Model Boat Mayhem

Mess Deck: General Section => Chit-Chat => Topic started by: polaris on January 02, 2009, 06:41:01 pm

Title: The weight of things..........
Post by: polaris on January 02, 2009, 06:41:01 pm

Dear All,

If a ship might have enough space to be able to accommodate another ship inside it (a tanker for example), and if the ship inside the 'mother vessel' is floating on water captured inside the other vessel (viz., the captured vessel can float in and out if needs be - but be sealed in if required), what is the weight of the whole? Since the vessel inside the 'mother vessel' is floating on water, I presume that displacement and weight is proportional? - the water was there in the first place, so this is a constant... it just has something floating on it. If the water was to be pumped out, the weight of the inside vessel then becomes part of the 'mother vessels' weight'. So, the question is, with say a 'mother vessel' weighing 20k tons: what is the weight of the vessel with the inside vessel floating, and the weight of the inside vessel not floating??? - will the 'mother vessel' weigh more with the inside vessel not floating within it? - the inside water weight is 'irrelevant' in that if the inside vessel can come and go as it pleases, the 'mother vessels' weight does not alter as this vessel has a 'constant' so to speak... but with the water pumped out the inside floating vessel then becomes a dead weight - the inside vessel only becomes a burden on the 'mother vessel' if the water supporting the former is pumped out! - but the 'mother vessel' will then float higher and have added buoyancy to support the inside vessel??? - in other words it will counter act... the inside water only becoming a weight when it's sealed off from the outside water and pumped out.

Regards, Bernard
Title: Re: The weight of things..........
Post by: Ghost in the shell on January 02, 2009, 06:49:17 pm
thats one for archemedes
Title: Re: The weight of things..........
Post by: polaris on January 02, 2009, 06:57:15 pm

Dear Ghost,

It was just something I was thinking about on/off during the day while doing other things. It will get some little grey cells working somewhere I'm sure! - but it is an interesting little matter! %%

Regards, Bernard
Title: Re: The weight of things..........
Post by: polaris on January 02, 2009, 07:07:35 pm

Dear All,

To further complicate this: if the inside vessel was to weigh, say, 150k tons gross and still float inside a vessel weighing 20k tons, in theory if the water was pumped out the whole lot would sink!!! %%

I am sure there is an easy answer to this... somewhere! Too much physics for my liking!

Regards, Bernard
Title: Re: The weight of things..........
Post by: das boot on January 02, 2009, 09:19:51 pm
Someone has been watching far too much James Bond.... %)

Rich
Title: Re: The weight of things..........
Post by: toesupwa on January 03, 2009, 12:36:36 am

If a ship might have enough space to be able to accommodate another ship inside it (a tanker for example), and if the ship inside the 'mother vessel' is floating on water captured inside the other vessel (viz., the captured vessel can float in and out if needs be - but be sealed in if required), what is the weight of the whole? Since the vessel inside the 'mother vessel' is floating on water, I presume that displacement and weight is proportional? - the water was there in the first place, so this is a constant... it just has something floating on it. If the water was to be pumped out, the weight of the inside vessel then becomes part of the 'mother vessels' weight'. So, the question is, with say a 'mother vessel' weighing 20k tons: what is the weight of the vessel with the inside vessel floating, and the weight of the inside vessel not floating??? - will the 'mother vessel' weigh more with the inside vessel not floating within it? - the inside water weight is 'irrelevant' in that if the inside vessel can come and go as it pleases, the 'mother vessels' weight does not alter as this vessel has a 'constant' so to speak... but with the water pumped out the inside floating vessel then becomes a dead weight - the inside vessel only becomes a burden on the 'mother vessel' if the water supporting the former is pumped out! - but the 'mother vessel' will then float higher and have added buoyancy to support the inside vessel??? - in other words it will counter act... the inside water only becoming a weight when it's sealed off from the outside water and pumped out.


Bernard... Ease off on the paint / epoxy / CA fumes if i were you..  %%
Title: Re: The weight of things..........
Post by: polaris on January 03, 2009, 10:06:44 am

Dear Rich,

You are perceptive. I didn't watch the film this time round (but knew it was on), and it was indeed that film that started me thinking about this on/off quite a few years ago.

