Displacement vs. Weight

[leaotis]

Pop Quiz-

Lets say you could measure the water level of a lake with infinite accuracy.

On the lake there is a small boat with 2 guys in it. They throw the anchor overboard without attaching the rode.

Does the water level in the lake raise, lower or stay the same?

One (average American size) guy goes overboard and swims underwater to retrieve the anchor.

Does the water level in the lake raise, lower or stay the same?

[paladin]

Just a quick stab..case 1 the lake lowers...case two..lake rises...

[Charles Burgess]

Hi, Ross....of course you are correct...but I think we were playing a semantics game....

And a rather enjoyable game too.

[Charles Burgess]

Just a quick stab..case 1 the lake lowers...case two..lake rises...

Here's an interesting take...the lake rises in both cases.

Remember all the media coverage of the Chinese spy scandel at Los Alamos during the Clinton administration?

Well, the Chinese got hold of some of our research where we used micrometer RADAR from satellites to detect and track submerged submarines in realtime. How? The submarine displaces water toward the surface (actually in all directions) as it moves through the water, and the satellite was able to detect its path (amounting to just a few millimeters difference in water surface...even with waves) ...sort of like watching a high-speed gopher or mole tunnel just under the surface of your lawn.

[Charles Burgess]

...snipped...

Any motionless object floating in still water will displace a volume of water which weighs exactly the same as the object.

...snipped...

Thanks for bringing up the point of measuring displacement of a motionless object...in our case boats.

When the boat goes into motion then the energy that propels it is transfered into additional displacement of the water that is equal to the force of propulsion, whether power or sail. It is the displacement wave of larger vessels that swamps small boats and sends its occupants for a swim...the reason for wake restrictions in harbors.

The issues you bring up about trim on iron ships ... the reason for the existence of trim tanks - something you never find on wooden vessels (although this was sometimes done with ballast stones). The History Channel has had some good shows about early snafoo's with building iron steam ships.

[Lucky Luke]

Here's an interesting take...the lake rises in both cases.

Just a quick stab..case 1 the lake lowers...case two..lake rises...

Paladin: 1
Charles Burgess: 0



Some seriously wrong reflexions here, dear Mr. Burgess...

In the present case, the man would be naturally floating, but had to force himself down by swimming, hence increasing his displacement compared to when he was in the boat. So: lake rises, OK.
But when they throw in the water an anchor, which naturally sinks, and which was previously displacing exactely its weight while it was in the boat (equivalent to *floating*), it now displaces a smaller volume, while boat floats higher: of a volume corresponding to the weight of the anchor. Lake level lowers.

Concerning motionless displacement vs. quantity of water moved by wake, which is vastly higher, it is your terminology that is wrong: although putting water into motion *displaces* it, and the wake is water in motion, it is not the word displacement that is used in N.A. in this circumstance due to the confusion it would create, but dynamic motion.

Would have been fun if the anchor was attached to the boat by a section of chain followed by a length of polypropylene...how is your *pill* condition, Chuck?

[paladin]

I just took the entire load and one shot of meds 30 minutes ago....so I check in here until I get a little woozie again......they had the pain injections in my back which worked long term as a local anesthetic, but the damn oxycodone
every four hours plays hell with your head when you're trying to think.....and trying to get co-ordinated typing this takes a bit of time and heavy concentration.......

[Tom M.]

Burgess: 0, Paladin: 0



Tom: 1

Case 1, the lake lowers. Case 2, it depends on if the swimmer floats or sinks. In other words, does the swimmer displace as much water as he weighs?

[leaotis]

I guess this question isn't much of a challenge for you guys.

The entrance exam also ask: You have two cylinders, both weight the same but one is hollow. How do you find which one is solid by using a piece of plywood? No fair tapping on the cylinders.

[Lucky Luke]

Burgess: 0, Paladin: 0



Tom: 1

Case 1, the lake lowers. Case 2, it depends on if the swimmer floats or sinks. In other words, does the swimmer displace as much water as he weighs?

The question specified: swims underwater
BAAAAD reading of the question: Tom= 0.00

[Lucky Luke]

I guess this question isn't much of a challenge for you guys.

