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Author Topic: What do they mean by displacement?  (Read 1461 times)

tonyH

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What do they mean by displacement?
« on: September 11, 2020, 07:32:24 pm »

Good Evening All,

When I'm contemplating a new build and I look at the numbers involved for the possible ship it's relatively straightforward sorting out the length, beam and draft BUT when it comes to the approximate target weight I have been caught by the various figures given for the weight. I'm sure it's been asked before and answered somewhere on Mayhem but can someone point me in the direction of what all the possible terms for displacement/load/tonnage etc. mean?
Yours, fed up with adding lead, Tony {:-{
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Taranis

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ANDY
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Colin Bishop

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Re: What do they mean by displacement?
« Reply #2 on: September 11, 2020, 08:24:24 pm »

For modelling purposes, displacement is the key measurement as it is what a ship (or model) weighs. If you are building a scale model then you can use the displacement tonnage of the original vessel to work out how much your model will weigjh. (as long as you build it accurately!)

Colin
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Jerry C

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Re: What do they mean by displacement?
« Reply #3 on: September 12, 2020, 01:37:58 am »

A body when floating displaces it’s own weight of whatever it’s floating in. ( Archimedes, eureka and all that). It’s weight in nautical terms is displacement.
Jerry.

Des

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Re: What do they mean by displacement?
« Reply #4 on: September 12, 2020, 02:43:41 am »

I find it intriguing that before making a single cut of material for a model, I know, and indeed I need to know, what the finished model will weigh.  If the weight of all the materials, motors & drives, paint, fittings, etc isn't enough then I need to add ballast.  If the weight of all that stuff is greater than the pre-determined finished weight then I have a problem.  Hence I weigh the model throughout the build, 'cos if I have to remove some of the structure then it is much easier to do so before I put the deck down rather than after.
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Jerry C

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Re: What do they mean by displacement?
« Reply #5 on: September 12, 2020, 03:04:19 am »

Look up “Box Coefficient”. This is a simple method of determining approximate displacement of various types of vessel. LWLx Beam x Draft x coefficient, will give volume of displacement. eg. A box shaped vessel coefficient is 1. A bulk carrier approx 0.85. A general cargo ship 0.75 and a destroyer 0.6. These are guesstimates from faded memories.
Jerry.

Des

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Re: What do they mean by displacement?
« Reply #6 on: September 12, 2020, 04:10:50 am »

Yes, I found that method of calculating displacement - but it is a guess as to the 'k' factor, especially for tugs of differing hull shapes.  Bu tit is close enough for a first, rough, ball park guess.

I develop my hull Lines on CAD, then develop the frame shapes needed to achieve the desired hull shape.  It is then relatively simple to calculate the area of each frame below the water line, and then estimate the displacement between frames.  Then total them all up.

Des
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derekwarner

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Re: What do they mean by displacement?
« Reply #7 on: September 12, 2020, 04:43:13 am »

As Jerry says......'guesstimates from faded memories' ........ {-)


.......lets not forget in recorded history, displacement became important when the British Kings wanted to ensure Wine Excise Tax were paid in accordance with the number of Tuns [containing Spanish wine] that a vessel could bring back


Yes.......a Tun was a very largeish wooden barrel........so if the vessel could carry 100 Tuns,......it became that the vessel had a capacity of 100 Tons displacement


Derek
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Derek Warner

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Jerry C

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Re: What do they mean by displacement?
« Reply #8 on: September 12, 2020, 06:51:05 am »

Yes Des, CAD should do admirably. Old fashioned method I was taught for Masters Ticket was to use Simpsons Rules which is simplified (for simple sailors) calculus. Very good for areas and volumes.  In real life “tonnage” has various interpretations. Displacement Tonnage is the only one that specifies the actual tonnage in Tonnes (1000kg). Gross tonnage is the actual measure of internal space in a ship 100f3/ tin, note spelling. Net tonnage is the internal space with exempted spaces deducted, ie crew spaces, machinery spaces etc. Measured in 100ft3/ton. Deadweight Tonnage is the amount of cargo a ship can carry measured in Tonnes (metric tonnes). Before we went metric tons were 2240lbs.
Jerry.

Colin Bishop

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Re: What do they mean by displacement?
« Reply #9 on: September 12, 2020, 08:49:38 am »

Quote
.it became that the vessel had a capacity of 100 Tons displacement

I think that should be capacity only Derek. You can't have a capacity of displacement!

Colin
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Colin Bishop

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Re: What do they mean by displacement?
« Reply #10 on: September 12, 2020, 09:51:05 am »

As Jerry points out above, there are lots of 'tonnage' definitions but displacement tonnage, or what the vessel would weigh if placed on a big enough set of scales, is the only one relevant to modelling.

