Stability?

[Roger Long]

I'm stuck here this snowy afternoon babysitting a steambox, not something fiberglass boat owners do often.

http://www.rogerlongboats.com/09Winter.htm#Steambox

I've even done the dishes, I'm so bored. Anyone interested in another technical discussion?

I've many times given a lecture on stability to sail training classes and groups in which I demystifyed the subject and presented it in a way that provides a direct feel for how it works as opposed to the math. If you think it's a matter of GM or that GM is important at all, then you don't really understand the subject. GM is just a calculation convienience.

It all begins, like most things, with the basics. I always started my lectures by asking the audience what makes a boat float. Only once over the years, did I get the right answer. Anyone here want to try?

 

[jibes138]

I'll give it a shot

Equilibrium. This is a statics problem. The boat will sink until the sum of the forces in the vertical direction equal zero. So let's say you have a 100 pound boat. You put it in the water. It starts to sink. The force making it sink is f=ma with a the acceleration due to gravity and m = the mass of the boat. So the boat is pushing down on the water with force F1. Now water exerts pressure on everything around it. As you go deeper in the water the greater the pressure. The inside and exposed hull is subjected to atmospheric pressure of 14.7 PSI approximately depending on the weather that day. 33 feet down the pressure is 2 atmospheres. So as the boat sinks there is a force on the hull depending on the depth of any given point. The sum of the pressure on the entire hull in the vertical direction pushes the boat up while gravity and atmospheric pressure push the boat down. When these forces balance out the boat will float.

 

[Roger Long]

Re: I'll give it a shot

You almost get the tee shirt. What makes the water pressure?

 

[Ross]

Gravity. In a zero gravity environment floating wouldn't put water and boat into the same sentence.

 

[Bill Roosa]

My turn

The summation of forces on the boat must always be 0 or the boat will begin to accelerate in the direction of the non zero force. Basic statics and dynamics.
To make her float the downward force from the weight (mg) must equal the buoyant forces (PyAy) on the hull.

mg = PyAy Where y indicates the vertical direction. P is pressure and A is area.

The weight acts (mathmaticaly speaking we assume this) through the center of mass of the boat while the buoyant forces act through the center of buoyancy. Now if the center of buoyancy finds itself above the center of gravity there is no tendency for the boat to overturn because the bottom one is weight and pulls down while the top one is buoyancy and pulls up.
If on the other hand the center of gravity finds itself above the center of buoyancy the boat will heel till either the torque caused by the now offset centers is equal to the torque caused by the non symmetrical (WRT the center of gravity) boyant forces or she takes on water. More on this later as this tends increase stability.

To answer your question "what makes the pressure in the water?" Water has weight and presses down on all the water it finds below it. The water below presses upward to support the water above with the same force. Since the summation of forces must be 0 or the particle will begin to accelerate we know these forces have the same magnitude. Being a liquid we can also figure that the pressure force acts horizontally. The proof is a little more involved though. So the pressure of the water at a point in the water is equal to the weight of the column of water over it. In PSI that is a 1" by 1" by the length of the column with the length being the depth you are interested in. So at 15 feet the weight of a 1"x1" column would be.

62.4 lb/ft^3 * 1/12 ft * 1/12 ft * 15 ft or 6.5 lb and since it is 1" x 1" we can say 6.5 lb/in^2

 

[Roger Long]

Yes. The boat makes a hole in the water. If you lift the boat out, gravity, pushing down on all the other water, will fill in the hole. It's attempt to do this when the boat is there is the water pressure on the hull.

Gravity is one of the very few real forces, the shape of the universe, an exchange of elemetal particles, whatever. It's basic stuff like electromagnetic forces. Buoyancy is just one of those many thought convieniences we call "forces" to assist calculation and visualization. It's really just gravity at work. It helps to think of buoyancy this way to understand how stability actually works.

Oh yes, I forgot to mention to Jibes138 that air pressure has nothing to do with it.

 

[Ross]

If the boat were floating in a pool of mercury all of the calculations would have to change.

 

[Bill Roosa]

What???

Roger! Are you trying to tell me that all those little arrows I learned about in college where lies and the universe is not what the college profs said it was?:naughty:
Pressure is a real force. Shoot a CNG bottle and see if it don't start to move. The electromagnetic force is to blame here. So water is just a convenient way to convert gravity forces (weight) to electromagnetic forces (pressure). And it does it at E=mC^2 efficiencies!!

 

[Bill Roosa]

Sailing in mercury

It would probably capsize due to the greater density of mercury before you got all the math done Ross.

 

[Ross]

At least it wouldn't sink.