There is a double buoyancy factor: something of a specific weight/buoyancy floating on water, with something of a far greater weight floating within it. This is the bare bones of the equation. In theory, a vessel weighing 20k tons, capable of 'containing' a floating vessel of 150k tons, would mean the 20k ton vessel would only have to move it's own weight - viz. 20k tons.

If, however, one has a hull of a vessel, you place another ship within it, and place weight within that, and then make it float by putting water in the hull, the whole and total gross weight will increase proportionately. However, if the water level within is kept at a constant with the water the main hull is floating on, the vessel within becomes a zero weight (viz., the inside vessel/weight weighs nothing!).

Interesting isn't it! I have and claim copyright!!!

Regards, Bernard
Title: Re: The weight of things..........
Post by: malcolmfrary on January 03, 2009, 10:56:19 am
If the 20K vessel, complete with water, gains another vessel, and the water contained remains aboard, then the weight of the added vessel is gained.  If the contained water has a connection to the outside world, the added vessel will displace its weight in water to the outside world, and its total displacement will be unchanged.
Assuming that the 20K vessel is a HUGE hollow box that is capable of holding, say, 200K of water without sinking, then adding a 150K vessel to the inside (assuming 150K is its total displacement) and allowing the excess water to escape will leave the total displacement unaffected.
The Stirling lift wheel works on this principle, it has two open chambers filled with water.  A barge from the high canal floats into the top one, displacing its own weight in water back into the canal.  The gate is sealed, the wheel turns and chamber, water and boat are lowered.  at the same time the lower chamber, with the identical weight, is raised.  Whatever the weight of cargo, the two chambers are always the same weight.  The only way a chamber could be made heavier than its mate would be to drive an object into it that was heavier than the water it displaced, and that would not, by definition, be a boat.

Having seen the film, I wonder at a submarine probably capable of 25kt on the surface and with the ability to dive getting caught by a 20kt (being generous) bulk tanker almost incapable of maneuvering.  How the bow doors remained attached is a mystery as well.  I did like the fittings in the escape pod, though.
Title: Re: The weight of things..........
Post by: Reade Models on January 03, 2009, 12:20:03 pm
Archimedes Principle - for that is what is being discussed here, states in simple terms that a buoyant vessel will displace its own weight (mass) of water.

'Displaced' is probably most easily defined as "moved to somewhere else".

Thus a floating boat placed in a tank of water (or any other fluid for that matter, regardless of its specific gravity) that is full to the brim, will cause the tank to overflow.  The weight (mass) of water, or other fluid, that overflows will be equal to the the weight of the boat.

Any object that does not float i.e. is sunken, displaces its own volume.

http://en.wikipedia.org/wiki/Buoyancy (http://en.wikipedia.org/wiki/Buoyancy)

Malc
Title: Re: The weight of things..........
Post by: Mankster on January 03, 2009, 04:10:02 pm
It isn't as complicated as it first seems. If the water within the "mothership" can freely enter and leave then it is akin to a moonpool so the mothership is essentiallly a tug, there is no change in displacement or  weight of the mother ship. If water in the flooded part of the mothership can be pumped in and out then the mothership is a semisubmersible with a big ballast tank that can be filled with water, another ship or a combination there of. Displacement will change according to how much ballast (water, second ship) is taken on board.
Title: Re: The weight of things..........
Post by: polaris on January 03, 2009, 06:54:37 pm

Dear Mankster,
 
This was my conclusion, and we are both thinking on the same lines, but I thought the solution couldn't be that easy!!! One would think that the potential for fuel saving could be significant, since there is the possibility that if the engineering could be overcome a 20k ton shell with a small engine might be capable of moving a great weight that would otherwise need a massive engine - which most cargo/container ships have. I know this thinking is very much 'in brief', but the possibility is there all the same - indeed, I might go so far in saying that it will happen one day.
 
Interesting isn't it.........

Regards, Bernard

(Malc, thanks for your tech. Post).
Title: Re: The weight of things..........
Post by: Colin Bishop on January 03, 2009, 06:57:35 pm
Don't make the mistake of confusing weight with mass/inertia! The International Space Station may be weightless, but you still need a lot of force to move it!
Title: Re: The weight of things..........
Post by: Reade Models on January 03, 2009, 07:16:01 pm
Don't make the mistake of confusing weight with mass/inertia!