To repeat a comparison I have made previously:

The commonly quoted ‘size’ Comparison for passenger ships is gross tonnage which is actually a measurement of internal space. Queen Mary was 81,000 tons while QM2 is no less than 151,000 tons, almost twice as much. However, when it comes to displacement tonnage, which reflects the actual weight of the vessel, the two ships are much the same and in fact the earlier ship is quoted as 80,000 tons as opposed to QM2’s 76,000 tons. (The big American Nimitz class aircraft carriers displace over 100,000 tons!). Queen Mary was 1019 feet long (311m) with a beam of 118 feet (36m). QM2 is 1,132 feet long (345m) with a beam of 135 feet (41m).  But Queen Mary had a deeper draught of 39 feet (11.9m) compared with QM2’s 33 feet (10.1m) and a rather fuller underwater hull form. Basically what this means is that QM2 offers a huge amount of extra usable space on a similar displacement compared to Queen Mary which reflects shipbuilding progress over the last 70 years.

The photo below shows the two older Cunard Queens and the current QM2 to the same scale and illustrates this.

Colin

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Jerry C

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Re: What do they mean by displacement?
« Reply #11 on: September 12, 2020, 11:04:07 am »

An excerpt from one of my Discharge Books. Note Tonnages for Mobil Eagle in the left hand stamp. They never mention the deadweight tonnage which was 284,089dwt, that’s the weight of crude oil she carried. Draft loaded 28.78 M
The largest ship I sailed on.
Jerry.

tonyH

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Re: What do they mean by displacement?
« Reply #12 on: September 12, 2020, 11:37:27 am »

Yup Colin BUT if you're building something from scratch and the reference books give varying terms which some could read as "weight" ........?If you look at early books, such as Rees's Naval Arcitecture from 1819, they worked out from the King's Rules and then a Mr Parkyn of Chatham came up with a better version so, for example Victory was calculated at 2162 tons by KR, 1839 by PR and 1840 actual. Adamant with 50 guns was 1044 tons by KR, 870 by PR and 886 actual. Tonnage then was specified to be at the light watermark.A further check was made, where possible, by reverse engineering the hull model.
Going later, one of the standard references, Janes, for 1898 doesn't give any displacement figures atall. Conway 1860-1905 give displacement for most, displacement (load) for some and displacement (normal?) or displacement(standard?) for others. Janes WW1 either give the same 3 options or just the number of tons with no explanation.
What actually caused me to ask the question was the current build which is a simple steam trawler from 1905/6.A general reference book gave 276GT, Register tonnage, as part of a mortgage, was 306, Admiralty weights give 360t light and 547t load.
OK, general guidelines say you can always add weight and in this case I've run out of lead flashing BUT if you're building, for example, a model of a WW1 Z class destroyer at 1:100. The tolerances can be so little as to make the boat fall over, or worse, as soon as you hit the wet stuff BUT if you change from using the Europlans at 1:100 to 1:96 to match your opposition you get a 13% uplift in available weight.
Thank you for your input sofar. I hope you don't think it a futile exercise but I'm sure you know that we wouldn't want to be overweight :-))
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Colin Bishop

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Re: What do they mean by displacement?
« Reply #13 on: September 12, 2020, 12:30:10 pm »

Tony,

Yes I agree that even when working with displacement tonnage you need to be very careful about which ‘tonnage’ units are being quoted and how they are being applied such as ‘standard’, ‘light’ or ‘deep’ displacement for warships. And of course the likely accuracy of the measurements.

Gross tonnage is often quoted as displacement by people who should often know better as well as those that don’t. For vessels such as cruise ships the difference can be huge as in the case of QM2. In earlier times the difference could be a lot smaller such as for the old Cunard Queens and this is possibly why the two sets of figures came to be seen as roughly interchangeable for popular comparison.

It is good to air these issues as it helps promote understanding (hopefully!)

Back in the early 1980s I built a model of the Isle of Wight ferry Shanklin. I purchased the plans from the National Maritime museum for £50!
The plans were 1/4in. = lft. scale which would have resulted in a model 50in. long and 12in. beam - a bit large for my purposes – I wanted something about 3ft. long. At l:72scale, 3/16in. = 1ft.; the length went down to33in. and the beam to 8in. This was also near enough to model railways OO scale. This size was O.K. but there was a snag. Shanklin has a very unusual hull shape and a glance at the lines made it glaringly obvious that there wasn't very much hull underneath the water to hold the rest of the ship up. The Denny List gave displacement tonnage as 820 (gross tonnage was 833!) and at 1.72 scale this resulted in a model weight of 78oz. or just under 5lb. Weight of radio, batteries and motors, etc., came to 39oz. leaving a further 39oz. for hull, superstructure and fittings. Obviously it could be done but construction would need to be kept fairly light. A good point was that the 'fish platter' shape of the lower hull meant that the weight needed to bring about a given amount of sinkage increased rapidly with immersion giving in practice a further 8oz. to play with. As it turned out, when ballasted to the correct waterline the all up weight was 74oz. – not far off the mark!

Colin
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