The entrance exam also ask: You have two cylinders, both weight the same but one is hollow. How do you find which one is solid by using a piece of plywood? No fair tapping on the cylinders.

Question seems incomplete: same dimensions also?

[leaotis]

Both cylinders look identical

[Bruce Taylor]

Incline the plywood and roll 'em down it?

[P.I. Stazzer-Newt]

That'll do it,

First one down is solid.

[Lucky Luke]

Both cylinders look identical

Then only distribution of masses and radial inertia changes....thus rotation of the hollow one will need more energy.....but I see that others have (correctely) answered already .

[leaotis]

You guys would have gained entry to a prestigious English collage.

Maybe some of you did.

[Bruce Taylor]

a prestigious English collage.

[paladin]

Berkeley ain't English.....

[Charles Burgess]

Weight of a boat VS its Displacement....Round 3 (or is it 4 or 5? I lost count...LOL)

Water can be easily displaced and the amount displaced exactly equals in weight the under-water portion of any substance floating upon it. That is the theory.

A simple experiment: take a bowl of water and a goldfish; the water in the bowl is level with its brim; add the fish and the water overflows. The weight of the of the amount of water that overflowed and the weight of the fish are supposed to be exactly the same...but in reality the fish weighs more than the water.

Take a block of lead that is one cubic inch and weighs 8 onces, add this to that same bowl of water. The lead cube will make a hole in the water that is exactly one cubic inch...by displacement. But the water displaced will not weigh 8 onces - actually considerably less.

To account for this phenomena we must consider the fact that objects lose a portion of their dry weight due to hydrodynamic bouyant forces (pressure) that act to reduce the weight of any object...submerged or floating.

Thus it is a popular fallacy to suppose that the water displaced weighs exactly the same as the object so displacing it.

[Charles Burgess]

That'll do it,

First one down is solid.

And that answer is why Galileo almost was burned at the stake when he showed that objects of different masses, when dropped from the same height, will hit the ground at exactly the same time...he used a grape and an orange if I recall correctly.

[Lazy Jack]

"Thus it is a popular fallacy to suppose that the water displaced weighs exactly the same as the object so displacing it."

But it IS true for FLOATING objects. The vast majority of a goldfish IS made up of water along with some bone which is denser (mineralized) and fat which is less so to cancel it out.

Does oil DISPLACE water?
Does air?
Does a hovercraft?

[Bruce Taylor]

What happens as a powerboat gets up on plane

uh oh...

[Tom M.]

The question specified: swims underwater
BAAAAD reading of the question: Tom= 0.00

Ohhhh, ohhh! Lucky Luke minus 1.

I get another point for being right still. So that's 2 for Tom, 0 for everyone else Maybe 1.5 for me for not explaining myself better.

Yes, I know the swimmer dives underwater. But it still matters whether the swimmer displaces as much water as he weighs. Remember that this thread's purpose is to distinguish between displacement and weight (mass). If the swimmer, while doing nothing, floats on his own with positive boyancy (fat guy), then his mass is less than the amount of water he displaces while he's completely underwater. Thus, the water level rises when he dives for the anchor. On the other hand, if the guy sinks without doing anything (lean guy), then his mass is greater than the amount of water he displaces (negative boyancy), so when he dives in, the water level drops. If the guy is kinda fat and kinda lean, he'll have zero boyancy, and the water level will remain the same.

I didn't explain what happens to the boat's displacement as these fat and skinny guys dive in, but I hope its clear enough Points man, I'm after points.

[Lucky Luke]

Ohhhh, ohhh! Lucky Luke minus 1.

I get another point for being right still. So that's 2 for Tom, 0 for everyone else Maybe 1.5 for me for not explaining myself better.

...hum...I am afraid you may be the only one giving yourself these points...others might do a slightly different counting....

Yes, I know the swimmer dives underwater. But it still matters whether the swimmer displaces as much water as he weighs.

Displacement is a volume, weight is a mass x the accelerartion of gravity. Your system of measurement is unconsistent. Archimedes principle compares the weight an object with the weight of a volume of liquid. That is consistent.

Remember that this thread's purpose is to distinguish between displacement and weight (mass).