I wasn't...

Malc




Title: Re: The weight of things..........
Post by: Colin Bishop on January 03, 2009, 07:46:30 pm
Just a general comment Malc!
Title: Re: The weight of things..........
Post by: Max Power on January 03, 2009, 07:58:43 pm
Polaris, you have obviously had too much time on your hands over the festive period! (A perhaps a like too much libation <*<
What is important is surely not the wt of the larger (mother ship) but the maximum displacement it is capable of when fully laden. If this does not exceed the unladen weight (20T in your example) plus the weight of any cargo (ship carried inside it) it will obviously sink. While the interior hold of the mother ship is open to the ocean and the vessel inside it is obviously floating itself. I doubt very much if it is possible to build a ship big enough to contain a vessel of 150T without the larger vessel having a possible displacement well in excess of 150 + 20 T. In fact now that I think about it again I am certain of it! Your whole question is a teaser and a red herring and I have just spent ten minutes trying to answer it <*<
Title: Re: The weight of things..........
Post by: polaris on January 04, 2009, 06:48:13 pm

Dear Max,

Wish what you said re time was true... it's always the time when I can catch up on the things that can't be done at other times - and I very very rarely touch alcohol these days (so both factors wrong in this instance! :P ;D :-)

It's always best to read all Topic Posts: you will notice Mankster get's very close to the principal/idea, and I quote again what he said to save you looking back - see end of this Post.

10 mins. thinking about it... that all... shame on you! I have been pondering it on/off for a year or two. It's an interesting theory that needs hard maths/physics - which is beyond me! %%

Regards, Bernard

Mankster's Post
It isn't as complicated as it first seems. If the water within the "mothership" can freely enter and leave then it is akin to a moonpool so the mothership is essentially a tug, there is no change in displacement or  weight of the mother ship. If water in the flooded part of the mothership can be pumped in and out then the mothership is a semisubmersible with a big ballast tank that can be filled with water, another ship or a combination there of. Displacement will change according to how much ballast (water, second ship) is taken on board.
Title: Re: The weight of things..........
Post by: polaris on January 05, 2009, 10:34:01 am

Dear Colin,

I have wondered about this in the past from time to time. Whilst something in a vacuum still has a weight, without gravity it doesn't - because it is 'floating' - but it has it's own weight all the same - but no resistance in it's movement apart from it's own weight... but without gravity there is no weight! It mustn't be forgotten that the Space Station is not out of range of gravity, and is constantly doing it's best to get back home! - so, yes, it does have weight... proportionately more than if it was in gravity free deep space. Once something is given a good 'kick' in deep space, huge speeds are obviously achieved due to no resistance... the matter is more how to slow things down before hitting something in this instance! - near perpetual motion! The 'slingshot' principal of using a bodies gravity to boost a space vehicle was/is a superbly brilliant piece of engineering physics.

A space vehicle motor relies on thrust obviously... but it always makes me wonder what the motor pushes on in a vacuum to achieve movement! Obviously thrust alone of course. Lot's of physics that was sorted out quite a while ago... mind you, things are moving onto ion/particle drive... makes one wonder how far this tech. will reach one day! - ships maybe???!!!

What has all this to do with ships floating on external water in ships floating on water! Well, they said it was impossible to fly, that the motor vehicle wouldn't catch on, that it was impossible to travel faster than 30mph in a train as one would die from the speed, and that there would only be a market for a handful of computers!!!!!!!!! I have always believed in the philosophy of looking at how things can be done as opposed to why they can't!!! - the former being the essence of all invention..........

Anyway, break over, warmed up again, so back to work!

Regards, Bernard

Title: Re: The weight of things..........
Post by: Colin Bishop on January 05, 2009, 11:15:19 am
Bernard, rocket motors as used in space rely on ejecting gas at high velocity to give them thrust. The gases go one way, the motor goes the other in direct reaction. Ion drives chuck out ions to give the same effect. Both "burn" fuel so they don't last forever.

