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Go Back   SailNet Community > On Board > Boat Review and Purchase Forum > Sailboat Design and Construction > Zero buoyancy
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Topic Review (Newest First)
06-08-2009 09:08 PM
KeelHaulin Well, Skipper, unfortunately you're wrong on so many levels with that last post there is no sense in responding to the in-duh-vidual attacks.

Keep believing that you can be negatively buoyant; maybe someday you will be.

As for questioning what I do for a living; I'm a Mechanical Engineer, double majored in college, which required study of physics and applied science for 4 years, and interned at a national lab while attending college. I'm pretty much certain I know what I'm talking about on this subject; do you Skip?
06-08-2009 07:50 PM
J36ZT
Neg Buoyancy...

I reject your reality and substitute my own... NEGATIVE BUOYANCY does exist and is a commonly accepted term.

I don't know you, Keelhaulin. I've never met you and don't know your background. I suspect you are an AIG executive or worked as a financial manager for G.M. If I'm wrong, with your logic, you should consider applying for such positions. I think you'd do well! Heck, you might even be delusional enough to think the taxpayers love giving you bonus money!

Here is a very basic example. You have two apples. You promise to give a friend three apples. How many apples do you really have? ... (Basic example of a NEGATIVE number!) You keep saying you have a positive number of apples, and the rest of us believe the correct answer to be "-1."

I'm pretty sure you've never taken a SCUBA diving class, or even been below the surface of water where you were less than neutrally buoyant. For anyone considering passing a SCUBA class; you have to believe there is a term for when you sink...that term would be "Negative buoyancy." If you don't understand the term or don't believe it exists, you simply will not be able to pass the class and obtain certification.

Now, if you are arguing semantics and think the term "Negative Buoyancy" doesn't exist...please do an internet search. When I Googled to define the term, there were 995,000 results...including a very good WIKIPEDIA result.

However, I believe the term "Zero Buoyancy" is not correct and "Neutral Buoyancy" is the term that should be used.

Sorry I had to attack you so badly Keelhaulin, but when you posted we should teach out children something that is incorrect I had to come back to correct things.

Skipper, J/36 "Zero Tolerance"
06-05-2009 06:06 PM
KeelHaulin
Quote:
Originally Posted by sailingdog View Post
an object has negative buoyancy when it weighs more than the water it displaces, and as such will sink.
Unfortunately this is not correct. It is a term used to describe a condition of weighing more than the water you displace; but there really is no such thing as being "negatively buoyant". It's an oxymoron. A rock is buoyant, just not buoyant enough to float. The buoyant force is ALWAYS in opposition to gravity and as such it is ALWAYS non-negative. People who dive have coined the phrase "negative buoyancy" but unfortunately it is an incorrect term and kids who are trying to learn what buoyancy is get it totally wrong because of this nomenclature (believe me, my GF is a science teacher). Don't teach this; your children will be doomed to answering incorrectly in their school studies and on standardized tests if you do.


Take this example:

A diver who is neutrally buoyant tries to retrieve a friend's weight belt from the bottom. The weight belt weighs 10 lbs. When he picks the belt up off of the bottom; the diver cannot swim to the surface because he now has a total weight that is more than his buoyant force. Did the diver LOSE any buoyancy?

The answer is:

No, he did not lose buoyancy. He gained mass and his buoyant force remained the same. His buoyancy never became "negative"; he was no-longer buoyant enough to be able to return to the surface. If the diver had an air bladder he would increase his buoyancy by adding air to the bladder; offsetting the additional weight.
06-04-2009 07:08 AM
sailingdog KH-

an object has negative buoyancy when it weighs more than the water it displaces, and as such will sink.
06-04-2009 12:44 AM
KeelHaulin Again; the physics of it is that there is no such thing as negative buoyancy. Buoyant force is always in opposition to the pull of gravity; and it's regardless of the density of the item that is submerged. Salt water weighs ~8 lbs per gallon; so if you displace a volume of 1 gallon whatever is taking the place of the water will weigh ~8 lbs/gallon less in water.

