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[mac.one]

Hi all
              Is it possible to gauge what sort of lift you would get from twin 600ml ballast tanks (bladders) I.E. what weight in kilos would they lift. Cheers Julian.

[snowwolflair]

It is the difference of the weight of water they can hold minus the weight of air they can hold.  As the weight of air is too small to consider it’s the equivalent of the weight of water.

[mac.one]

Hi So what you are saying then is if i weigh 1.2l of water that would be the potential lift Julian

[dreadnought72]

Yes.

Air weighs ~1.23 kg/m³. Water ~1000 kg/m³. As snowwolflair says, the air is negligable.

Andy

[mattycoops43]

if that's too technical, 1.2L of water weighs 1200g. 

[Subculture]

You shouldn't go by weight, go by displacement (volume).

Weight works to a point if using plastics, as most plastics tend to be around the same density as water. It won't work with wood or metal which tend to be lighter or heavier than water respectively.

[Mankster]

2x600ml tanks will displace 1.2L water. So as Andy points out, if your sub is made of plastic you can 'lift' 1.2kg of plastic. If it is made of foam, you may only be able to lift 100g. And if it is made of lead you could lift closer to 12kg.

[snowwolflair]

If you are lifting superstructure out of the water water displacement no longer applies.  you are displacing air not water.  So weight applies.

[mattycoops43]

You shouldn't go by weight, go by displacement (volume).

Weight works to a point if using plastics, as most plastics tend to be around the same density as water. It won't work with wood or metal which tend to be lighter or heavier than water respectively.

Sorry, I thought the question was how much the water that is displaced would weigh, I realise I know nothing about the black art that is subs! How does it work anyway? If you have a sub that weighs 10kg on land for example, how much does it weigh in water when fully submerged?

Matt

[Subculture]

Guys please go and Google Archimedes principle. It is somewhat counter intuitive to understand I know, but I stand by what I said. I realise you can go much more in depth regarding this discussion, but you will have to trust me that if you go by weight, you will find yourself in trouble when sizing ballast tanks.

[mattycoops43]

I will happily google archimedes principal. I studied physics to A level and understand about displacement and upthrust, but where I know how a boat floats, I was asking someone who knows subs if they could explain it in a simple laymans term? Obviously not!

Matt

[hollowhornbear]

I would trust and listen to Subculture and mankster, they know their subs, trust me.

[Subculture]

Sorry, I thought the question was how much the water that is displaced would weigh, I realise I know nothing about the black art that is subs! How does it work anyway? If you have a sub that weighs 10kg on land for example, how much does it weigh in water when fully submerged?

Matt

It will weigh 10kg minus the volume of water it displaces. How much this is depends on the material it's constructed from.

[Mankster]

When a subs ballast tank is full of water it is designed to be neutrally buoyant. Buoyancy is equal to the displacement of the sub. So to be neutrally buoyant the displacement of the sub must equal its weight (when used in fresh water of course - where 1000ml of water weighs 1kg).
Now when the ballast tank is blown, the displacement of the submarine increases (because water from the pond no longer fills the tank and it is replaced with air). the sub which was submerged underwater is now Positively buoyant (displacement/buoyancy  exceeds weight) so the sub rises to the surface and breaks the water. Now the sub still weighs the same, but as the superstructure breaks the surface it no longer contributes to the buoyancy of the sub. When enough of the superstructure leaves the water, the buoyancy of the sub reduces to a point where the  displacement (when surfaced) equals the subs weight, and the sub come to rest on the surface at its designed water line. Now we know that 1 cubic meter of foam displaces the same amount of water as 1cubic meter of plastic or 1 cubic meter of lead. These 3 will all be equally buoyant but weigh different amounts. Hence as ballast tank size is related to the displacement of the sub -not its weight, it matters not what the sub is built of, only the volume. If you want to keep your ballast tank volume small, you need to keep the volume(not weight) of the superstucture that breaks the surface as small as possible. Those of you that have converted the Revel Type VII Uboat will have noted that you need a bigger ballast tank if you went for the lighter but thicker wooden deck compared to the much heavier but thinner(less volume) brass deck. So a model sub built identically with a hollow perforated hull, 1 made of Fibreglass and one made of lead will require exactly the same sized ballast tank to achieve a scale waterline when surfaced (Of course the lead hulled sub will require a lot more floation foam on the inside to overcome the extra weight of the lead).

[mattycoops43]

Thank you. Very clear!

[mac.one]

hi so displacement is volume of wtc Julian

[Subculture]

No it's not just the WTC, it's everything that submerges including the hull.

[Davy1]

Very nice description of a tricky topic, Mankster.
We really should make more sub superstructures in materials like sheet brass. They have a more scale like appearance too - they appear as a thin plate.

[Mankster]

Yes, its the same with 1:1 subs. The structures that are above the water line are all made hollow and free flooding with thin sheet. The solid/high volume water tight part of the sub always remain below the surface; otherwise the ballast tanks would need to be unfeasibly large to get the sub to dive. Of course you want to make the superstructure out of light materials as well to keep the centre of gravity low to improve stability.

[Subculture]

I think it's much easier to visualize the way buoyancy, gravity and displacement works if you think in reverse.

Lets take our imaginary model submarine. For arguments sake we will say it is made from brass (a material about eight times the density of water) and let us take for granted that we have calculated the displacement of the structure above the intended surfaced waterline is 100ml, which equates to a 100 grams of (fresh) water or approximately 800 grams of brass.

In order for our submarine to float at the right waterline, we would need to provide buoyancy to support 800 grams, including the ballast tank, and this would all be placed below the waterline. The ballast tank would equal the displacement of 100ml.

If you used weight, then you would have built an 800ml ballast tank, and most certainly your boat would submerge, albeit with just a tiny amount of water in the tank.

In practice trying to calculate the right amount of volume for your tank using mathematics and a pocket calculator is very difficult unless the boat you are modeling is of extremely simple shape. The best way to find the volume is to calculate a rough estimate, then use empirical methods and even then you should allow yourself 10-20% over to account for different water density and mineral content.