You can see the effects of low resistance very easily on Earth though. Say you have a boat weighing 4 tons or so sitting on a trolley. The chances are that no matter how hard you push you will not be able to overcome its inertia and move it. However, if the boat is alongside a quay in still water you can start pushing and in a minute or two the boat will begin to move slowly as the water offers little resistance. Be sure to jump on board before the gap becomes too great. ok2
Title: Re: The weight of things..........
Post by: Roger in France on January 05, 2009, 12:03:39 pm
Colin's explanation/example is a good one as it also illustrates inertia. You may not be able to start an object moving because of its inertia but once it is moving you need significantly less force/power to maintain or increase movement.

With space travel it is not useful to think of "thrust" despite that term being popular. As Colin describes, it is a reaction to the engine emission.

Thinking about weight and mass, it helps to remember that anything which exists has mass but it has no weight unless acted upon by gravity. The weight of a floating object in water appears to decrease because gravity is acting downwards on the water. Which is why you need a relatively less amount of force to lift an object from a depth of water but much more force to lift it clear of the water.

Roger in France.
Title: Re: The weight of things..........
Post by: polaris on January 05, 2009, 01:44:43 pm

Dear Roger,

Re weight under/above water. Yet a one kg. lead weight underwater (whatever depth), weighs exactly the same as the same above water. One little additional equation in this though is the added weight of the lifting line attached to same... the deeper it is the more the gross will be (& visa versa).

I wonder how much the mean sea level would drop if all floating vessels were taken out of the water at the same time!

Regards, Bernard
Title: Re: The weight of things..........
Post by: dreadnought72 on January 05, 2009, 02:44:08 pm
Quote
I wonder how much the mean sea level would drop if all floating vessels were taken out of the water at the same time!

About the same distance as the width of a spider web strand.

This page (http://en.wikipedia.org/wiki/Oil_tanker) tells me that (in 2005) 36.9% of the world's fleet were oil tankers, and that they displaced 960 million DWT. So I'll suggest that the total mass of all ships displaced by the oceans is around the 2.6 billion tonne mark.

The oceans cover 3.61*1014 square metres, so on average each square metre displaces 0.0072 kg.

Since 1kg is equal to 1mm of ocean per square metre, 0.0072 kg is seven thousandths of a millimetre.

Andy #1963#
Title: Re: The weight of things..........
Post by: polaris on January 05, 2009, 03:14:36 pm

Dear Andy,

Thankyou for a very interesting input/Post. Didn't expect to get such a thorough tech. reply as that! :-))

Regards, Bernard
Title: Re: The weight of things..........
Post by: dreadnought72 on January 05, 2009, 03:45:35 pm
Not at all. Hopefully it beats listening to me droning on about the potential difference between inertial and gravitational mass, and how this ties in with Einstein's Equivalence Principle.

...Which I nearly did.  %%

Andy #1963#
Title: Re: The weight of things..........
Post by: polaris on January 05, 2009, 05:42:50 pm

Dear Andy,

I know there are some flaws in my ship-within-ship idea, not least of which is what happens in a rough sea(!), but, as always, and with a bit of determination and 'where there is a will there is a way' thinking, there is a way around everything. About 40 miles away there is a long and deep railway cutting: when the rly. was proposed many so called experts said it was 'impossible'... needless to say David Davies took no notice and simply did it! - Brunel did such things numerous times: one of the best that springs to mind being the Maindenhead Bridge (one of the widest low arched brick bridges ever built - still is I think).

Drone away! What were you going to say re inertia etc.?

Regards, Bernard
Title: Re: The weight of things..........
Post by: malcolmfrary on January 06, 2009, 09:51:02 am
Quote
Re weight under/above water. Yet a one kg. lead weight underwater (whatever depth), weighs exactly the same as the same above water.
No.  It weights whatever it weighed above the surface minus the weight of water displaced.  It will have the same mass as on the surface for the purpose of working out inertia.
Dangle a lump of lead from a spring balance, take a reading, then dunk it and take another reading.
If lead is about 11 times the density of water, there will be a 10% drop in the measurement.
Title: Re: The weight of things..........
Post by: polaris on January 06, 2009, 06:18:39 pm

Dear Malcolm,

All suitably deep outside test water is under three inches of ice! {:-{ Will however ask a physicist colleague/friend for opinion - specialises in fluid flow and all things elec. (his Father was a scientist in Germany during WW2 and was thankfully one of the few bagged by us and not the Americans! - the latter being somewhat very focused on rockets they missed quite a few other very important people in gnrl. science! :-))).

