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  #11  
Old 05-28-2009
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I think the zero boyancy is just to lighten it up as much as possible for a speed run, or just to show it can do it. Every other pic I've seen of it, along with the other videos I've found show a pair of 'pontoons' or another type of flotation hull on it.
Really don't think that it is designed to be regularly sailed without the floats, the very end of the video I linked to earlier shows a pretty violent landing, even when it's cushioned by the pontoons. Violent enough to fold at least one pontoon.

Ken.
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  #12  
Old 05-28-2009
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Zero Buoyancy means that you have no buoyancy and have sunk.
What you should be asking about is positive buoyancy... Then you will be afloat.
You also want positive stability to stay upright.
Neutral stability is the stability of a ball. Any point on that sphere is up.
Negative stability is where you will capsize sooner or later... usually sooner.
Hopefully I have started you on your studies of Ship's stability.

Now I believe you want to lighten ship for a speed run. Which is a different animal all together. But when you do, pay strict attention to your stability or you may just capsize as you start out on your speed run.
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  #13  
Old 05-28-2009
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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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  #14  
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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"
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  #15  
Old 06-02-2009
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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).
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Old 06-02-2009
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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
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  #17  
Old 06-03-2009
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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"
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Old 06-04-2009
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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.

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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.
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Old 06-04-2009
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an object has negative buoyancy when it weighs more than the water it displaces, and as such will sink.
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  #20  
Old 06-05-2009
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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.

Last edited by KeelHaulin; 06-08-2009 at 10:01 PM.
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