Buoyancy Calculator - Net Buoyancy and Buoyancy

Hi, I’m new here. Looking at the buoyancy calculator link on this page, I don’t quite understand the different between the two values. Can someone please explain to me on which value should I take into account when I want to calculate my ROV weight in water.

Hi @kudo-liar, welcome to the forum :slight_smile:

‘Net’ describes an amount that remains after all deductions have been made. The buoyancy of an object is a force (pushing the object upwards in reaction to the weight of some mass it has displaced), and the relevant ‘deduction’ being considered here is the weight of the object itself.

Net buoyancy is the negative of “weight in water” (which is net weight).

  • If an object has a positive net buoyancy (given a particular water density) then it will float upwards in that water, so its “weight in water” is considered to be a negative value.
  • If the net buoyancy is negative then the object will sink (unless it has additional forces pushing or pulling it upwards, like thrusters or a tensioned rope/tether).
  • If the net buoyancy is zero then the object is not inclined to float upwards or sink downwards - it just stays where it is until affected by some additional force (e.g. from water currents, thrusters, etc).

For calculations, these equivalencies may be useful:

\begin{align} buoyancy_\text{object} &= weight_\text{water displaced by object}\\ weight_\text{net (object in water)} &= weight_\text{object} - buoyancy_\text{object}\\ buoyancy_\text{net} &= buoyancy_\text{total} - weight_\text{object} = -weight_{net}\\ \end{align}

While both weight and buoyancy are forces (which should technically be expressed in units of force, like Newtons), it is quite common to express them both in units of mass (e.g. kilograms) and ignore minor variations in gravity.

Hi Eliot,

Thanks for your brief reply.
I want to be sure that my assumption here is correct.
Say I have a buoyancy block with following detail:
Weight in air = 234 kg
Density of the block = 415 kg/m3
Density of sea water= 1025 kg/m3
Volume of block = 0.564 m3

So, from all above info and formula you have given, I will get following answer:
buoyancyobject = 0.564 x 1025 = 578.1 kg
weightnet(object in water) = 234 - 578.1 = -344.1 kg
buoyancynet = -weightnet = 344.1 kg

So when my other total ROV structures weight in water (exclude the buoyancy) is (let say) approximately 300 kg, the weight of whole ROV assembly in water is:
ROV weight in water = 300 - 344.1 = -44.1 kg (floating)

Am I correct to say like that?

If the net weight of an object in water is equivalent to 300kg, and you attach to it a piece of foam with a buoyancy equivalent to 344.1 kg in the water being operated in, then yes, the net weight of the combined assembly in that water is -44.1 kg, which would cause it to float at the surface (or float up to the surface if it’s first propelled or pushed below the surface).

If the mass of an object is 300kg then its equivalent (net) weight in water will be less once its buoyancy is accounted for, so if you then add on some foam with a buoyancy of 344.1 kg the total assembly would have

\begin{align} weight_\text{net (assembly in water)}&=\left(300 - \left(V_\text{object}\cdot \rho_\text{water}\right)\right)-344.1 \end{align}

Hi Eliot, great! Really appreciate your help.

1 Like