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# A thoroughly nautical question for salty Jetblasters.

Jet Blast Topics that don't fit the other forums. Rules of Engagement apply.

# A thoroughly nautical question for salty Jetblasters.

18th May 2020, 21:20

Join Date: Apr 2014
Location: Scotland
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Originally Posted by kkbuk
In order to float, a ship has to displace a certain volume of water equivalent to the weight of the ship. This is true whether the ship is in a dock or not. The volume of the water in a flooded drydock will be more than if there was a ship in it. Put a ship of say 10,000 tons into that dock then 10,000 tons of water will be 'displaced'. As a drydock is flooded, the amount of water that enters the dock will be less if there is a ship in it than if the ship wasn't there. Archimedes wasn't a numpty!!!

Ok, but the point was that water volume doesn't actually have to be 'displaced' or even exist. Say for example, you dug a hole in the ground with a volume of 11,000 cubic meters and then constructed a ship in it with a volume of 10,000 cubic meters, if you fill it with 1000t of water, your ship will still float but you haven't 'displaced' anything. Pretty trivial, but it makes the point (that Archimedes, as you say, understood); 'displacement is an equivalent measure, but it is not the cause of the effect (i.e. buoyancy).
18th May 2020, 22:10
Psychophysiological entity

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Location: Tweet Rob_Benham Famous author. Well, slightly famous.
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In my yarn, the wondrous Maria Celeste had a mass of 83,000 tonnes. She spent most of her time a few light-seconds away from the planet. In all the years I've lived with her crew I never once wondered how they measured her bulk. However, a PPRuNe member did the calculations for the energy required to accelerate her to provide 'gravity', so I suppose it would be simple to deduce. Which brings me to the point. Now we're getting so nifty at sensing things, perhaps we could measure the weight of a ship by the forward load on the prop-shaft, vs the acceleration - factored of course for the temperature and salination of the water.
19th May 2020, 08:04

Join Date: Dec 2011
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All I know, as a traditional marine carpenter / boat builder is that the weight of a boat on papers or engraved on her beam only serves the purposes of Authorities and very rarely, never the same as Reality...and the readings on the gage of the crane!!!! Even varies from crane to crane!. The only weight that matters is, the pleasure she brings to her owner....and do not blame the shipwright if she is too heavy compared to the plans, marine architects are dreaming nuts..... (joke)
19th May 2020, 19:05

Join Date: Jun 2011
Location: Falkland Islands
Posts: 113
Quote:
Originally Posted by kkbuk View Post
In order to float, a ship has to displace a certain volume of water equivalent to the weight of the ship. This is true whether the ship is in a dock or not. The volume of the water in a flooded drydock will be more than if there was a ship in it. Put a ship of say 10,000 tons into that dock then 10,000 tons of water will be 'displaced'. As a drydock is flooded, the amount of water that enters the dock will be less if there is a ship in it than if the ship wasn't there. Archimedes wasn't a numpty!!!
Originally posted by recc Ok, but the point was that water volume doesn't actually have to be 'displaced' or even exist. Say for example, you dug a hole in the ground with a volume of 11,000 cubic meters and then constructed a ship in it with a volume of 10,000 cubic meters, if you fill it with 1000t of water, your ship will still float but you haven't 'displaced' anything. Pretty trivial, but it makes the point (that Archimedes, as you say, understood); 'displacement is an equivalent measure, but it is not the cause of the effect (i.e. buoyancy).
Easy to demonstrate that the “displaced” water does not need to exist to float an object - take two concentric mixing bowls, pour water into the gap between them until the upper bowl floats. I just floated a 350g bowl on 50g of water. The displacement is theoretical.
19th May 2020, 19:32

Join Date: Jan 2008
Posts: 11,567
Originally Posted by Ant T
The displacement is theoretical.
Though not if you fill the outer bowl before putting the smaller one in.
19th May 2020, 20:22
I don't own this space under my name. I should have leased it while I still could

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Take a submarine, HMS Dreadnought displaced 3,500 t on the surface but 4,000 t submerged.
19th May 2020, 21:02

