BIZOMAT MAKEOS · Hydraulics
Buoyancy and Submerged Weight
Will this body float, and how much does it weigh when submerged?
Buoyant force by Archimedes’ principle, the apparent (submerged) weight of a body, whether it floats, and the fraction of its volume below the surface when it does.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Volume of the body | L | Total external volume of the body (displacement when fully submerged). |
| Mass of the body | kg | Mass of the body. |
| Fluid density | kg/m³ | Density of the fluid (fresh water about 1000, sea water about 1025 kg/m³). |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Volume of the body
- What it is
- The total outside volume of the body, equal to the volume of fluid it pushes aside when fully submerged, including any sealed hollow spaces.
- Units and format
- Enter a number in L: litres. The form accepts 0.000001 L or more. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10 L as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The buoyant force rises in proportion.
- Get it right
- 1 L = 0.001 m³. For a hollow body use the outer envelope volume, not the volume of material.
What this means: Mass of the body
- What it is
- The mass of the body in air, including anything attached to it or trapped inside it.
- Units and format
- Enter a number in kg: kilograms. The form accepts 0.000001 kg or more. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 78.5 kg as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The apparent weight rises with the body’s weight; the buoyant force does not change.
What this means: Fluid density
- What it is
- The density of the liquid or gas surrounding the body, at its working temperature.
- Units and format
- Enter a number in kg/m³: kilograms per cubic metre. The form accepts 0.000001 kg/m³ to 30000 kg/m³. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1000 kg/m³ as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The buoyant force rises in proportion.
- Get it right
- Take the value for the actual fluid and temperature from a property table; sea water and oils differ from fresh water.
Method and formulas
- Buoyant force = ρ g V (weight of fluid displaced).
- Apparent weight = m g − ρ g V.
- A body less dense than the fluid floats with ρ_body ÷ ρ_fluid of its volume submerged.
- Example: 10 L at 78.5 kg has a buoyant force of 98.067 N in 1000 kg/m³ fluid.
Limitations
- Static fluid; flow, waves and lift from motion are excluded.
- Floating stability (metacentric height) is not checked.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Find the lifting force needed for a part underwater
- Check whether a float or tank will float and how deep
- Size ballast for a submerged structure
Who it is for
- Design engineers
- Rigging supervisors
- Process engineers
What you get
- Buoyant force when fully submerged
- Apparent weight in the fluid
- Floating draught fraction
Interpreting the result
- When lifting a submerged load, the crane takes the full weight as the load clears the surface.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 3027. Topic reference; no source tables or recommended settings are reproduced.
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