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Old 10-01-2012, 16:36   #1
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Anchor Riddle

Just got this one and it made me think of so many wonderful anchoring debates here on CF. I'll check back in with the answer in a while...

You're in a sailboat, which is in a large tank filled with water. You have an anchor on board, which you throw overboard (the chain is long enough so the anchor rests completely on the bottom of the tank). Does the water level in the tank rise or fall?
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Old 10-01-2012, 16:39   #2
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Re: Anchor Riddle

The water level stays the same. The weight of the anchor and chain already displaced water while they were still stowed in the sailboat. Whether they were inside or outside wouldn't matter.
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Old 10-01-2012, 16:44   #3
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Re: Anchor Riddle

+1 I agree
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Old 10-01-2012, 16:44   #4
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Re: Anchor Riddle

No, actually the water level will go down. The chain is way heavier than water, and when dumped overboard, displaces less than when on the floating boat.

BTW, if it did NOT hit the bottom, the water level wouldn't change.
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Old 10-01-2012, 16:56   #5
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Re: Anchor Riddle

I agree. The boat will rise... it weighs less.. the amount of weight of the anchor.
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Old 10-01-2012, 17:02   #6
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Re: Anchor Riddle

Wait, are we talking a "new-gen" anchor here or a plow?
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Old 10-01-2012, 17:05   #7
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Re: Anchor Riddle

Quote:
Originally Posted by shorebird View Post
I agree. The boat will rise... it weighs less.. the amount of weight of the anchor.
Not quite - this is a displacement problem. The anchor in the boat displaces more water than it does in the water (on the bottom). On the boat, it displaces an amount of water equal to its weight. On the bottom, it only displaces as much water is its volume.
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Old 10-01-2012, 17:08   #8
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Re: Anchor Riddle

the boat flys away
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Old 10-01-2012, 17:14   #9
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Re: Anchor Riddle

A 40 pound anchor will add 40 lbs to the weight of the boat. The boat will weigh 40 lbs less whether the anchor is on the bottom of the sea, put ashore, or removed from the boat by helicopter. The vessel will rise the amount of surface area at the waterline times the thickness of a layer of water equaling 40 lbs.
So says me anyway...
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Old 10-01-2012, 17:14   #10
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Re: Anchor Riddle

I don't think so Bill. Displacement is the amount of water displaced in lbs. And weight is calculated with mass x gravity. Since it is a closed system, I don't think the fact the anchor hits the bottom changes the displacement. But, I guess we will find out eventually.
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Old 10-01-2012, 17:23   #11
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Quote:
Originally Posted by bstreep
No, actually the water level will go down. The chain is way heavier than water, and when dumped overboard, displaces less than when on the floating boat.

BTW, if it did NOT hit the bottom, the water level wouldn't change.
+1 - water level goes down

Think about weight and volume - Archimedes requires we examine both. When on the boat the anchor is simply part of a large volume at x weight. When removed from the boat the volume of the boat stays the same, the weight is deceased so the boat rises and water level falls.

When you now put the high weight, low volume anchor in the water, the water rises but much less than the fall. If you distribute the weight of the anchor into a thin hull shape you could actually make the shape float. i.e. increase its volume at the same weight.

But it would rust, perform poorly and make a lousy sailboat...

Archimedes principle is about density.
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Old 10-01-2012, 17:31   #12
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Re: Anchor Riddle

Quote:
Originally Posted by shorebird View Post
A 40 pound anchor will add 40 lbs to the weight of the boat. The boat will weigh 40 lbs less whether the anchor is on the bottom of the sea, put ashore, or removed from the boat by helicopter. The vessel will rise the amount of surface area at the waterline times the thickness of a layer of water equaling 40 lbs.
So says me anyway...

But the question doesn't ask about the boat.

It asks about the level of water in the tank. but I guess if the boat rises the water has to go down in the tank.

To the bathtub where most of my sailing (and anchoring) skills have originated.
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Old 10-01-2012, 17:32   #13
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Talking Re: Anchor Riddle

Quote:
Originally Posted by GeoPowers View Post
I don't think so Bill. Displacement is the amount of water displaced in lbs. And weight is calculated with mass x gravity. Since it is a closed system, I don't think the fact the anchor hits the bottom changes the displacement. But, I guess we will find out eventually.
Think about it this way. Say the anchor weighs 50 lbs, and you are in a 20 foot boat (so we can use extremes). Hang the anchor off the bow, holding on to it. Will the boat rise when it hits the bottom?

BTW, for the OP, "you suck!" This is keeping me from working!
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Old 10-01-2012, 17:33   #14
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Re: Anchor Riddle

Quote:
Originally Posted by bstreep View Post
Not quite - this is a displacement problem. The anchor in the boat displaces more water than it does in the water (on the bottom). On the boat, it displaces an amount of water equal to its weight. On the bottom, it only displaces as much water is its volume.
This gets my vote

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Old 10-01-2012, 17:36   #15
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Re: Anchor Riddle

Agree with Bill. The anchor has a greater density than the additional displacement of the anchor on board.
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