Buoyant force or water pressure?

Ross · Jan 20, 2009

jibes138 said:
Many of you seem to believe there is some magic force called the "bouyant force." I assume you all will agree that water has a pressure based on the pressure head of the column of water above the measurement point? We can also all agree that water exerts pressure in all directions. So let's say some magic force called the buoyant force exists in the water, I submit that this force will be dissipated across all of the water and what is a force except pounds applied per square inch. This is also known as PSI. PSI is also the designation for water pressure. So I propose that water pressure is the force that is keeping boats afloat.

Jibes , if I start loading your boat with sand bags it will be forced deeper into the water and the pressure on the hull will increase. Now if I continue to load sandbags there will come a point where there is not enough pressure to keep your boat buoyant even if we are able to keep the water out of your boat. At that time your boat becomes a submarine and continues to settle deeper in the water until it reaches a place called the bottom. If the bottom is not too far from the surface the water pressure will not crush the hull and all you need to do is remove some of the sand bags and your boat will return to the surface.
I still prefer to believe that gravity affecting all of the elements involved is the probable cause.

Don S/V ILLusion · Jan 20, 2009

much more interesting than anchoring or shaft seal debates...

still waiting for someone to mention water displacement.

Todd Smith · Jan 20, 2009

jibes138 said:
Many of you seem to believe there is some magic force called the "bouyant force." I assume you all will agree that water has a pressure based on the pressure head of the column of water above the measurement point? We can also all agree that water exerts pressure in all directions. So let's say some magic force called the buoyant force exists in the water, I submit that this force will be dissipated across all of the water and what is a force except pounds applied per square inch. This is also known as PSI. PSI is also the designation for water pressure. So I propose that water pressure is the force that is keeping boats afloat.

And you don't seem to understand that the words "bouyant force" are equivilant to the words "sum of upward water pressure on the hull".

Todd

RanAweigh · Jan 20, 2009

As I recall the collection of forces that keep a vessel afloat are referred to as the single term "bouyancy".

But then again ... What do I know?

DanW9494 · Jan 20, 2009

You let go of two mylar film balloons, one filled with helium and the other with air to equal volumes, and a piece of lead weighting the same as the empty balloons at the same time. One goes up fast, one goes down slowly, and one goes down fast.

It's not gravity because the gravity is the same for all three. But gravity is necessary to make this work.
It's not air pressure, because the air pressure is the same on all surfaces, pushing up and down equally
It's not displacement because the displacement of the two balloons is the same.
It's not weight because they all essentially weigh the same, and the heaviest is the balloon with air and the lightest is the lead weight.

So it's obvious it is density. The helium balloon is less dense "lighter" than air while the other two are more dense than air. Density seeks it's own equilibrium. The helium balloon will eventually stop rising, until it opens. Then, the helium goes much higher to the edge of space, while the mylar falls onto a powerline.

Water is just a more dense medium than air and the principles are the same.

ITMaster · Jan 20, 2009

Don S/V ILLusion said:
much more interesting than anchoring or shaft seal debates...

still waiting for someone to mention water displacement.

I did yesterday Don. To me, to sum it up in a single word, that is what I consider it as, Displacement. Glad to see someone thinks the same way I do

Thanks, tom

DanW9494 · Jan 20, 2009

To finish my thought, I was interrupted.

The buoyant force is just the resulting vector of the greater gravitational pull on the denser air or water medium compared to the lesser pull on the less dense object. The object has to go up to allow the denser, heavier air or water to go down.

petersea · Jan 20, 2009

jibes138... long time since I heard that name ! How are you ! hope thing are well with you !

Jim Kolstoe · Jan 20, 2009

I would refer to my response (#76) to Roger Long's posting on stability. Briefly stated, when you put an object in water, the water is displaced. Since its already filling all available space at or below its initial level, it has to go up. The power to lift the water comes from the force of gravity acting on the mass (i.e. weight) of the boat, and is therefore limited to the weight of the boat. Obviously the water resists being lifted by pushing back against the boat with a force equal to the mass (weight) of the water lifted by the volume of the boat extending into the water, balancing out the force acting on the boat.
Jim Kolstoe, h23 Kara's Boo

Don S/V ILLusion · Jan 20, 2009

ITMaster said:
I did yesterday Don. To me, to sum it up in a single word, that is what I consider it as, Displacement. Glad to see someone thinks the same way I do

Thanks, tom

Tom
we must have both been awake in hydrodynamics class that day

Bill Roosa · Jan 21, 2009

Summerizing

Hi all
Pretty interesting discussion. I always though this was a moderately easy first year engineering question but apparently I was wrong.
I would have to note that, aside from the one fellow that I think said air pressure is holding my boat in the water (I had tears in my eyes around that part of the thread), most of us seem to be in agreement with each other. We are certainly not saying things in a consistent fashion or using a common language however. I think this is what is causing most of the discussion. Not sure how attached everybody is to their semantics but I propose the following for the groups consideration:

1) The boat has weight due to it sitting in some sort of gravity or other acceleration field. I say we limit ourselves to gravity since I don't think any of us has or ever will sail in a centrifuge. But I will admit ANY acceleration field will work.

