Stability?

[Bill Roosa]

Foam math

That depends Franklin. Are you talking closed cell or open cell foam. Foam that fills with water (open cell) has almost no buoyant force once it is saturated, it could have negative buoyancy in fact. Closed cell cannot technical "support" weight above water. It has to be submerged in the water to support weight.

Assuming that what you are saying is a block of closed cell foam can displace 60 lb of water just as it is completely submerged we can proceed as follows:

The cube 1'x1'x1' of foam will experience a) the weight suspended below it which I will assume is a point so I don't have to calculate the buoyant force on it and can concentrate on the foam, b) a pressure upward on the bottom of the cube, and c) a pressure downward on the top of the cube.

From above posts the pressure water exerts is:

Pw = x * 62.4 where x is the depth and we are working in fresh water.

I'll assume the foam is not compressible and that it still has the 1'x1'x1' dimensions when submerged to 2'. This is not true at lower depths BTW. The upper surface is at 1.5 ft below the water and the lower surface is at 2.5 ft below the surface so summing forces we get:

Pw on the lower surface times the area of the lower surface - Pw on the upper surface times the upper surface = weight supported.
or
2.5 * 62.4 *1 - 1.5 *62.4 * 1 = W= (2.5-1.5) * 62.4 = 1 *62.4 = 62.4 lb

So the 60 lb point weight the foam block supported when "just submerged" can be increased by 2.4 lb when you push the block's center of buoyancy to 2 ft deep and the block will be at neutral buoyancy.

Back to you Franklin:
If we assume the weight is not a point but a cube 2"x2"x2" and is made of suitable material to make it weight 62.4 lb will the foam block sink or float upward once I mechanically submerge it to 2 ft deep and what weight will have to be added to /subtracted from the weight to achieve neutral buoyancy at 2' if I maintain the 2"x2"x2" size?

 

[Bill Roosa]

Same argument Brian

Mass tells space how to warp, space tells mass how to move.

We still don't know how space and mass communicate over a distance which for gravity is pretty large.

Another thing I have always wondered is if I take a generous mitten full of energy and "make some mass" how long does it take the rest of the universe to figure out there is some more mass effecting it. I suspect the question is dumb as Mr. E determined that mass and energy are equivalent. A less technical way of asking is what is the speed of gravitational propagation?

 

[Franklin]

I just remember reading somewhere a guy who claimed to have made his boat unsinkable. I believe it was a monohull. So I was just thinking the other day if I put foam in all the cracks and corners that I don't use, would that make it unsinkable. Definately not by these calculations. I figure I could get at max 50 cubical feet of foam in. No where near the 300 I would need.

 

[Roger Long]

I think what Roger is trying to say is that buoyancy is an abstraction of the electromagnetic force.

No, I didn't say that.

 

[Bill Roosa]

Foam in unused corners

Hi Franklin
Boy you must have a big boat! Unused corners, what a concept.

It is hard to beat air as a flotation device, light, cheap, easily replaceable and not hard to confine. Perhaps you could seal all those unused corners so they are air tight. Foam actually has a slightly lower buoyancy because it does weight more than an equivalent volume of air.

Roger
Sorry, not trying to put words in your mouth. Am I to ass u me that you are of the belief that buoyancy is not a real force?

 

[Brian Schwerdt]

Mass tells space how to warp, space tells mass how to move.

We still don't know how space and mass communicate over a distance which for gravity is pretty large.

Another thing I have always wondered is if I take a generous mitten full of energy and "make some mass" how long does it take the rest of the universe to figure out there is some more mass effecting it. I suspect the question is dumb as Mr. E determined that mass and energy are equivalent. A less technical way of asking is what is the speed of gravitational propagation?

This is not exactly true. It has been found that gravity is communicated over distance by means of particles known as "gravitrons". They happen to travel at the speed of light (roughly 3x10^8 meters per second)

 

[Roger Long]

Roger, Sorry, not trying to put words in your mouth. Am I to ass u me that you are of the belief that buoyancy is not a real force?

Depends what you mean by "real". This whole thing has gotten derailed over semantics and a (thankfully rare around here) Usenet like frenzy to prove who is the biggest dog by parsing the phrasing in absurd ways rather than simply trying to figure out what the poster meant.