There again: weight and mass are different things: mass is invariable, weight depends of the acceleration of gravity: you weight less on the moon while you mass remains the same.

If the swimmer, while doing nothing, floats on his own with positive boyancy (fat guy), then his mass is less than the amount of water he displaces while he's completely underwater. Thus, the water level rises when he dives for the anchor. On the other hand, if the guy sinks without doing anything (lean guy), then his mass is greater than the amount of water he displaces (negative boyancy), so when he dives in, the water level drops. If the guy is kinda fat and kinda lean, he'll have zero boyancy, and the water level will remain the same.

Then again: no good reading of the question as it has been formulated : Average American size it says.

I didn't explain what happens to the boat's displacement as these fat and skinny guys dive in, but I hope its clear enough Points man, I'm after points.

...and you lost all the points you were trying to demonstrate!

[Lucky Luke]

Weight of a boat VS its Displacement....Round 3 (or is it 4 or 5? I lost count...LOL)

Water can be easily displaced and the amount displaced exactly equals in weight the under-water portion of any substance floating upon it. That is the theory.

I am having a little fun there...although I should be designing (boats) and not play here...
1: be precise: liquid water. Just say: a liquid: that will do
2: [/I] any susbstance[/I]: Wrong: be precise again (a foundament of any engineering): somewhere herabove, one cites oil on water: oil is a substance. If the oil entirely covers the surface of water, it does not displace any.

A simple experiment: take a bowl of water and a goldfish; the water in the bowl is level with its brim; add the fish and the water overflows. The weight of the of the amount of water that overflowed and the weight of the fish are supposed to be exactly the same...but in reality the fish weighs more than the water.

Wrong again: fish (sharks, rays, aso...excepted) change their buoyancy by the contraction of their swimming bladder. They can have exactely the same density, in their whole, as water.

Take a block of lead that is one cubic inch and weighs 8 onces, add this to that same bowl of water. The lead cube will make a hole in the water that is exactly one cubic inch...by displacement. But the water displaced will not weigh 8 onces - actually considerably less.

At least, that is right, but.....

To account for this phenomena we must consider the fact that objects lose a portion of their dry weight due to hydrodynamic bouyant forces (pressure) that act to reduce the weight of any object...submerged or floating.

....but that is wrong! their weight will not change. W= M * g. Period. It is the resulting force between the Archimedes force (exerted by the liquid) and the weight that is lower than weight of the object .

[Thus it is a popular fallacy to suppose] that the water displaced weighs exactly the same as the object so displacing it.

Who said that??????

Confusion...confusion....

[Lucky Luke]

"Does oil DISPLACE water?

If it covers the entire surface: no. If it does not: yes.

Does air?[

Are we talking statics or dynamics?
If statics: no.
If dynamics: can be, if non laminar.

Does a hovercraft?

One could say: Yes, whether stopped (becomes archimedian) of moving: see hereabove...and look at the foam around it, which is water. It sure moves some! Planning hulls also move water, but in all those cases remember that we should talk of motion (dynamic), not displacement (static).
Basically: cinetic energy of any object in motion: = 1/2 M v2 . Hydrodynamic force(s) is(are) due to the acceleration(s) of the fluid(s) and local gravity.
Applies to planning hulls, hydrofoils, propellers, wings, jets, aso....but viscosity(ies) and turbulence(s) (incl. limit layer and wake) make it a little ...complex!

OK. enough for today.

[Tom M.]

Displacement is a volume, weight is a mass x the accelerartion of gravity. Your system of measurement is unconsistent. Archimedes principle compares the weight an object with the weight of a volume of liquid. That is consistent.

There again: weight and mass are different things: mass is invariable, weight depends of the acceleration of gravity: you weight less on the moon while you mass remains the same.

Then again: no good reading of the question as it has been formulated : Average American size it says.


...and you lost all the points you were trying to demonstrate!

Yer pickin' nits that don't matter, but I guess that's part 'n parcel of a lot of this thread. "Average American size" doesn't matter. (did I say that?) His boyancy matters. And I know weight and mass are different, but it doesn't matter in this case.

