# Calculating displacement for scale model ship

Discussion in 'Boat Design' started by Peter Binns, Mar 25, 2021.

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### gonzoSenior Member

It is easy on a model by using Archimedes method.

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### TANSLSenior Member

Archimedes enunciates a principle, not a method. But, leaving semantics aside, what does Archimedes' "method" consist of?. Is it only applicable to models or does it also apply to large boats although it is more difficult?

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### Tiny TurnipSenior Member

Sorry, should have been clearer if I'd said hull *displacement*. We don't have the displacement, so I don't understand how Archimedes can be used directly. However, using Ad hoc's information for gross tonnage, and the constant in the George Moorsom method, there might be sufficient information in the drawings that Peter (OP) has to allow a reasonably accurate estimate of the below waterline volume.

Would the waterline in the drawings be fully laden?

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### TANSLSenior Member

Gross tonnage (Moorsom tons) is not the same thing that displacement metric tons.
Archimedes has much more to do with the ship displacement than Moorsom has.

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### Tiny TurnipSenior Member

Yeah, that's my point. Using Moorsom, we can extrapolate a gross volume to deck (and on deck structures I think) from gross tonnage, and from the OP's drawings, it might be possible to estimate the volume below waterline by subtracting the volumes above waterline, which are rather simpler (if we know if the waterline is lightship or fully laden or what?) but we can't get directly from gross tonnage (Moorsom) to displacement.

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### gonzoSenior Member

When you make the model, submerge it to the waterline in a tank that was filled to the top. Remove the model and use a measuring cup to measure how much water was displaced. That is the displacement of the model.

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### Tiny TurnipSenior Member

Thanks for the explanation Gonzo - yes, I understand Archimedes principle. However, the OP doesn't have a model yet, he is in the planning stages and needs an *estimate* of the displacement for planning the steam power plant. I am suggesting that as the gross tonnage of the full size vessel is available, and was likely calculated by George Moorsom's method at the time, he might get a reasonably accurate estimate by converting from gross tonnage to volume, and then subtracting the above waterline and [edit: adding below waterline] non cargo volumes, which, from the limited information he has available, will be easier to work out than the below waterline volume. It is then a relatively trivial matter to scale and convert to displacement for the model.

Last edited: Mar 27, 2021
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### TANSLSenior Member

I don't remember how the moorsom tonnage is calculated but I think it has more to do with the useful space for cargo than with the total weight of the boat. If I'm wrong, you kindly tell me.

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### gonzoSenior Member

The gross tonnage is a rating for taxation, crew requirements and other legal purposes. It has no relation to the displacement of a ship.

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### Tiny TurnipSenior Member

That's quite correct - see the link in my earlier post. I'm suggesting that, from the scant information available (old photos, poor drawings) as the (Moorsom)gross tonnage is available, it might be possible (easier) to convert from gross tonnage to maximum available cargo space, subtract above waterline volumes (more information on the drawings and photos than for below waterline) add any below waterline non cargo volumes to get an estimate of the below waterline volume, which of course can yield the displacement.

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### Tiny TurnipSenior Member

If you read my posts and the links therein, you will see that the Moorsom gross tonnage is directly calculated from the available cargo volume.

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### gonzoSenior Member

I would use one of two methods. The first, more tedious, is to calculate the immersed volume from the plans. The second is to carve a hull model, close enough, out of styrofoam and use the Archimedes method.

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### gonzoSenior Member

That is a common tonnage calculation. However, it is a theoretical empty volume, which is only useful to compare similar ships.

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### TANSLSenior Member

But what is that method? I'm already starting to get intrigued. After so many years, I had never heard of him, which would surely have made my life easier.

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### Tiny TurnipSenior Member

Gross tonnage is theoretical, sure, but it is derived from a measured volume. Only one ship involved - no comparison of dissimilar ships needed. If the OP manages to source lines of this vessel or a similar one, great - he can model the hull, but at the moment, the available information is looking pretty thin.

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