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And now, for a good laugh, let's introduce the concept of Mass, which has displaced weight in aviation for reasons I never understood. What's the Mass of the QE2, for example?
19th May 2020, 21:02

Join Date: Feb 2007
Location: Spain
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Originally Posted by Ant T
Easy to demonstrate that the “displaced” water does not need to exist to float an object - take two concentric mixing bowls, pour water into the gap between them until the upper bowl floats. I just floated a 350g bowl on 50g of water. The displacement is theoretical.
You really do not grasp this, the volume of water that would have to be put into the larger bowl in order to fill that bowl if the floating bowl was missing would weigh 350 grams. When you removed the floating bowl did the water retain the shape of the removed bowl? No, of course not.
19th May 2020, 21:08

Join Date: Feb 2007
Location: Spain
Age: 77
Posts: 55
Originally Posted by Pontius Navigator
Take a submarine, HMS Dreadnought displaced 3,500 t on the surface but 4,000 t submerged.
That may be because in order to sink, the submarine had to take on board a great deal of water, about 500 tons or so!
19th May 2020, 21:17

Join Date: Jan 2008
Posts: 11,567
Originally Posted by old,not bold
And now, for a good laugh, let's introduce the concept of Mass, which has displaced weight in aviation for reasons I never understood. What's the Mass of the QE2, for example?
Bring back slugs.
19th May 2020, 21:26

Join Date: Jun 2011
Location: Falkland Islands
Posts: 113
You really do not grasp this, the volume of water that would have to be put into the larger bowl in order to fill that bowl if the floating bowl was missing would weigh 350 grams. When you removed the floating bowl did the water retain the shape of the removed bowl? No, of course not.
No - I really do understand this! I totally understand that the volume of water that would occupy the space that the part of the bowl below the waterline now occupies, would be the same as the weight of the bowl. All I am pointing out, (in support of recc’s comment above), is that the water does not have to have existed in order for the bowl to float.
20th May 2020, 01:06

Join Date: Jan 2008
Location: Denver
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Oddly, I just had to explain this - more or less - to someone with regard to floatplanes. How large the floats must be (how much water they must displace) to hold a plane of weight X out of the water with a given amount of freeboard.

I started out, "Just imagine each float as a tiny, tiny battleship....."
22nd May 2020, 16:36

Join Date: Dec 2007
Location: Shangri-la
Posts: 49
Tonnages: The Old Way

This is going back to 1950's technology so may have changed over all the decimalisation years.
Gross tonnage: A measure of the ship's internal capacity of all permanenlty enclosed spaces, measured at the rate of one hundred cubic foot to the ton,
Nett tonnage: A measure of the ship's cargo capacity, with all non earning spaces deleted, measured at the rate of one hundred cubic foor to the ton.
Deadweight tonnage: A measure of the ship's carrying capacity, i.e. the difference between light dispacement (with fuel and stores on board) and full load dispalcement. Tankers are usually measured in DWT. A ton of water is 35 cubic foot of sea water at SG of 1025.
Displacement tonnage: The weight of a ship and her contents in actual avoirdupois tons, which will change depending on her cargo,stores and all other weights on board. A retangular barge 200 ' in length 20' beam and a draft of 10 ' would displace 200 x 20 x 10 divided by 35 = 1142.8 tons. Ships of course are not barge shaped, they each have a unique Co-efficient of Fineness, so a fine lined sailing vessel with the above dimensions and a Coeff of .488 would displace 200 x 20 x 10 x .488 divided by 35 = 557.7 tons
Full Load Displacement: The maximum displacement, as determined by her Load Line or Plimsoll Line, which does vary according to the season and area.
Warships of course are not subject to the load line regulations, but no naval designer wants his ship to break up, so limits are placed.
And of course, these apply to container ships but they are usually measured in TEU (Twenty Foot Equivilent Units)
I do not imagine anybody really wants a serious answer, but here it is, as in the Fifties!
E&OE

Last edited by very old flyer; 23rd May 2020 at 10:03. Reason: another typo

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