Regards, Bernard
Title: Re: The weight of things..........
Post by: polaris on January 06, 2009, 06:24:47 pm

p.s. My last, the equation of line v. lifting weight is one of the important factors in lifting weights through mine shafts. You might have a cage weighing ten tons, but a mile down the 'rope' might weigh 30 or so tons! The 'rope' being in graduations of diameter to reduce overall winding weight... the 'rope' needs to be strongest nearer the top when at full extension obviously... just a further bit of gnrl. interest info.! B.
Title: Re: The weight of things..........
Post by: Reade Models on January 06, 2009, 06:45:44 pm
Before slack rope detector switches were introduced on South African gold mines (my home turf) the weight of shaft ropes was responsible for almost as many deaths in the gold mining industry as rock bursts.

Occasionally a cage would jam in a shaft, but the winder would continue paying out rope until the jammed cage couldn't bear the extra weight of rope on its roof any longer.  The cage would suddenly break free and go hurtling down the shaft to a certain death for all of the occupants.

We used to drop test 1000KVA transformers. We knew from experience that you could roll a transformer into a cage at ground level (on a short rope) without any problem.  Take the cage down 3.5 Kilometers and try rolling the transformer out onto the landing - as the weight of the transformer transferred from the cage to the landing, the cage would shoot back up the shaft as the rope contracted neatly 'flipping' the transformer through a couple of somersaults before it landed (usually upside down).  Needless to say, we were all very adept at keeping out of the way...

Malc


Title: Re: The weight of things..........
Post by: malcolmfrary on January 06, 2009, 06:51:52 pm
My lake is similarly afflicted, but an enquiring mind should direct itself to either a hammer, or, for the more scientific, a bucket of water indoors.
For a suggested use of a tapered rope, have a read of "Fountains of Paradise" by Arthur C Clarke (inventor of the communication satellite so we are told)
Title: Re: The weight of things..........
Post by: polaris on January 06, 2009, 06:56:00 pm

Dear Malc,

Good to see another mining man on here! Thought I was alone! Hope you found the blasting pics. of interest? Have refrained from Posting more.

Regards, Bernard
Title: Re: The weight of things..........
Post by: polaris on January 06, 2009, 07:09:19 pm

Dear Malc,

Re hammer and ice, have enough to do without going to such extremes! ;D :} There are much warmer ways of doing things! :-)) As to a bucket of water: done this years ago, and it never proved anything!

Regards, Bernard
Title: Re: The weight of things..........
Post by: Reade Models on January 06, 2009, 07:56:12 pm
Dear Malc,
Good to see another mining man on here! Thought I was alone! Hope you found the blasting pics. of interest? Have refrained from Posting more.
Regards, Bernard

Hi Bernard

I'm not sure if I've seen the blasting pics?  Were they opencast?  I used to work for Johannesburg Consolidated Investments, part of Anglo American, first off on Platinum mines in the Rustenburg/Thabazimbi area, then later on shallow coal mines at Witbank and on deep level gold mines, Randfontien Estates and Western Areas - my only opencast experience was with drag lines on the highveld coal mines, though I was involved at Palabora Mine near Phalaborwa (on the edge of the Kruger Park) with open pit mining of copper ore for RTZ.  Plenty of blasting there! We had a herd of elephant resident on the plant, leopards too.  A group of baboons used to ride the run of mine conveyor sitting one behind the other - hilarious!

A world away from my life now....(Google Earth coordinates below).

Malc

  2359'30.75"S
  31 7'34.08"E

Title: Re: The weight of things..........
Post by: polaris on January 06, 2009, 09:23:48 pm

Dear Malc,

Blasting links at end of Post. I partic. like the Glory Hole rounds.

You have been about! JCI good Co. to be with - good old BarneyB! Did a bit of cons. with CG and DeB, and later a some for INCO, followed by gen. with 'privates'. Had my own opers., but not on that scale! - all in a prev. life now! - still miss it now and again <:( - but much less stress these days :}! Was a C. of IMM until the IoM3 came along, now still C. with Iom3 but no fees now!!! - quite happy with this as would not otherwise be worth the fees - the latter now only a mere shadow of IMM as you know.

