Subsea calculator
ROV buoyancy and ballast calculator
Work out whether a vehicle floats, sinks or hovers from its mass and displaced volume, and how much syntactic foam it needs to reach the trim you want.
This is an estimate for early design work. It uses the method and assumptions shown below the result. It leaves out some things a real build has to allow for (see “Assumptions and limits”) and it does not replace an engineer's sign-off. For a figure you can build to, send us the requirement.
Result
Why it matters
Track buoyancy from the start.
Every part adds mass and displaced volume, and in different proportions. A sealed aluminium housing adds a lot of both. A solid steel bracket adds mass and almost no volume. Foam adds volume and very little mass. A vehicle designed without a running total turns up at trials nose-down or unable to hold depth, and gets lead bolted on the night before.
Aim slightly positive. A few hundred grams to a couple of kilograms of positive buoyancy means that after a power or thruster failure the vehicle floats up to be recovered, and it is small enough that the thrusters are not fighting it for the whole dive.
How it is calculated
Archimedes, then foam.
The upward force is the weight of the water displaced: displaced volume × water density. Net buoyancy is that minus the vehicle's mass in air. Positive floats; negative sinks.
Foam lift is water density minus foam density. A 0.40 g/cm³ foam in seawater lifts 0.625 kg per litre, because each litre displaces 1.025 kg of water and weighs 0.4 kg itself. Denser grades rated for greater depth lift less per litre; that is the price of surviving the pressure.
Displaced volume is the outside volume of everything below the waterline: frame, housings, thrusters, foam and cables. Do not use the inside volume of the housings. Measuring it by how much water it displaces in a tank is more reliable than adding up CAD volumes, which tends to miss the tether.
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