Quote:
Now measure the force the block is pulling on the line and you have the force of NEGATIVE BUOYANCY!
No, you have the difference between the weight of the object and the buoyant force that acts against it. Let's say you have a 1 gallon container full of molasses; and say it weighs 10 lbs (6 lbs water 3 lbs sugar). In salt water the pull on that string would be 2 lbs (10lbs - 8 lbs). The buoyant force is 8 lbs; it will always be 8 lbs for 1 gallon of displacement, whether the object floats or not is dependent on it's density relative to the surrounding water.
06-03-2009 04:36 PM
J36ZT
Negative buoyancy does exist

OK, you take a block of aluminum, attach a line onto it and drop it into the water. The block will sink until the line stops it. Now measure the force the block is pulling on the line and you have the force of NEGATIVE BUOYANCY!

Now, immagine you are a SCUBA diver in a wet suit. The wet suit contains small bubbles of air. You dive to a given depth and then begin returning to the surface. The closer you get to the surface, the less pressure is exerted on the bubbles and the more they expand (the more bouyancy your wet suit will have). In other words, the closer to the surface the faster your ascent will be. But, SCUBA divers have this thing called a BCD (Buoyancy Compensation Device) which is really nothing more than a vest that can be inflated or deflated. A SCUBA diver, wearing a wet suit, knows about buoyancy and will let air out if the ascent needs to be slowed.

SCUBA divers quickly learn about buoyancy--positive, neutral, and NEGATIVE--because it takes energy to overcome the forces.

KeelHaulin, not meaning to attack you...but you would have us believe a graph of buoyancy would be a parabola, or a curve (in the least) that would never reach zero. From having done diving, I can not accept this. As a matter of fact, just yesterday I was Neutrally Buoyant at a depth of eight feet. If a took a deep breath I ascended, and if I breathed shallow I sank...yep, I could make myself NEGATIVELY buoyant.

Back to our watercraft... Because the air contained within the craft can not compress, depth would not change its characteristics. Since the pictures show said craft, at rest, with a portion of it sticking out of the water (not just at the surface)...it has to have some Positive buoyancy in the liquid it was placed. It is possible the watercraft would have positive buoyancy in salt water but sink in fresh water. I wonder if there are warnings stating which type of water to launch the thing into?

Again, just my thoughts...as I've witnessed things...

Skipper, J/36 "Zero Tolerance"
06-02-2009 06:07 PM
peikenberry Positive buoyancy wins. It would not hold up the weight of itself and the crew if it did not have positive buoyancy. If the boat had nuetral Buoyancy then the weight of the crew would sink it even lower untill the water pressure increased to where it would go no further.
it is obvious the people who said zero buoyancy do not understand buoyancy. Maybe we should introduce them to Archimedes
06-02-2009 02:56 PM
KeelHaulin
Quote:
Originally Posted by J36ZT View Post
Negative Buoyancy means it will sink (at that depth).
There is no such thing as "negative buoyancy"; you are either buoyant (and have "reserve buoyancy"), neutrally buoyant (weigh the same as water) or the buoyant force is less than your weight and you sink.

In example if you put a block of solid aluminum in the water and it's weight is 10 grams, and the volume of water it displaced weighed (appx) 5 grams, it would sink because it needs 5 grams more buoyant force. If you had a scale on it it while submerged it would register 5 grams because the buoyant force is still there but only providing 1/2 of the needed force (it's not negative, it just is not big enough).

Take the same mass of aluminum as a foil and shape it into a shell or "boat" shape and it will float; because the shape allows there to be a displaced volume of water with air in it's place (much less dense than water). The displacement of the water allows the aluminum shell to float and it's the amount of water displaced is exactly the weight of the shell. If you add weight to the aluminum boat it will sink gradually with the additional displacement equaling the amount of weight added; and that's considered "reserve buoyancy".


The frame-only sailboat appears to be close to neutral buoyancy. The tubular carbon fiber displaces water and allows it to just barely float. The styrofoam pontoons are probably for reserve buoyancy (to be sure it won't sink).
05-28-2009 11:43 AM
J36ZT
Sailingdog is right

Neutral Buoyancy means it will stay at a given depth (even if right at the surface). Negative Buoyancy means it will sink (at that depth).

By this strict definition, I'd say the "boat" actually has a slight Positive Buoyancy. If it truely had "Zero Buoyancy," no part of it would be above the surface of the water...not even the mast.

Just my thoughts...

Skipper, J/36 "Zero Tolerance"
05-28-2009 09:13 AM
sailingdog Zero buoyancy is neutral and it will neither sink nor float, but stay where it is put until other forces work on it. To sink, without external forces working on it, it would have to have NEGATIVE buoyancy.
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