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Old Mar 03, 2013, 08:58 PM
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Water density and buoyancy

just a quick question about water density, depth and buoyancy;

after playing with my little toy sub in the tub, i have found that i am a little confused about how to hover. it seems like the sub behaves reverse of how i thought it should, at a certain ballast point it will float up if its within 2-3 inches of the surface, but if its about 4-5 inches down, it will sink. if the sub is using a sealed hull and the displacement via ballast only changes from the remote, shouldnt it go down to a depth that it is equal weight to the surrounding water and wont go farther?
in my head, once it displaces a specific weight of water, it should go to the depth where it is equal and that will result in a hover.

i can understand that if the ballast is open or exposed to the outside of the pressure hull, the amount needed is variable as depth increases. but being that this one compresses the inside to reduce displacement, i would think it should be easier to get a real hover out of it.

so what am i not understanding about this, please sub masters, teach me the ways of hovering
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Old Mar 04, 2013, 04:13 AM
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Quote:
Originally Posted by derk View Post
just a quick question about water density, depth and buoyancy;

after playing with my little toy sub in the tub, i have found that i am a little confused about how to hover. it seems like the sub behaves reverse of how i thought it should, at a certain ballast point it will float up if its within 2-3 inches of the surface, but if its about 4-5 inches down, it will sink. if the sub is using a sealed hull and the displacement via ballast only changes from the remote, shouldnt it go down to a depth that it is equal weight to the surrounding water and wont go farther?
in my head, once it displaces a specific weight of water, it should go to the depth where it is equal and that will result in a hover.

i can understand that if the ballast is open or exposed to the outside of the pressure hull, the amount needed is variable as depth increases. but being that this one compresses the inside to reduce displacement, i would think it should be easier to get a real hover out of it.

so what am i not understanding about this, please sub masters, teach me the ways of hovering
Hi, your sub response just right because I think that your sub are not form-fast I mean that your little toy sub becomes a bit compressed (and getting a smaller volume) and start sinking, and above a certain depth your Sub is getting a bit bigger and the volume is increasing and the Sup will go up.

I have a Lafajette Sub with a volume of 9,5 KG submersed and its reacting when I sail her in our local pond when entering the shadow side of the pond or leaving that area in to the sunny side of it.
I need to trim here then to keep it hovering at the same depth, so only a few degree in temperature will give a great reaction at the buoyancyof a Sub.

I have seen it with my Sub in a swimming pool that when I go deeper then 2 meter I had to trim her lighter to keep her at the same depth or she was sinking faster.

Model RC submarine Lafajette under water view (3 min 21 sec)


So as far I understand your question, for me it make sense what you experience.

I hope I helped you out a bit,

Kind regards,

Jan,
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Old Mar 04, 2013, 05:01 AM
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it just sounds backwards that a sub will sink faster in deeper water if its total displacement isn't changing as it goes deeper. on subs that have an external bladder or use gas systems, i can totally see why it would do that, as it goes deeper the air in the ballast bladder or central tank compresses so it needs more air to displace the compressed volume. but in subs like this little one and your big one that use a piston system, it should only go so far down before pressure is equal and it stops sinking. almost like the reverse of a balloon going up, it gets bigger as pressure decreases, but if you built a balloon with a solid shell, it could only go so high before its lift is equal to its air displacement.
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Old Mar 04, 2013, 06:44 AM
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Just a few comments

I don't know if it is theoretically possible to find a neutral point of buoyancy. I realize that the density of water remains constant (depending on the mixture of salinity) throughout the water column, with pressure increasing on the order of 1 atm every 10 m. Water is incompressible (not too sure about this at great depths?) And therefore not a factor regarding buoyancy. Buoyancy is simply the amount (weight) of water displaced by the volume of the object or submarine depending on its shape. A cubic foot of salt water weighs approximately 64 pounds (density as Mass over volume), and fresh water less than that. So a rowboat in salt water would ride a little higher than the same rowboat in fresh water. The shape of the rowboat would determine its displacement.

Assuming for example you had an incompressible glass sphere to which you could add water you would think that you could find a neutral point. Theoretically the weight of the glass sphere partially filled with water would have to displace an equivalent weight of water. The ratio of mass to volume be one and this would be the same density as the water and therefore it would be neutrally buoyant. This would be difficult to accomplish on a practical level because there would always be an infinitesimal force on the sphere either up because of buoyancy or down because of a lack of buoyancy. Even if that force were only a few tenths of a gram.

But my question essentially is-can it be theoretically possible to adjust the weight of the sphere to a true neutral buoyancy? The "force" of buoyancy becomes less significant the larger the object e.g. submarine. For example consider an upward force of a few hundred pounds on a large submarine. For all practical purposes this would be nothing. However this same force would have more influence on say for example a smaller one man submarine.

If you could adjust the sphere to a neutral buoyancy it should remain in place at any depth in the ocean regardless of pressure. It should stay where you place it.

In addition if there's any motion in the submarine the use of dive planes can stabilize the subís depth as though it were neutrally buoyant. Also water temperature and density can affect buoyancy. There is a thermocline in the ocean where the water temperatures vary and you can actually see particles and debris sitting on top of the thermocline rather than sinking through it.

But I still don't know if it is theoretically possible to find true neutral buoyancy? Intuitively it seems to me that if you could add or subtract infinitely small amounts of water to the glass sphere example you could find neutral buoyancy. On a practical level I don't think you could be very successful.
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Old Mar 04, 2013, 07:14 AM
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okay, i think i see the flaw i had in my head now, INCOMPRESSIBLE fluid! it makes much more sense if the water doesn't weigh any more or less with depth, at least not without great depths involved.
for some reason, i was attributing air's compressibility with water, farther down more weight per cubic volume which is totally wrong!
thanks for the help guys, now i will continue with this information in mind next time
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Old Mar 06, 2013, 07:43 PM
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And just to add a little bit to Carcharadon's excellent explanation of buoyancy, we have to remember that our models are actually made of compressible sections. The "solid" water-tight compartment lid will deflect inwards just a little bit as the sub goes deeper. The exact amount of deflection will depend on the material used as well as its shape.

A 1 atmosphere increase in pressure is about 14 psi, not a figure to be trifled with.
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