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  #21  
Old 08-27-2012
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Re: Whats the reason for the huge differences in displacements in similar sized boats

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Originally Posted by Squidd View Post
I'm not arguing.... just having a hard time wraping my head around it...
I'll take a crack at it from a different angle.

The misunderstanding is that a sunk mass of weight can displace as little water as it can based on its density, but boats do not sink. They float.

So... think about it this way... a sling suspends a boat in the air. The ocean, or lake, or any body of water also suspends the boat. It take the same amount of lifting force to hold the boat up off the bottom of the ocean as it does to hold the boat up in the air. Your choice, straps or water.

If you put a hole in the boat, it no longer displaces the water and the water enters the cavity of the boat. The only displacement is now the thickness of the hull and deck etc. So you've kept the same weight, but you've reduced its displacement... Therefore, no more water pushing UP on the hull and hence the boat sinks to the bottom. Similarly, if you cut the strap on the crane, there is no more upwards force and it will sink. The boat doesn't weight any more but there is not as much force pushing up, so it falls.

Hmmmm.... Explanation by analogy. Perhaps this is easier.
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  #22  
Old 08-27-2012
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Re: Whats the reason for the huge differences in displacements in similar sized boats

Getting back to your original question, lets try that by analogy too.

Take two identical juice glasses, cylindrical in shape. You put a handful of gravel in one, and nothing in the other. One is obviously heavier than the other, yet if you put them in the sink, they both float. One displaces more water than the other. When you pull them back out of the water, they each have identical external dimensions.

This is just like your boats above... they each have similar external dimensions, but when you float them, they displace a different volume of water... more weight means more displacement.
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Old 08-27-2012
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Re: Whats the reason for the huge differences in displacements in similar sized boats

Thanks Donna. I didn't tip over but I certainly came close, again. I stay in the class so the rest of the students can feel good about themselves. I'm the oldest guy there besides my instructor, Master Chou. He's 89 years old. He's truly amazing. I speak Chinese to him and he understands. Tonight, in front of all the other students, some of whom have been "learning" for 19 years, he praised me on my Mandarin tones. I felt so good. I earned "face". I have tried so hard to learn to speak Chinese that a wee bit of praise makes me happy and a bit proud. And praise coming from Master Chou makes me really, really happy. He's pretty much deaf as a stump. But handsome. Tai Chi can be a bit humbling but it's one thing I do with my wife that lets her laugh at my efforts. That's kind of fun.
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Old 08-28-2012
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Re: Whats the reason for the huge differences in displacements in similar sized boats

Crimeny Jordan, now I'm confused.
Could we try some wine in those glasses instead of gravel?

Sometimes the easiest concepts are the very hardest to explain.
I can't explain Archimedes to Squidd so I sure as hell am not going to try to explain Newton. Boats sailing of the edge? Gravity man. Gravity!
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Old 08-28-2012
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Re: Whats the reason for the huge differences in displacements in similar sized boats

Can I try?

I make a crown out of balsa wood and paint it with gold paint and make sure it weighs exactly 1 lb it has to be very thick and tall because balsa is so light.
I fill up a tank until the water is overflowing.
I put the wood crown in the tank and push it down until it is just submerged.
The tank overflows by the volume of the water this large one pound wood crown displaces.

Now I make another crown out of gold.
It also weighs one pound but it is very thin and short as the gold is much heavier.

I start over with another full tank and put the crown in the tank.
Again water overflows by the volume of the gold crown.
But their is much less water because even though the gold crown weighs one pound it is thinner and smaller so takes up less water space.

Archimedes did exactly the same thing as their is a difference between weight of a cubic inch of gold and an cubic inch of silver. Not as much as the difference between a cubic inch of balsa and a cubic inch of gold to be sure but enough of a difference to be measurable.

Now lets take a hollow cube exactly 1 foot square made out of steel plate that weighs 10 lbs and other one made out of carbon fiber that weighs 1 lb.
Put them in the full tank and push them down until they are submerged and they will both overflow the tank by the exact same amount. They will both displace 1 cubic foot of water or about 64 lbs of salt water.

But let them float and the steel cube will sink just enough to displace the same amount of water that would weigh 10 lbs.
The carbon fiber cube would only sink enough to displace one lb of water.

So the steel cube would float lower in the water.