We have seen a few Baboons sitting on moving surfaces etc. over the years haven't we!!!!!!!!!!!!!!!!!! O0 :}  - all heading the same way as well!!!!!!!!!!! {-) %% Have lot's of fond memories!!!!!!!!!!  ;) LOL!!! {-)

Hope you like the Links... technically I like the Glory Hole rounds - mind you, there are some extreme cases of v.high tech. competence amongst this lot.

Regards, Bernard

http://uk.youtube.com/watch?v=xHldEwMvY70&feature=related

http://uk.youtube.com/watch?v=44tm26Fhqr8&feature=related

http://uk.youtube.com/watch?v=9DmUitYmxIM&feature=related

http://uk.youtube.com/watch?v=XM2TbddOhN0&feature=related

http://uk.youtube.com/watch?v=G4z-xUdroys&feature=related

http://uk.youtube.com/watch?v=G4z-xUdroys
Title: Re: The weight of things..........
Post by: Reade Models on January 06, 2009, 09:54:03 pm
Hi Bernard

Do you know, if I had half a chance, I'd be back on the plane tomorrow...

Great blast video's - you wouldn't want to be anywhere close to that lot!  I particularly like the coal dust fireball - I keep harping on about the dangers of dust explosions in safety meetings, It's a bit like talking to a brick wall, you don't appreciate what can happen  unless you have first hand experience, but that video is a great example...

Malc



Title: Re: The weight of things..........
Post by: polaris on January 06, 2009, 10:05:58 pm

Dear Malc,

Know what you mean. But, you know, I am pleased in numerous ways to be out of it now... things are so different these days. The rapid advance of tech. means that one would have to be v.more on ones toes now than before... it was bad enough then! %% - but somehow easier.

Re dust. It has been a 'thing' of mine for many years since I have asthma (not what they gen. call it now, but the hereditary type), and dust and fumes in working environs. always does for me so I am partic. tuned to the prob.. Hope that partic. footage might be of use to you then.

Regards, Bernard
Title: Re: The weight of things..........
Post by: dreadnought72 on January 07, 2009, 09:48:17 am
My lake is similarly afflicted, but an enquiring mind should direct itself to either a hammer, or, for the more scientific, a bucket of water indoors.
:-)

The easiest way to think about this is to consider balloons. Helium or hot air, they displace air equal to their volume. If the mass of the balloon is less than that of the mass of the air they displace, they'll rise until they reach a point where the masses are equal.

Ships work in the same way. If a hull is too high in the water, it'll sink; too low in the water, it'll rise. When it's found its level, the volume of water displaced masses the same as that of the ship.

A solid lump of metal submerged in water similarly displaces a volume equal to itself - so the (land weighed) 7800kg cubic metre of steel WILL weigh 6800kg in fresh water. (And, to be pedantic, it would weigh about 7801.2 kg in a vacuum, since a cubic metre of air masses around 1.2kg.)

There's a section in Das Boot, I think, where they trim the sub to "hang" via the periscope - effectively making it a surface vessel, albeit one with a minimal hull-above-water.

For a suggested use of a tapered rope, have a read of "Fountains of Paradise" by Arthur C Clarke (inventor of the communication satellite so we are told)
An excellent example of tapering. The wiki article (http://en.wikipedia.org/wiki/Space_elevator#Cable) on the cables required for orbital towers is a good read.

Andy #1963#
Title: Re: The weight of things..........
Post by: Max Power on January 07, 2009, 10:57:39 am


Dreadnought said "If a hull is too high in the water, it'll sink; too low in the water, it'll rise. When it's found its level, the volume of water displaced masses the same as that of the ship."

This is obviously correct. Consider the case of a floating vessel with a pool of water contained within the hull. If the pool of water is drained the buoyancy of the vessel will increase and the vessel will rise in the water. This is true even if there were an object floating in the pool before it was drained and even if that object were another ship. This obviously applies to the scenario posed in the original post.

Consider the case in the original post. At the instant of closing the sea doors in the larger ship it must contain not only the 150T vessel but the 150Ton water required to float it. As the water is pumped out the buoyancy of the vessel increases by 150T and, as Dreadnought pointed out, it will rise in the water.
Title: Re: The weight of things..........
Post by: polaris on January 07, 2009, 11:02:58 am

Dear Andy,

Thankyou for your Post. Interesting and quite right.