Tom: 3
Unlucky Luke: -2

Yes, I give myself points. That's the point...er....joke I mean

[JimD]

Does Lisa displace anything?

[JimD]

How about Stacy?

[JimD]

I bet water moves aside for Jennifer:

[paladin]

Nope...they is on dry land.....

[JimD]

This poor thing has run aground. Must have out of date charts.

[Lucky Luke]

wouaouh! personnal floating devices!!!
However (sorry if I may be hard on them), they have not covered the subject.

[pippo]

Actually the resulting force for a free floating object is zero, otherwise the object would be moving.

It is the resulting force between the Archimedes force (exerted by the liquid) and the weight that is lower than weight of the object .

[Lucky Luke]

Actually the resulting force for a free floating object is zero, otherwise the object would be moving.

Ooooooops!!! missing piece in the phrase: ! Should have been:
it is the resulting force between the Archimedes force and the weight of a sinking object that is lower than the weight of the object. (I did not say : free floating). The original phrase actually meant nothing: ...and the weight that is... The weight of what???

Effectively: it moves: it sinks!

Did I say *confusion*...???....hum!!!

Well, with all these babes around

[Dimitris]

Forces cause acceleration, not motion. If no forces are applied on an object, then it will not change its state. A moving object will keep on moving straight at constant speed, a stopped object will stay stopped.

[Lucky Luke]

Forces cause acceleration, not motion. If no forces are applied on an object, then it will not change its state. A moving object will keep on moving straight at constant speed, a stopped object will stay stopped.

gnak gnak gnak gnak!
Hey, folks: I may want things to be precise, and eventually like to nag a bit in a discussion, just for the fun!
But I have noooooooo time to continue debating, for example (as you perfectely well know, Dimitris) that without a force at least having been exerted during would it be a short time (Plank s time, as a ultimate minimum - is that OK?) , a movement will simply not exist...because it will not even have started!

Anyway, you have forgetten antagonist forces (like the one caused by water s viscosity) that need *some force * against them fro the movement to continue to exist (don t start with the story of the man runnning in front of the train, OK?)

Beddy by babies! Over and out!

[Lucky Luke]

Oh! I just had a look at your *profile*, Dimitris! It says:first post!!!

Then: WELCOME IN THE MAD WORLD OF THE WBF!!!!

.....and I should add: forget all what you have learned: it will not apply here!

[Thames]

Here's another analogy: Take two men weighing exactly the same, 250lbs, one is very lean muscled and the other has mostly body fat and thus has much more waistline girth than the lean&mean muscled guy - the man with the larger girth (because of body fat) will displace more water when swimming.

The important consideration here is whether an object floats or sinks which is defined by its density.

Most humans with air in their lungs float, and these men are described as swimming, so we assume that both these men are breathing and floating. The fat one will float higher because they both displace the same amount of water.

With objects that sink it's a different story. One cubic meter of lead weighs 11,340 Kg but will only displace one cubic meter of water (1000 Kg) because it sinks.

The same cubic meter of lead will float on a lake of pure mercury displacing 11,340 Kg of mercury because mercury is more dense than lead.

Put the cubic meter of lead inside a large boat on the sea and the boat will displace an extra 11,340 Kg of water.

So when an object is floating its weight and displacement are the same, when it sinks they can be different. When it's floating it displaces its weight, when it sinks it displaces its volume. Boats are supposed to float.

[MN Dave]

Don't think it was **arguing** exactly....more like BS'ing, or guessing, or brainstorming.

With me as a primary offender on this thread, I'd say the score is:

Laws of Physics - 10
WB Forumites - 2

;0 )

No, we're geezing. In American slang anyway.

The NAs got it all wrong. They broke the first rule of explaining technical detail to Sr. Management using email. You have to explain it in short words and less than 3 short paragraphs. Nobody will read past that and understand anything, which makes you wrong in spite of what you said. But thanks for the otherwise well written and informative explanations anyway.

So displacement is the weight* of the boat plus the weight of the stuff you think should** be in it at certain times. A floating boat displaces a mass (oops) of water equal to the actual as-built mass of the boat plus the actual weight (oops) of everything in it at the moment, including bilge water and beer. So the displacement (a calculated prediction of the mass) is not the volume of water that the boat actually displaces at the moment, except by accident. No problem if I can wash down Paladin's happy pills with enough beer.