So far, we are one minute into what was the hour long lecture I used to give and people are posting that I haven't proved my point yet. The gravity business was just a throw away I used to use to get the attention of smug students who though they could sleep through the lecture because they did the reading the night before. Works great. Look what it stirred up here.

The students would do the reading, memorize "If the center of gravity is above the metacenter, the boat will be unstable. If below, it will be stable." They would then think, Cool, I can remember that and spit it out on the test. I can sleep through the lecture tomorrow.

I'd ask, "What makes a boat float?" A bored, smug voice would say, "Buoyancy".

I'd ask "What causes buoyancy?" 30 deer in the headlights. By the time the lecture was over, they all knew that, not only had they not understood the reading, the conventional explanation (sort of like lift) is incomplete and misleading. They would also have a good gut level feeling for the forces actually at work when a ship resists wind and other forces.

Buoyance is a real force but it is just a simple mechanical effect of gravity at work on a liquid. Gravity itself is a an entirely different class of force being one of the most fundemental things in the universe. For the nit pickers, you could have the same dynamics in the swimming pool of a big space station rotating to produce simulated gravity but we and our boats are here on earth so that's just Usenet noise.

Maybe I'll get back to this after I stop laughing and rolling around on the floor over how this whole discussion went.

Can anybody tell us why there is a metacenter and what force it represents?

 

[dvideohd]

I don't think there is a "graviton"... Makes the theories get more complicated... Speed of light is not so constant either...

I saw (TV) an experiment to raise boats with Ping Pong balls... Seen a fellow who filled the "nooks and crannies" space with foam, too.. Seems to work well with Boston Whalers and MacGregors....

While eating my next generous meal of "Sea Kittens", I would like to see that "generous mitten" as well...

--jerry

 

[Todd Smith]

...The gravity business was just a throw away I used to use to get the attention of smug students who though they could sleep through the lecture because they did the reading the night before. Works great. Look what it stirred up here.

...

Can anybody tell us why there is a metacenter and what force it represents?

Maybe if you didn't start off with a throw away line, you wouldn't end up with an absurd conversation. There are different conditions here than a real-time lecture hall.


The "why" of a metacenter is that it helps a naval architect determine and describe the stability characteristics of a ship. And it represents bouyancy !

 

[Bill Roosa]

Metacenter

Well I can't speak for others but I use it to simplify the analysis. The same reason we use CG and CB.

If you extend a line vertically upward through the CG of a boat that is not inclined the MC will always be on that line. It will also be on the line extended vertically from the CB vertically upward and be positioned where the two lines meet. So for a heeled boat the MC will be directly over the CB along the (formerly vertical but now angled) line through the CG. A more useful observation would be that once you know where the meta center is and heel the boat the CB will be directly below that point. You don't really care where the CB is on the line it is the line that is important. If you draw a horizontal line from the line dropped vertically from the MC so that it intersects the CG you can measure a distance called the righting arm. This is a handy number to know BTW but Roger is teaching the class.

 

[bffatcat]

Correct me if im wrong

I dunno guys, I always thought a keel boat would sink until It displaced water equal to its
weight and at that point it would float. Im no physicist and I dont understand how air pressure plays a part in this.

 

[Ross]

Too often our education gets in the way of our intelligence. Thomas Edison once gave a group on potential hires a complex blown glass container and asked them to determine its volume and provided them with calipers, rulers, and other tools. Then he left and when he returned later for their answers they presented him with all of their measurements and calculations. He picked up the test piece ,went to a sink and filled it with water and poured it into a measuring flask to check their answers.

 

[novelman17130]

The simple answer is that the boat floats when the weight of the part of the boat displacing the water it floats in is less than the weight of the water it displaces.
novelman

 

[Bill Roosa]

Floating and stability

But Novelman
The fact that a boat floats may or may not be a good thing. As our catamaran fellows are so apt to point out, their boats float upside down. They are stable (very stable, too stable for me) in that condition. So stable they have a door in the bottom of the boat!
I'd prefer to understand how stable my boat is and means of calculating some of the properties that effect it.

