[size=78%]So let's see If I understand correctly:[/size]

- in order to know how much water I need in Ballast Tank I have to CALCULATE the Volume of ALL is ABOVE the Water Line ? .

- how about hes weight as a Sub , I mean If weight is, let's say **7,5 KG** and the Volume of everything is on above the water line is **0,8KG** I need a ballast tank with **8,5KG** with those 10-15% ?

1. Yes, you can roughly calculate the volume above water. And that will correspond to the volume your tank must be able to take in + some more maybe 10%.

2.

Ballast and ballast tank is two separate things.

You have your submarine. If it weighs in total

**10KG** (EVERYTHING), you need

**10 Liter** of volume

below the surface to keep it afloat.

(If the submarine is

**10 Liter** and your sub only weighs

**5KG**, you will need

**5KG** of

ballast, for example lead.)

If the volume above surface is

**1 Liter** (an air filled Tower for example), your ballast tank will need to take in

**1 Liter** of water before the whole submarine is submerged (neutral buoyancy).

If the

**0,8KG** above waterline equals

**0,8 Liter**, you will need

**0,8 x 1,15 Liter** in your ballast tank.

The weight of everything "above water line" is only interesting if you know what the exact density of the material is.

Extreme example:

If you have

**0,8KG** of styrofoam above water you will need

**0,8 x 1,15 x 50 Liter** of water in your ballast tank. Because styrofoam roughly is

**0,02KG/Liter**.