*Don't even try to explain local gravity, weight, density of the water in that location and mass.
**Can't say will because you know it won't, although it may. Should is probably too strong a word there. Maybe it should say could. There is a MIL-STD that lays out shall, should, will, can, may, must and will in detail.

[keyhavenpotterer]

Lets look at the Caledonia Yawl.

When you finished building it, sat there on the lawn painted, rigged and sails on. It will weigh 250 kgs if you put it on a weigh scale.

If you then put it in the water it will displace 250kg of water while you stand there on the beach looking at its aching beauty.

Mr Oughtred didn't design it to be pilotless. He designed it to be carrying 3 people, probably with winter clothing on and carrying a picnic and a tent.

3 people = 80kgs each = 240kg. Extra clothing, tent, picnic, 6 cans of beer = 250kg all told.

The 3 people get in and load the tent, beer and picnic.

The boat and people and stuff adds up to 500kg now. So the boat is displacing 500kg of water sitting there with three people sat in it and their stuff.

When Ian Oughtred drew the boats lines and calculated the stability and sail area, wetted area and performance, he had in his mind all the time that it would be sailed by three as a weeken picnic boat and would be displacing 500kg of water. So he shaped the hull lines and drew the sail area etc to push it accordingly. So that when the baot is down into the water at its designed waterline, the whole thing (boat, 3 people, picnic etc) is 500kg. So the designed waterline (DWL) is the line at 500kg.

So the boat weighs 250kg (and this indicates how many kg of wood you going to have to buy).

Its designed displacement with the anticipated crew and picnic is 500kg.




But, say you are always solo sailing. You weigh 80kg and take 10kg of picnic and beer.

Your boat is 250kg plus you and the picnic (90kg) thats 340 kg all up.

At this displacement she won't be at her displacement waterline of 500kg. She's 160 kg short. She'll be sitting a bit high. Less of the boat will be in the water.

There are advantages to this: less in the water = less wetted area, so she'l goast along in light air and there is less wave making resistance as less water has to pushed out of the way as she goes along. Also she'll have a bit more freeboard above the water. For small planing boats, lighter boats can plane earlier.

But there are dissadvantages, the boat and crew and picnic are displacing 340kg of water. If the boat heels in a gust the sides of the boat push down on leeward side into the water. The boat at 340kg can be pushed over more easily as there's less water there underneath resisting it. So she'll be more tender in gusts and blow over more easily. Wave action may jostle her more fore and aft also because she's lighter.

So, finding her a bit tender in heavy weather, the thing to do is put back in 160kg of weight to get her down to her designed displacement so all the stability calculations work and she will sail better in gusts and ride waves as she was designed.

That 160kg can be water ballast, lead or iron ballast screwed to the floor, or anticipating solo sailing, a builder may choose to build it from heavy materials.

Heavy materials will give a very durable boat, and there is an argument that putting weight all over the boat like that helps to prevent pitching in short seas inshore.

Water ballast has advantages - you can empty it out for the advantages of light hull in light air or when your rowing.

Lead gets the weight lowest in the bilge, but putting it all in the middle can allow pitching (although this helps prevent nose diving) and helps right the boat as she heels to carry sail.

Putting in ballast is sub optimal though for small boats as the lead only starts to exert its affect when the boat is healed over quite away. Having crew quickly out on the gunwale in a gust exerts an affect quickly and more positively than lead ballast and boats built and so crewed are faster. Things are a bit different with keel boats as the lead can be put 5-6ft down concentrated in a bulb, but that's not the situation with small boats here.

Most boats here a cruised solo, so its often the case that Ness Yawls or CY are found to be a bit tender until they have ballast put in to take them down to the designed displacement (usually the following year) when all the calculations work properly and the boat heels less and works as it should.

Some small round the world cruising boats like say John Welsford's Sundowner have a big difference between boat weight and displacement at correct WL because in this case their is anticipated a huge inventory of equipment, food and water that will be added, additional to the boats weight. John predicted and anticipated this so its designed displacement when all the caluclations are optimal is 3 tons when the boat weighs only 2 tons.

