Naval Architect Brain Teaser

Another question asked during newbie interrogation here at work...

A barge loaded with concrete blocks is floating in a tank full of fresh water.

Including its cargo, the barge has a displacement of 300 tonnes (300,000 kg).

The barge unloads five concrete blocks, each weighing 10 tonnes (10,000 kg) into the tank of water.

Does the water level in the tank go up, down, or stay the same ?

id say down as the barge isnt displacing as much water, stays

stays the same surely?

id say down as the barge isnt displacing as much water

if the blocks were taken off and put somewhere else. but the blocks have been put into the water so surely the displaced water would be the same, no?

if you mean water level on the boat it stays the same, if you mean water level on the glass it goes down.

if you mean water level on the boat it stays the same, if you mean water level on the glass it goes down.

how can it stay the same on the boat?

surely if you remove blocks from the boat it would be lighter, displace less water and thus sit higher in the water?

id say the level in the tank would stay the same. and the boat would rise.

Yeah I reckon the boat rises but the water level stays the same.

Unless of course it's a clever riddle of some kind with a piece of vital information missed out in some stupid deception using the wording of the question.

I think the boat will now sit higher in the water due to less weight. Therefore it will now displace less water so the water level on the side of the tank will go down.

Edit: Just read it again and noticed the blocks are going to be placed into the water 1 litre of water weighs 1 kg, therefore the weight removed will in relation displace the same volume of water when placed in the tank. Water level on tank remains the same.

I think lol

Is it testing your maths or knowledge of boats?

I think the water level in the tank will go up.

is it something to do with the fact its fresh water and not salt water?

id say the level in the tank would stay the same. and the boat would rise.

Im going with the water level staying the same but the boat rises too

When do we get the answer?

I think if it was salt water you could question the saliency at different points in the tank and stuff like that, it just clears up any annoying questions by saying it's fresh.

if the block were taken off the barge , and put outside the tank , then it wouldnt displace as much water .

but it says the blocks are put in the tank , so yes the barge isnt displacing as much , but the level is raised by the blocks in the water. so staying the same . come on ewen put us out of our misery

The water level in the tank would go down as the barge is now displacing less water.

come on ewen put us out of our misery

He doesn't know, he's doing his finals on his mobile asking us!!

same

I can't get my head around wether a block has the effect of displacing more water in the boat or on its own. I mean, it's not like all the blocks are in the water when they're on the boat. I keep thinking about surface areas and things.

I'd say it stays the same, unless there's some odd thing about mass and displacement, i.e. the concrete blocks in the barge are above water and it's their weight, not their mass, thats causing the barge to displace water.

That's probably all bollocks though.

What if the concrete absorbs water?

I think that 10T of mass on a boat will displace 10T (equivalent) of water.

Take it off and the water level will drop by however far 10T of water is (dependant on the size of the tank etc obviously).

Put it back in the water and it still displaces 10T of water, the same as before, so the water level will go back up to where it was, the boat will be higher in the water than before (by 10Ts worth of water in the shape/volume of the bottom of the boat).

OOH change my mind, it'll be 5Ts worth of water higher than it was originally? The boat will also be 5T worth higher in the water?

I think that 10T of mass on a boat will displace 10T (equivalent) of water.

Take it off and the water level will drop by however far 10T of water is (dependant on the size of the tank etc obviously).

Put it back in the water and it still displaces 10T of water, the same as before, so the water level will go back up to where it was, the boat will be higher in the water than before (by 10Ts worth of water in the shape/volume of the bottom of the boat).

That's what I think but it can't be that easy can it? There must be a catch.

No-one mention the Jet-plane on the conveyor belt runway!

Its not a trick question, knowledge of boats not necessary.

Couple of you have the right idea....bonus points if you can explain your answer.

Clue 1...boat floats, boat carrying concrete floats too, but concrete sinks.

Clue 2...1 cubic metre of fresh water weighs (approx) 1000 kg...1 cubic metre of concrete weighs (for the sake of this exercise) 2500 kg.

I'm assuming that you can answer this fully with the inf given, but I'm getting stuck on the fact that while in the barge the blocks will be displacing a mass of water equal to thier own mass (i.e. a greater volume than their own) but once in the water they will sink and so displace a volume of water equal to their own.

So the blocks displave more water while they are still in the barge, and less when they are in the water. Also the barge displaces less water when the blocks are off loaded.

Holy moly - thats it. The water level will drop. Because both the barge and the blocks are displacing less water.

It is the volume of an object that determines the amount of water that the object displaces.

Clue 2...1 cubic metre of fresh water weighs (approx) 1000 kg...1 cubic metre of concrete weighs (for the sake of this exercise) 2500 kg.

Basically you're swapping water for concrete?