Knowing the how objects behave in the water is an intrinsic skill of the supervisor.

•Upthrust       =      Volume       x   Density
                     of liquid displaced                of the liquid

–Density of Seawater    =  1030 kg/m³ = 1.03 t/m³ (= 64.38 lb/ft³)

–Density of Freshwater    =  1000 kg/m³ = 1.0 t/m³  (= 62.5 lb/ft³ )

•Calculating volumes:

–Volume of cube 

•length x height x width.

–Volume of a cylinder:

• π r²h

–Volume of a sphere

•4/3 π r³

EXAMPLE

A steel pontoon is 6m x3m x 3m is floating with a draft of 1.5m in Sea Water. What is the weight if the pontoon ?

Upthrust = Volume displaced x density of seawater

Volume of water displaced = 6 x 3 x 1.5 = 27m³

Density of sea water = 1.03 t/m³

The pontoon is floating therefore weight = upthrust

Therefore 27 x 1.03 = 27.81 tonnes

Answer pontoon weighs 27.81 tonnes 

• IMCA D022-ver2 2016 DIVE SUPERVISORS MANUAL.  Section 2-43
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