2) Water is a liquid that exerts pressure on anything immersed in it provided statement 1) is true.

3) Boats float when put in any of the various types of water we find around the planet.

4) Once floating, a boat will "sink slightly" when additional weight (or mass for you purist but statement 1 should satisfy) is added and "rise slightly" when weight is removed from the boat.

5) The amount of sinking or rising in 4) is proportional to the magnitude of the weight. Larger weights produce larger values of sinking or rising.

6) Anything that is immersed in water moves (displaces) the water out of the way. 4) and 5) would seem to imply that more water is moved out of the way when you add weight to the boat and less water is moved out of the way when you take weight from the boat.

With all that said I'm going to ask; are there any more important things that should be included in the above list? and, Should any of the above statements be removed?
Please provide your RATIONAL.

Also please note that none of these statements is meant to answer the question "what makes boats floats". I'm just trying to find out what the issue seems to be.

Maine Sail · Jan 21, 2009

It's

Displacement...... I really don't see how this turns into anything more??

If the object/boat is less dense than water meaning it floats on the water, it then displaces a weight of water equal to the weight of the object/boat. If the object sinks in water, it simply displaces a volume of water equal to the volume of the object..

Anyone remember Archimedes of Syracuse or the Archimedes Principle??

Bill Roosa · Jan 21, 2009

Thought experiment

If you have 2 hemispheres, one 1' and one 1.1' in diameter so that you can nest them.
If you put a volume of water equal to the difference between the two volumes in the larger and force the smaller hemisphere into the larger the water will fill the space between them.
Now ballast the smaller so that it has the weight of an equivalent weight of the water it would take to fill it. Hey look the smaller one is floating in the larger!!!!!
When you pull the smaller out you will see that it is not the amount of water that is displaced but it is the fact that a volume is being suspended in a fluid.

You could float a battleship in a teaspoon of water with a properly shaped container and I'm pretty sure a battleship does not displace 1 teaspoon of water.

I'm not saying that boats don't displace an amount of water equal to their weight. You have to ballast the smaller hemi with exactly that amount of weight to get it to float in the larger hemi.

What I'm trying to point out is that of course the amount of water you would bail out of the inner is equal to it own weight. But the boat does not float on the water INSIDE the boat it floats in the water OUTSIDE the boat and, in this case, it clearly does not take as much water to float the smaller hemisphere.

By your assertion the smaller should never float as ballasted because it can't displace (the water volume/weight is not there to begin with) a sufficient amount of water.

Comments?

John T1594 · Jan 21, 2009

Weightless

Given the forces involved that cancel each other out, a boat would appear to be weightless when floating. That seems to be borne out by the fact that even a small ripple or wave will cause a boat weighing several tons to rock & roll. Even the inertia of the heavy mass of the boat seems to overcome.

Bill Roosa · Jan 21, 2009

Not weightless but

Hi John
I agree totally. It is the forces on the boat (called buoyancy forces) that make it float. The fact (and it is a fact) that a hole shaped exactly like my boat hull in the water could be filled with water and that water would weight as much as it weighs is not what floats the boat. It is the forces (provided by the solid hull) of the water on the hull that float the boat.

The statement "A boat displaces a weight of water equal to its own weight" is not false, is convenyant when weighing stuff of odd shapes but it is useless in floating a boat as the boat floats in the rest of the water not the water it displaced.

Ross · Jan 21, 2009

Bill , I have several stainless steel mixing bowls that demonstate your example very nicely. Finish with the largest and put it in the sink and fill it with water to let it soak. Add another smaller bowl floating on the water in the larger and fill that one also with water. Filling the second bowl causes the the first bowl to overflow until the second bowl sinks and the water is shutoff. The water level in the smaller bowl is always just a little lower than in the larger bowl reflecting the dry weight of the smaller bowl. If there is a load of spoons and other small stuff in the second bowl the difference in the relative heights of the water in the two bowls is greater.

Bill Roosa · Jan 21, 2009

Now pull the smaller bowl out

I agree that putting the smaller bowl into the larger filled with water will displace the volume. But that water is now going down the drain and does nothing to float the smaller bowl. Take the smaller bowl out and see how much water it takes to float the smaller.

It is not the displaced water that floats the boat. That water is gone!
It is the water that is left and that, by inspection, is of a smaller volume than the small bowl. You can check me by pouring the water in the larger bowl into the smaller and noticing that it does (if the bowl are not too different in size) NOT fill the smaller bowl.

So how can an item float if it does not displace its weight in water? Test by replacing the bowl and note that it does indeed still float.

I'm telling you it is not the DISPLACING that produces the force that caused floating it is the force of gravity causing water to have weight.

newly anonymous · Jan 21, 2009

by rote:

Maine Sail said:
Anyone remember Archimedes of Syracuse or the Archimedes Principle??

The way I was taught it: "An object immersed in a liquid is buoyed up by a force equal to the weight of the fluid displaced."