I have just this moment come off the phone with my physicist friend (recently retired Lecturer/Research Physicist, now doing geophysics research consultancy) - opportune as the Univ. goes back tomorrow so bagged him at home! As usual when asking a tech. question/query of him, I had the normal fascinating and invaluable Lecture... so, what was intended to be only a five minute call lasted over half an hour! - enjoyed every minute!

As you and Malc(Fray) point out, the weight does indeed decrease the further a weight is immersed in water, and once fully immersed, any given weight is indeed proportionately less. This applies whether the weight is on the seabed or suspended in water, since displacement is pertinent whatever be the case. Gravity plays it's part in giving all/everything it's weight obviously, so one must be careful in measuring such an experiment since there are gravitational differences (local and gen.), around the planet - only small of course, but would affect fine measurement all the same, and need computations done to adjust for localised gravitational variances. This is all a bit OTT for our purposes I suppose, but if we are talking about it we may as well get it all as 'right' as poss.! - similarly as your interesting Post.

Horst raised other 'sideline' factors, this being that the 'power' needed to 'accelerate' 1 ton(shift, drag, or move), would be proportionately the same anywhere - whether on the moon, underwater, or anywhere else. This all down to mass and gravity equations. There is obviously more to this, and I have a page of notes from the conv. and my mind is buzzing trying to remember other things he said... but I think we have gone into fine techs. far enough now!!! O0 :} - and I must do some work this am.!!! :((

As to my 'ship-in-ship' thing, he saw something else against it connected with the 'free water' within, so I am going away to think on this when-&-as over the coming months!!!

Thankyou to all who have taken the time to participate - certainly helped blow away the Festive cobwebs!!!!!!

Regards, Bernard
Title: Re: The weight of things..........
Post by: tigertiger on January 07, 2009, 11:17:33 am
This thread is way over my head.

I can think of a simple solution to the problem.

Get a washing up bowl and fill it with water.

Take a buoyant object that will fit in washing up bowl. Weigh it, it needs to be fairly heavy.


Take a good set of scales. Digital bathroom at least. Weigh the bowl and water. Then insert boyant object.

What does it weigh now.

Then work out why.
Title: Re: The weight of things..........
Post by: polaris on January 07, 2009, 11:18:46 am

Dear Max,

Thankyou for your Post.

But if the water is not pumped out, and the vessel within is floating on same, and the water within is still kept freely open to the level of the outside water, the weight of the inside vessel is still floating, BUT, the water within still has to be moved. How much vol. of water within the mother vessel for the within vessel to float in/on is important, since it must still be physically moved - even if the within vessel were only floating on/in six feet of clearance 'all round'. As I mentioned before, the idea is relatively ok while in calm weather/water, but not so good in rough conditions!

I must go and do some work now!!!!!!!!!! :((

Regards, Bernard
Title: Re: The weight of things..........
Post by: polaris on January 07, 2009, 11:20:02 am

Dear Tiger,

Quite right.

Regards, Bernard
Title: Re: The weight of things..........
Post by: malcolmfrary on January 07, 2009, 01:22:13 pm
Hi Tiger
You have described my idea of laboratory conditions - indoors and warm.  Working outdoors in the winter of 61-62 was a great incentive to staying awake on day release and trying to ensure an indoors job with minimal heavy lifting.
Title: Re: The weight of things..........
Post by: Max Power on January 07, 2009, 02:33:54 pm

Dear Max,

Thankyou for your Post.

But if the water is not pumped out, and the vessel within is floating on same, and the water within is still kept freely open to the level of the outside water, the weight of the inside vessel is still floating, BUT, the water within still has to be moved. How much vol. of water within the mother vessel for the within vessel to float in/on is important, since it must still be physically moved - even if the within vessel were only floating on/in six feet of clearance 'all round'. As I mentioned before, the idea is relatively ok while in calm weather/water, but not so good in rough conditions!

I must go and do some work now!!!!!!!!!! :((

Regards, Bernard

I assume that the 20T weight of the mother ship refers to its unladen dry weight, (the sum of the weight of all its component parts out of the water). When in the water and containing a 150T vessel floating inside the hull, even when open to the sea, the space for the internal vessel must accommodate both the 150 ton vessel and in excess of 150 tons of water. In order for this to happen the "20T" mother ship will have to be ballasted so that it sits lower in the water. This ballast will have to be in excess of 300T! Without the ballast it would not be able to carry out the task.