So now you know how boats and crowns work.

Last edited by davidpm; 08-28-2012 at 12:28 AM.
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Re: Whats the reason for the huge differences in displacements in similar sized boats

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Originally Posted by Squidd View Post
I guess that darn Archimedes Guy was messing me up...

I thought he said two objects could "weigh" the same, but have different displacements...???

Which has nothing to do with Bouyancy....
Oh I get the confusion now.

Two objects can certainly weigh the same and have different volumes like wood and lead for example. If forced under water they will displace the amount of water equal to volume of water they displace.

The cubic foot of lead will displace 68 lbs of water and sink

A cubic foot of wood that weighs 10 lbs will require a force of 58 lbs to hold it underwater to displace the 68 lbs. Otherwise it will float and only displace (sink) about and inch and 3/4.

Last edited by davidpm; 08-28-2012 at 12:49 AM.
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Old 08-28-2012
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Re: Whats the reason for the huge differences in displacements in similar sized boats

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Originally Posted by Squidd View Post
Isn't displacement the amount of water the hull shape "displaces" IE: cubic volume...

and has nothing to do with actual "weight" of the boat or construction.
Study up on Archimedes principle - a boats displacement and its weight are the same - the water it displaces weighs the same as the boat.
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Re: Whats the reason for the huge differences in displacements in similar sized boats

I suspect what has Squidd confused is that boats float due to positive buoyancy, but there remains the concept of neutral and negative buoyancy as well. An object is buoyed (upward floating pressure), by the weight of the fluid displaced. A boat will float at the level that it has displaced a fluid of the exact weight of the vessel. But there is a bunch of boat above the waterline. Therefore, the entirety of the boat is positively buoyant. Just like a piece of wood or an iceberg. If it were precisely neutrally buoyant, you could place it at any depth and it would just stay there, neither rising nor sinking. Neutral buoyancy is what divers attempt to achieve by inflating or deflating their buoyancy compensators, which changes their displacement. If you are displacing fluid that weighs less than the weight of the object, it will sink. However, if the weight of the object is one pound greater than the weight of the fluid displaced, it will seem to weigh only one pound when sinking. In this negatively buoyant example, you could hold a freight train in your hand underwater, if it was only one pound negatively buoyant. So yes, the density of an object affects its buoyancy, but that supposes that the entire object is underwater.

Not sure that helped.........
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Re: Whats the reason for the huge differences in displacements in similar sized boats

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Originally Posted by bobperry View Post
Crimeny Jordan, now I'm confused.
They don't teach you the glass and gravel theory in Boat Design 101? ;-)

Quote:
Originally Posted by bobperry View Post
Sometimes the easiest concepts are the very hardest to explain.
I explain complex concepts, which are often virtual in nature, to clients all the time. They can't see it, feel it, smell it, taste it or touch it so grasping the concept is difficult. I've found that by explaining the concept from as many angles as I can often works; Eventually one method is able to be visualized and *click* they get it.

I am sure that is the case here. I'm sure squidd just had a 'moment' and the light bulb will go on.

I agree with Minnewaska that the confusion is the difference between being buoyant and not being buoyant. I attempted to explain that through an alternative method which may or may not help with the *click* moment.

Seriously, if you think I'm off my rocker, try using my little gravel experiment with a student that has never learned physics; two equal cylinders, add gravel to one, demonstrate that they both still float ('positively buoyant', just for Minnewaska), but point out how much lower the heavy glass sits in the water. Voila, the student will understand the difference how two boats of the same dimension will still float.

My other attempt at explaining this would be on a whiteboard. Some people learn better visually.

Now lets stop beating up squidd. I'm sure it was just a minor moment and now that he's slept on it will understand.
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Re: Whats the reason for the huge differences in displacements in similar sized boats

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Originally Posted by RichH View Post
The strategic differences are 'scantling difference' which determine the target service.

A simplified explanation of the engineering/design process that results in various and very different 'weights":
A boat designed for inshore (lake / river/ sheltered bay) service will typically have structure at 1.5 times the historical strength needed to insure survivability in that service; a coastal boat at 2 times the strength; an offshore / open ocean boat at 3 to 4 times the inbuilt strength.
Is that number a matter of public record for each boat?
If so where do you find it?
If not how would you calculate it on an existing boat?
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