Ed

[wizbang 13]

Yer gonna need more beer

[orbb]

Beer should not be counted as part of the displacement because it always ends up over the side of the boat, albeit indirectly.

[176inches]

OK guys, this has been a very amusing thread, but I'm not sure the original question has been answered. Of course actual weight=actual displacement (unless you sink), as Archimedes found out 2300 years ago. BUT...If a boat has, say, a design displacement of 1000 lbs., that means that it displaces 1000 lbs at the design waterline, so it can carry up to 1000 lbs of gross weight (hull, rig, people, kit, beer and all) without riding lower than that waterline. If you add another 500 lbs of persons or beer, say, then it would be riding lower in the water than the designer thought was good for it (handling, safety, etc.). Am I right?

[Bram V]

OK guys, this has been a very amusing thread, but I'm not sure the original question has been answered. Of course actual weight=actual displacement (unless you sink), as Archimedes found out 2300 years ago. BUT...If a boat has, say, a design displacement of 1000 lbs., that means that it displaces 1000 lbs at the design waterline, so it can carry up to 1000 lbs of gross weight (hull, rig, people, kit, beer and all) without riding lower than that waterline. If you add another 500 lbs of persons or beer, say, then it would be riding lower in the water than the designer thought was good for it (handling, safety, etc.). Am I right?

It will not carry that; it's own weight, plus what it will carry will add up to 1000 pounds. In one definition of displacement. Sadly, it would be better not just to have one number, but at least three, Designed empty weight (one could argue that this should also include everything you'll have with the boat anyways, susch as a full fueltank, rigging, oars, and an emergency bottle of thinking juice), designed optimal displacement, and designed maximum displacement. A lot depends on the boat once you go over it, some will have low enough freeboard to increasingly have water ballast after each wave, some will look at the weight and simply ride a bit deeper, it might even be that the designer knew his builders a little and the actual optimum displacement is a bit higher than the numbers indicate, to balance a bit of foolishness

EDIT, somehow I did not read your provision for the hull and rig and stuff

[keyhavenpotterer]

176,

You are correct. But in your example your overloading it by 50%. As a vague rule, its thought not to overload a boat by more than 25%.

Beer is adjustable water ballast and I am the pump.

Shackleton understood displacement. He knew he had to make his boat stiffer to cross the southern ocean, so he filled it with shore pebbles after dragging it accross the iceflo, and then part filled it with water to make it heavier in displacement, and thus stiffer for the conditions (if I remember it right). They also had to knock off ice that was on the boat and rigging, to stop the boat suddenly weighing more, displacing too much and losing too much freeboard (apart from chafe and sailing issues).

Ed

[wizbang 13]

500 pounds of beer sounds about right. but not lite beer.

[176inches]

It will not carry that; it's own weight, plus what it will carry will add up to 1000 pounds.

That's what I said: "it can carry up to 1000 lbs of gross weight (hull, rig, people, kit, beer and all)"

Edit: Correction noted. We agree. And your idea of listing three displacement numbers (empty, optimal and maximum) is very sound.

[Hwyl]

A thread that contains a resurrected Paladin floats my boat.

[Bram V]

You are correct.

Beer is adjustable water ballast and I am the pump.

Shackleton understood displacement. He knew he had to make his boat stiffer to cross the southern ocean, so he filled it with shore pebbles after dragging it accross the iceflo, and then part filled it with water to make it heavier in displacement, and thus stiffer for the conditions (if I remember it right). They also had to knock off ice that was on the boat and rigging, to stop the boat suddenly weighing more, displacing too much and losing too much freeboard (apart from chafe and sailing issues).

Ed

Beer is ballast that is not so well chosen, it starts out low in the boat, but goes to your head, which makes for a higher point of gravity, making you less stable (that is why the way home from the pub is longer then the way there, lots of tacking to be done, if your point of gravity is higher, you heel more, which I think is bad for your pointing ability). The ice on the boat and the rigging is similar, the weight might be desirable, but low in the boat.