So far as the mother ship moving through the sea with the smaller vessel still floating, this would not be a good idea. Once the mother vessel was moving at a steady speed everything would be fine; but accelerating up to speed, slowing down and turning would be cause problems. The internal floating vessel would crash into the sides of the containment space, unless it was very firmly moored, and the water would slosh about quite a bit.

We ourselves have the effects of inertia ably demonstrated to us if we are unfortunate enough to be standing up in a bus when it brakes {-)
Title: Re: The weight of things..........
Post by: dreadnought72 on January 07, 2009, 03:48:14 pm
... if we are unfortunate enough to be standing up in a bus when it brakes {-)

Which has to be the perfect point for me to link to this famous Billy Connolly routine (http://uk.youtube.com/watch?v=PyW0riD54nY). (Warnings for language and all the rest of it to those who might be offended by such things...)

Andy #1963#
Title: Re: The weight of things..........
Post by: polaris on January 07, 2009, 05:55:26 pm

Dear Max,

You see, my idea is that the mother vessel is only a 'ship' in that it would only be 'acting' as one. The whole idea is that it is simply a powered containment: it need not necessarily have a bottom as one is not really needed other than to give support to the whole structure. The bottom would, if one was used, be deep in draft to enable other ships within. Ok, I hear people ask, what keeps all this afloat... easy, float modules along the sides since in theory it only needs to support it's own weight - again, the weight within is floating. Ballast would only be needed to 'balance' the mother vessel structure and not the vessel within - which is free floating. As has already been mentioned, the front loading doors present all sorts of tech. probs.. Yes things would move inside, but as I said prev., it all depends on how much internal water space is allowed. As someone else mentioned before, the mother ship is basically acting as a deep sea tug.

I think I might have to build a model!

Regards, Bernard
Title: Re: The weight of things..........
Post by: Bryan Young on January 07, 2009, 07:08:17 pm
Another subject to ponder. My first pondering was as to why the question was asked in the first place. As it seems to be a bit of a no-brainer to start with, why all the exotic explanations? A little tinkling of thought would surely have equated total weight, mass, free flooding,deadweight and so on into one coherent "Eureka" moment. No? Oh, dear. How sad, but never mind. Another weird notion will pop up soon I'm sure. Cheers. BY.
Title: Re: The weight of things..........
Post by: polaris on January 08, 2009, 09:32:28 am

Dear Bryan,

Quick note before going out.

To get different perspective and new views by throwing the subject to debate... to be afraid of putting an idea forward and not doing so is not conducive to invention.

Regards, Bernard
Title: Re: The weight of things..........
Post by: Bryan Young on January 08, 2009, 04:04:24 pm

Dear Bryan,

Quick note before going out.

To get different perspective and new views by throwing the subject to debate... to be afraid of putting an idea forward and not doing so is not conducive to invention.

Regards, Bernard
No offence intended, and to be honest the debate is (was) sort of interesting. The original question was quite straightforward, but the brainpower that went into some of the replies made the subject more difficult than the question! Reminded me of the silly answers to the "wrong way round" steering as seen in the "Titanic" film. Keep up the lateral thinking...makes the world go around in a slightly better way. BY.
Title: Re: The weight of things..........
Post by: polaris on January 08, 2009, 05:57:09 pm

Dear Bryan,

Thankyou for your Post. None taken.

"...but the brainpower that went into some of the replies made the subject more difficult than the question!...". How many times have I seen this happen over the years! O0 - the 'posed more questions than answers' syndrome!!! :-)) - like that situation as it shows something 'exists', as opposed to definitely doesn't! :-) The trouble is the original question was relatively straight forward, but, inevitably, it was bound to become more difficult as the subject enlarged. Ok, I had to explain the gnrl. idea a few times (all with slight difference), but each time this happen it obviously expanded things that bit further. However, the circle has been done, and thinking can be more straight line now.

Regards, Bernard

(If anyone new reads this and wants to reply, please read all the Posts as we don't want to go over anything already gone over! :-))