For instance if I wish to make my boat more stable will moving the 120 lb of batteries in the aft starboard lazeret to the keel sump actually give me noticeably stiffer boat? Just saying it displaces x amount of water does not help me out here.

 

[DanW9494]

Re: Floating and stability

What makes a boat float? Lower density of mass than the water around it within a gravitational field.

What keeps a boat floating? A design that makes it possible to keep water out of the boat adequately to maintain a lower density of mass than the water around it.

What make a boat stable? A design with static and possibly dynamic righting forces to minimize the forces of the water around it.

 

[tsmwebb]

...
So far, we are one minute into what was the hour long lecture I used to give and people are posting that I haven't proved my point yet. ...

I'd ask "What causes buoyancy?" 30 deer in the headlights. By the time the lecture was over, they all knew that, not only had they not understood the reading, the conventional explanation (sort of like lift) is incomplete and misleading. They would also have a good gut level feeling for the forces actually at work when a ship resists wind and other forces.

Buoyance is a real force but it is just a simple mechanical effect of gravity at work on a liquid. Gravity itself is a an entirely different class of force being one of the most fundemental things in the universe. For the nit pickers, you could have the same dynamics in the swimming pool of a big space station rotating to produce simulated gravity but we and our boats are here on earth so that's just Usenet noise.

Roger, I have a great deal of respect for you. I read what you have to say carefully and generally find that very rewarding. But, I've got to say that I don't understand your take on this issue. You started a nearly identical conversation on rec.boats.cruising with nearly identical results: you got a bunch of very good answers to your question and created a lot of confusion with your assertion the buoyancy is imaginary and then retreated to the position that it was all really just a throw away line in the aid of some pedagogical device for a lesson unrevealed. Let me, with respect, suggest that the audience here is not just a bunch of sleepy students. I think we all come here because we're interested. You don't need to wake us up. If you're trying to make a larger point go on and make it. If you never intended to give the full lesson should we not feel used?

Finally, no we don't sail in space but even here on Earth buoyancy in the ocean will be in respect to a sum of forces, not gravity alone. Picture a raft riding a deep ocean swell on a windless day. Put a bubble level and a spinning gyroscope on the raft. As the waves pass under the raft the bubble level will stay centered but the gyro will rock back and forth. Down for the raft varies. So, too does buoyancy because it is not solely the result of gravity. In some dynamics this may be quite significant. How does your false assertion that buoyancy is just gravity help in understanding these very real phenomenon? This doesn't seem like nit picking to me.

--Tom.

 

[ITMaster]

Well, without making use of lots and lots of words, since I am trying to cut back on the use of them, I would say in one word that what makes a boat float is

Displacement

 

[Grizz]

Ouch! My head hurts and it's only Monday...

For the longest time I was under the (apparently mistaken) assumption that this was a sailing/boat owner/practical information site. What was I thinking? And today, after chewing through all of today's reply's (some required re-reading) containing answers, best-guesses, calculations, gravitons, string theory, thought experiments, Einstein, Thomas Edison and tons of 'back-and-forth', I'm left with the following question for the 'Great Rooska': so, what does happen when a 150 lb person boards the barge amidships?

Please tell me it snaps in two like the Edmund Fitzgerad and sinks...please?

 

[jibes138]

The final word

Air pressure matters because the pressure of the water at sea level is equal to the air pressure at sea level. Let's assume we have a mass less boat. The boat is very flat and thin. Kind of like a piece of paper but without mass. we put the boat on the surface of the water and observe that it floats exactly on the top of the water. The force of the air pressure pushing down on our flat boat is exactly equal to the force of the water pressure pushing up on our boat. as we add mass to our boat we are increasing the force in the downward direction so it sinks until the water pressure increases enough to push it up with the same pressure as force pushing it down.

 

[Jim Kolstoe]

Sailingdog, thanks for the correction on the density of saltwater, and yes, I was aware that it varies with the salinity. I used the weasal language because I have not needed to do buoyancy calculations since I took scuba certification as a high school kid in the early '70's and did not feel confident about my recollection. I was betting someone on this site would catch and comment on any error. Isn't that part of the fun?
Jim Kolstoe, h23 Kara's Boo