How to weigh a boat on land ?

[sarabande]

Given that you don't have an Iron Fairy to hand with a load cell on the boom, how would you go about finding the mass of a boat currently resting on its (long) keel on the ground supported by a steel cradle ?

I have a large (roughly 4m high, 5m wide) mobile gantry with a 3 ton lifting capacity in the middle of the cross strut, so idea #1 is to erect the gantry over the boat, arrange proper spreaders both athwart and fore and aft, gently raise said vessel till it is in the air, then lower even more gently on to an electronic weigh platform hired for the purpose. Am I right in assuming that if we lift the boat from the ends of the cross strut immediately above each of the A frame legs, then I can reasonably increase the lifting power to say 2 tonnes at each end ?

Yes, we do have the technical data giving the boat's official displacement, but it is fairly important to know the actuals, as that will impinge on the trailer design, and affect whether e.g. one leaves the mast unloaded to follow on afterwards.

Alternatives such as digging a hole under the CofG, and jacking up the boat with a load cell, or pace Mr Archimedes, using a very long lever and the bathroom scales will be welcomed. (For the latter method, if the boat is lifted 1m away from a fulcrum, the lever will have to be about 40m long )

The owner will probably favour a quick visit by a Chinook....



Ideas and suggestions please.

 

[lw395]

See the real waterline and integrate the volume below it?

Long lever is a possibility.
The short arm of the lever can be quite short. so the long arm more reasonable. A big steelyard.

A more practical method might be to lift it in two slings, with a short length of ss rigging wire at the end of each sling.
Then use a rig tension gauge to measure the tension in each. You'd have to correct if the wires are not vertical. And possibly calibrate the tension gauge.
How accurate do you need to be? If you are concerned about the weight of the mast that's a small %age....

http://www.ebay.co.uk/itm/sHeavy-Du...374122?hash=item5686e57cea:g:k5wAAOSwcvdXOs6h

 

[jsl]

If you are worried about lifting the whole boat at one go, you can do it in two parts, assuming you can arrange for all the weight to be borne on two (say) baulks of timber. Weigh first at one baulk (i.e. measure load between keel and baulk or between baulk and ground), then set the hull back on that baulk and repeat the operation at the other. Add the two weights together, subtracting weights of baulks if necessary, and that's your total weight. Logic says that all the weight not at one point has to be at the other, assuming you have not tilted the hull significantly, either sideways or fore-and -aft. I suppose if you had two weighing pads, you could just put them under the legs of your gantry and follow the same logic - but you would need to do some careful positioning to be sure that all the weight was accounted for.

 

[rogerthebodger]

Two thinks I would like to know is the approximate/maximum estimated weight of the boat and the profile of the hull. Shallow draft mobo or deep keel sail boat.

You are correct that the closer to the supports the greater load you can hang from the cross beam but you also need to consider the vertical supports as you could be exceeding the strut load of the vertical supports.

The other thing is stability. With a 2 point lift you must get the balance correct which may not be easy. With a 3 or 4 point lift it much easier.

If you can arrange a 4 point lift with 4 separate chain blocks you will have a lot of control and you could hire smaller load sells am add the reading from each to get the total.

The other way I have don this in the past is to jack the hull up on cross beams using heavy duty hydraulic jacks and putting the jacks on hired platform scales and adding readings.

I have used the beam and jack to load and unload heavy wood motor boats on to trailers and I used a 30 ton jack to lift my boat off its support frames for painting.

On my lightest motor boat I have used bathroom scales under each trailer wheel and add the 3 readings to get total weight then lift boat off trailer to weigh the trailer alone.

When you do this take pics and post out of interest

 

[ghostlymoron]

I think by doing that you would weigh some parts twice. I don't know if you can hire suitable load cells but you could lift each end and place a load cell under and take 2 readings simultaneously.

If you are worried about lifting the whole boat at one go, you can do it in two parts, assuming you can arrange for all the weight to be borne on two (say) baulks of timber. Weigh first at one baulk (i.e. measure load between keel and baulk or between baulk and ground), then set the hull back on that baulk and repeat the operation at the other. Add the two weights together, subtracting weights of baulks if necessary, and that's your total weight. Logic says that all the weight not at one point has to be at the other, assuming you have not tilted the hull significantly, either sideways or fore-and -aft. I suppose if you had two weighing pads, you could just put them under the legs of your gantry and follow the same logic - but you would need to do some careful positioning to be sure that all the weight was accounted for.

 

[lw395]

You can buy a small crane scale for under 100 quid.
Support it close to the balance point under the long keel, so that say 90% of the weight is on this support and the rest is on the front of the keel.
Lift the bow by the forestay attachment, using the crane scale.
Repeat with the main support moved say 200mm.
Lift the bow again.
You now have two equations and two unknowns, the distance of the CoG from the bow and the total weight.
From there it's o level physics and applied maths.
A* students will need to work out the error in W from an error in the 200mm.
Also the small error due to the CoG moving aft as you lift the bow a little.

 

[shan]

Borrow a trailer and use a weigh bridge?