulk waste, construction demo loads, etc. Drive on without boat on trailer. Go home, load boat, drive on again. Subtract...et voila!

No scales at your dump? Try a salvage yard/ junk yard where they take cars and metal. They'll have scales you can probably use as above for a tip or a case of beer.

Kevin

Kevin

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