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How to calculate ballast tank volume?

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Old 12-02-2004, 05:42 PM
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wingtip-RCU
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Default How to calculate ballast tank volume?

Any ideas on how to roughly calculate the required volume of a ballast tank... im guessing my total weight is gonna be anywhere around 11+ pounds...

a few pics are in my rc heli gallery at http://www.runryder.com/helicopter/gallery/2102/ towards the bottom of the page....


this is a compressed air system, with 2 onboard small compressed air tanks....
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Old 12-11-2004, 10:42 AM
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Default RE: How to calculate ballast tank volume?

Well, I know next to nothing about RC subs right now, but since you haven't gotten a response yet, here's how I'd figure it out...

First, recognize that neutral buoyancy means the weight of your sub must equal the weight of the water displaced by your sub. This means if your sub displaces two gallons of water when totally submersed, then it will sink if it weighs more than two gallons of water, and it will float if it weighs less than two gallons of water.

So the hard part is figuring out the volume of water your sub will displace. If your entire hull is water proof, then you just need to caculate the volume of the hull. If you have WTCs inside and flood the hull, you need to calculate the WTC volume (based on exterior dimensions, not interior) and guesstimate the total volume that the hull material takes up (maybe you can neglect this, especially if the hull material itself has close to neutral buoyancy).

So my simple minded approach is to figure out how much weight/ballast you have to add to get you total sub weight > water displacement weight.

So lets define some variables and write some equations...

WS = dry weight of sub (empty ballast tank)
WWD = weight of water displaced by sub
WB = weight of ballast

so neutral buoyancy is when:
WS +WB = WWD

and you need to calculate:
WB = WWD - WS

You said that WS = 11+ pounds for your sub, so you need to calculate or guesstimate WDW and then you can find WB.

I believe 1cc of water weighs 1g, so 1L weighs 1kg (1L = 1 cubic decimeter = 1000cc). I don't know the English unit equivalents (the metric system keeps things simple, as you can see), but you can just take your measurements in centimeters, or look up conversion factors.

So if your sub weighs 12lbs, and it displaces 16lbs of water, you need a ballast tank that can hold > 4lbs of water.

Hope that helps... maybe one of the experienced sub guys here will chime in and provide an easier way to figure it out!
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Old 12-12-2004, 12:20 AM
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Default RE: How to calculate ballast tank volume?

Hello, I have never been worth a darn at figuring ballast tank size out with a formula. I'm on my 4th sub attempt. Hoping this will work as I hope. This method came from Skip at SUBTECH a great guy! But here goes- Hope it helps.
Build your sub, complete it to where your WTC for electronics etc. finished detail, whatever are installed.Place it in a test tank( mine is the bath ) and ballast it to where it is just at neutral boyency. To where it is just under the surface, and if you touch it, it will sink a bit slowly. Now we have the sub at neutral, I use styrofoam that is 1 inch thick and comes in sheets from Home Depot. Place blocks of the foam under the keel of the sub, maybe 6" X 12" to where the sub is at surface trim and level. When you get it trimed at the surface add up how much foam it took and this is the cubic inches of a ballast tank is needed for that sub. Let me know if any questions. Good luck
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Old 12-12-2004, 11:31 AM
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Default RE: How to calculate ballast tank volume?

thanks alot blues man... i will try that method when i get to that point, not quite there yet. However a tub wont do for mine as its almost a foot and half tall lol...
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