 

[CalebD]

Uh, air pressure is due to the same force (gravity) that causes water to reach many times the pressure of 1 atmosphere (at various depths x 33' approx). If there were no air pressure there would be no gravity and thus the boat, the water it wanted to float on and everything else would float off into space.
Air pressure does not directly cause the water pressure that ultimately pushes up on the hull but both are created by gravity. OTOH, when a hurricane reaches a very low pressure it creates an upward surge of sea water beneath it (yes, winds have a lot to do with this as well).
You can make a boat that will float out of a material like steel (which does not float on it's own) and certain woods (Ipe or Ironwood) but the physics that allow boats to float is not necessarily dependent on their construction material.
The salinity of the water also effects the buoyancy of anything wishing to float upon it's surface so don't forget about specific gravity although specific gravity is not what make a boats hull float.

Oh yes, I forgot to mention to Jibes138 that air pressure has nothing to do with it.

 

[Ross]

Caleb , you are unnecessarily complicating this . As you stated without gravity there would be no air pressure and therefore no air, therefore no wind, therefore no hurricane. No gravity, no rain. No rain no ocean. No Hurricane and no water then no storm surge. No water then no boats.

 

[Franklin]

Ok....but lets take this to a meaningful discussion. Normally when we talk about Stability, we want to know as sailors how stable a boat is - how rocky will it be in choppy waves. Boats have gotten more stable in the past 20 years but it's my understanding that the righting ability has deminished.

So stability is important to make the ride smooth but righting ability is important to make comes back up after knockdown.

I believe the number is something like a 130+ is a good righting number but no boat has a 180. Which leads me to my question. If a boat is knocked totally upside down, say by a freak wave, and another isn't coming to help right it, will all boats stay upside down?

 

[jibes138]

Air Pressure Revisited

The pressure of water is the sum of the column of water plus the sum of the air column above it. At sea level the air is pushing down on the water with 14.7 PSI. The water is pushing up on the air with 14.7 PSI. At 33 feet the water provides another 14.7 psi so the pressure is 29.4 psi. at 66 feet it adds another 14.7, etc. So I submit that air pressure inside the hull is pushing down on the boat along with the mass of the boat accelerating due to gravity. Another way to look at it is the weight of the boat is equal to the mass of the boat times gravitational acceleration plus the weight of the column of air above it. You could try to tell me the air pressure on the outside of the hull balances the air pressure on the inside of the hull. This is true when the boat is on the hard. But when the boat is partially submerged can we still say this? Since the water pressure includes the air pressure pushing down on the water I suppose those forces cancel out if you ran the equations.
By the way the elusive higgs boson is still being chased and you can probably add a factor relating to dark energy. If you want I have a friend who chairs the physics department at the university of idaho. this could be a good final exam question for the undergrads.

 

[Ross]

Franklin, With a narrow hull and a reasonably tall cabin and sufficient ballast the boat will be self righting. Most boats are not built like this as form stability is desireable for comfort. A sphere fitted with a mast and ballast would represent the ultimate in self righting design but it would make a lously boat. So a Cylinder is the next choice but it would be hard to stand on deck, so as soon as we start to flatten the cylinder to form a deck we introduce the posibility of dual position stability.

 

[jibes138]

Physicist responds

My buddy responded to my question and in physics they are now considering a new force called the bf force. This force is very weak and it offsets the gravitational force which is also fairly weak. Another term for the bf force is the boatafloata force.

 

[Ross]

My buddy responded to my question and in physics they are now considering a new force called the bf force. This force is very weak and it offsets the gravitational force which is also fairly weak. Another term for the bf force is the boatafloata force.

This is always modified by the bs factor.

 

[Salish Sea Cruiser]

Sweat, elbow grease and money

We all know that any formula for keeping a boat afloat must include some combination of these.

 

[tsmwebb]

Gravity is one of the very few real forces, the shape of the universe, an exchange of elemetal particles, whatever. It's basic stuff like electromagnetic forces. Buoyancy is just one of those many thought convieniences we call "forces" to assist calculation and visualization. It's really just gravity at work. It helps to think of buoyancy this way to understand how stability actually works.

No Roger. Even being exceedingly pedantic you have to admit Strong, electromagnetic, weak and gravity. With just gravity the entire universe would collapse into a singularity. When the ocean is just sitting around waiting for us to float our boat the water is in equilibrium. Whats holding the water up if the only real force is gravity? Water is, we're told, mostly made up of space. And, of course, the atmosphere in all its layers floats for the same reason a boat or an airship or a submarine might.

Moreover, the fundamental force of gravity is not required for bouncy to exist. You can float things in zero gravity just buy taking advantage of a little angular momentum or any other convenient acceleration. I think this is important to understanding how stability actually works because in all except the most static case, the gravity one needs to know to understand buoyancy and stability is the local apparent gravity.

--Tom.

 

[newly anonymous]

in a word, what makes a boat float is...

"gravity." Otherwise, the boat angels would lift it right out of the sea.