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How much Bouyancy to ?? depth.. - Can't figure it out.

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Postby Robse » Sat Aug 23, 2003 5:44 pm

Hi.

I'm not sure how to frase this, but I'll give it a try:

I've been trying to find out how much negative bouyancy it would take to reach a certain depth, but cannot find the connection. The reason I'm wondering, is that if I give my sub just a TINY bit of negative B by pumpin' a little water into the ballast tanks, it'll dive to, let's say 3 ft, and stay there.... giving it a bit more, and it'll go a bit deeper, and stay there.. So far so good, but..

I'm aware that the sub must have a certain negative B to go to some depth, and that I can regulate the negative B using my MBT, but HOW much "does it take" to go to a certain depth? I'm about to build the MBT, and sure do not want to make it too small. I think that if I build the "variable bouyancy" too small, I'll only be able to go just bit down... makes sence?

I really hope someone can set me right on this, thanks in advance. :)
Yours Sincerely, Robert Holsting, Denmark
1/81 SSBN Ohio Class scratch builder, more at www.robse.dk

"Never be afraid to try something new; remember that it was amateurs who build Noah's Ark, and professionals who build the Titanic"
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Postby JWLaRue » Sat Aug 23, 2003 7:12 pm

Well....the quick answer to your question is that even the smallest amount of negative buoyancy will take the sub right to the bottom. Negative buoyancy is negative buoyancy.

I *think* you might be trying to ask how to adjust the buoyancy of a (model) submarine such that it can be balanced to stay at a given depth. This is the so-called neutral buoyancy that folks talk about. The goal is to get the trim of the sub in such a way as to take into account the effects of water pressure (squeezing the wtc), water density, etc. and achieve a balance....in effect 'hover' the boat.

In theory it is quite possible to trim a model submarine to hover at a given depth but even with a somewhat precise ballast system like the Engel piston, it has been my observation that this is difficult. The relative small size of a model sub means that if the trim is off by the smallest fraction, the sub won't hover.

The depth control electronics offered by a couple of our vendors could possibly be connected to the ballast system to provide 'active' control, but I don't think I've heard of anyone trying this. (Has anyone?) These electronics are 'normally' used connected to the dive planes to maintain a given depth while moving through the water, but it appears that this other application could work.

-hope this makes sense!

Jeff
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Postby Robse » Sat Aug 23, 2003 8:20 pm

Hi Jeff (And others)

Thanks for your reply. Well... yes, I got that down, but the problem is a big more complex:

Let's say my MBT can only hold 1 cup of water, and that that this takes my sub to about 3 ft. (For example), and ...say 5 cubs would take her down to about 10 ft, BUT I wanna go deaper. How do I then calculate how much water I must be able to take on in order to *prevent* my sub to hover at a too shallow depth?

In theory you could, "by accident", construct your MBT's so that they make the sub hover at, say 3 ft. while fully flooded.... That would sure make my day a bad one, as I wanna be able to go deeper.. makes sence?
Yours Sincerely, Robert Holsting, Denmark
1/81 SSBN Ohio Class scratch builder, more at www.robse.dk

"Never be afraid to try something new; remember that it was amateurs who build Noah's Ark, and professionals who build the Titanic"
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Postby KOEZE » Sun Aug 24, 2003 5:56 am

I think you missed the point a bit here. When the weight of the sub equals its displacement a state of neutral bouyancy is reached. This means that the sub is essentailly weightless under water. It neither sinks nor rises to the surface. Negative bouyancy is not coupled to the depth of the submarine. The goal is to trim the submarine so that when the ballast tanks are full you have neutral bouyancy.

With rudders you can then drive the sub to a certain depth. You can also use a trimtank to temporarily add more water creating negative bouyancy and expell that extra bit of water when you reach the depth you want.

The reason for wanting your ballast tanks full for neatral bouyancy is the fact that water is not compressable but air is. So if you take your sub down when there is still air in the tank this will be compressed by the rising pressure at greater depth. This will result in more water in your MBT and negative bouyancy. The sub will sink.

In short. Pumping more water into the ballast tanks only results in sinking your sub. Neutral bouyancy is what you want. Pumping more water into your tanks will not get you to a set depth other than the bottom.

EJK
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Postby Carter » Sun Aug 24, 2003 12:46 pm

It is my observation (and I think my book learning) that the deeper you go, the less bouyancy you have. As water pressure compresses the WTC (compressing the air inside the WTC as well), the volume of displaced water (bouyancy) decreases.

The deeper you go the heavier you get.

I have expereinced a neutral bouancy "sweet spot" on occassion. With a flooded ballast tank I run with a slight positive bouyancy, with the cap of the sail riding about 1/8" above the water. Sometimes, two to three feet below the surface, the sub is truly neutral. But sometimes this doesn't seem to be the case at all. Perhaps it's a matter of coasting the boat precisely to the sweet spot depth. With forward momentum, water currents and so on, it takes a bit of luck to hit it just right.

Is water temperature a factor at the scales we are operating at?
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Postby Robse » Sun Aug 24, 2003 1:04 pm

Hi guys.

Thanks for setting me straight... I must have gotten it in the wrong way at first. The increasing pressure will not compress my air in the MBT though, as the MBT will be fully sealed using valves when it's not gettin' either air or water.

I now have a better understanding of this, and will go ahead with the construction of my MBT, which will give me between +0,6 kg (+1,3 lb) and -1 kg (-2,2 lb) of bouyancy.
That should the allow me to dive, AND reclaim the world above.

Carter: The temperature would change the density of the water, and that will again be a factor in how deep the sub floates in the water, but I think it's a pretty small factor in model subs.. (Right, others?)

Thanks again, guys. :)
Yours Sincerely, Robert Holsting, Denmark
1/81 SSBN Ohio Class scratch builder, more at www.robse.dk

"Never be afraid to try something new; remember that it was amateurs who build Noah's Ark, and professionals who build the Titanic"
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