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Guesstimate Displacement?

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Old 23rd August 2015, 20:23
Almace Almace is offline
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Guesstimate Displacement?

Hello all,

Can anyone estimate the displacement of a ship based on its GRT, DWT, or measurements? For example, what might have been the weight of the RMS Media (13,345 GRT, 11,636 DWT, 531' long, 70' beam, 26' draught)?

I know I'm asking to guess weight from volume, which can't be done with any certainty or precision, but as a landlubber I'd like some idea of how big a ship is in terms I can easily grasp.

Thank you,
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Old 23rd August 2015, 21:59
randcmackenzie randcmackenzie is offline  
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Hello Eric.

There is no easy correlation between G/T and deadweight or displacement.

However if you transpose all the dimensions in to metres, it become a little easier.

So, underwater volume of the block = (length) 162 x (beam) 21.3 x (draught) 7.9 = 27,260 cubic metres.

You now have to guess the co-efficient of fineness, which is the relationship between the volume of the underwater hull and a block of the same dimensions.
Normally this will be found in the ship's hydrostatic tables.

The finest lined ships I was ever on were container ship at about 0.55 - it varies with draught, of course.

Let us take a guess of 0.65 for Media.

So 27,260 x 0.65 = 17,719 cubic metres, x 1.025 if the draught is in salt water = 18,162 metric tonnes. (17,875 long tons)

Displacement also equals deadweight + lightship.

The deadweight you are quoting is 11,636 - long tons since the dimensions you quote are imperial, say 11,822 metric tonnes.

If you now subtract that from the displacement we calculated (18,162 - 11,822) you get a light ship of 6,340 metric tonnes, which would not have been that very far from the truth.

Confused? Sorry!

Last edited by randcmackenzie; 23rd August 2015 at 22:11..
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Old 24th August 2015, 06:35
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Organisation: Merchant Navy
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It's that draught measurement that is causing the problem. Navy vessels, I believe are usually given as deadweight numbers, total weight of the vessle, stores, ammunition, fuel, etc. because there is usually very little difference in draught -- there is no large variable such as cargo. The vessel in question, however is given as "RMS", or Royal Mail Ship and therefore most likely a cargo vessel. The question then goes into many variables such as is the draught, the submerged depth of the vessel, given as referenced to which part of the Plimsoll marks, which will vary as to legal loading, and then applying to an even keel condition. If the vessel is trimmed by the stern a draught of 26 feet may be considerably less at the bow.

IMHO the best understanding of size is the GRT, (gross register tonnage) which is the interior volume of the vessel excluding the funnel and some deck houses, but including all accomodation. By definition one register ton = 100 cu. ft. of space. Net register tons is essentially the cargo carrying capacity (volume) of the vessel.

This way we can equate the size of the modern cruise ship (with huge superstructure), for example, with a cargo/container ship. The one exception that I am aware of is the tanker which carries a more or less uniform (volume of cargo of a more or less constant specific gravity) and is usually rendered as a deadweight tonnage, or the weight of cargo when fully loaded (excluding the weight of the ship), but again referenced to the applicable Plimsoll line.

I haven't been at sea for many years, so maybe one of our younger members can update my remarks, but out of the remarks so far, size doesn't always equate to weight as even the purpose for construction of any vessel will have a large bearing on the actual weight.
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Old 24th August 2015, 12:52
Almace Almace is offline
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Gentlemen, I thank you sincerely for your answers!
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Old 24th August 2015, 16:17
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NoR NoR is offline  
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Attached in 3 jpegs is one of the better explanations of tonnage that I have read.
Scanned from British Ocean Tramps Vol 1 by P N Thomas.

This book plus Vol 2 are worth getting hold of.
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File Type: jpg tonnage 1.jpg (246.6 KB, 14 views)
File Type: jpg Tonnage 2.jpg (145.9 KB, 8 views)
File Type: jpg Tonnage 3.jpg (244.0 KB, 7 views)
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