Center of Buoyancy Measurement Instrument

Why Measure?

Understanding mass properties, such as center of gravity, makes it possible to design and build stable objects and to predict their performance in a medium such as air or water. With aircraft, the center of lift is the point where the sum total of all lift generated by parts, principally by wings, control surfaces, and aerodynamic fuselage parts, balances out and the aggregate direction their force will act on a craft while in an atmosphere. Similarly, the performance of a submersible marine vehicle depends on its center of buoyancy. The center of buoyancy of an object is defined as the point that would be the center of gravity of the displaced fluid, not the submersed object itself. Until now determining the center of buoyancy was primarily done using calculations or indirect measurement.

Space Electronics has devised an instrument to experimentally determine center of buoyancy to an accuracy previously unattainable. Determining the geometry of the measurement system and validating the theory has been completed with a working instrument ready for shipment to the customer. The instrument is capable of measuring multiple axes with a center of buoyancy accuracy of 0.1".

Concept of Operation

Space Electronics’ CB Measurement Instrument suspends the test article from three load cells to first determine Center of Gravity in air. Measured CG in air is accurate to ±0.1 inch. The marine vehicle is then submerged and the load cell output is recorded. Differences between the load cell forces in air and in water indicate the Center of Buoyancy.

System Design

The CB measurement instrument is designed to support the marine vehicle from above by overhead crane. This allows measurement of any object within the crane’s capacity. Also, since the load cells are above the vehicle being tested, they remain above the water surface.

Other Design Advantages

  • Mass, Center of Gravity and Center of Buoyancy are measured using the same machine and setup.
  • Portable – All that is needed is a crane and a dunk tank or other body of water.
  • Ease of handling and loading.
  • Small waves do not affect instrument stability. This instrument is suitable to use in a non-benign environment.

Technical Specifications (typical*)

CG Measurement Accuracy0.0 ± 0.1 in
CB Measurement Accuracy 0.0 ± 0.1 in
Payload Weight Capacity 1000 lb
Test Article Length130 in
Test Article WidthUp to 70 in
Test Article Diameter Up to 20 in

* Alternate dimensions and capacities are available.

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