Subsea calculator
ROV thruster sizing calculator
Estimate how much thrust a vehicle needs to hold a given speed, and how many thrusters that takes once you allow for their mounting angle and a working margin.
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
Drag rises with the square of speed.
The number of thrusters shapes the frame, the battery, the wiring and the buoyancy budget. It is one of the first decisions on a build and one of the most expensive to change later, so a rough figure early is worth more than an exact one late.
Most first builds get the square law wrong. Doubling the target speed takes about four times the thrust. A vehicle specified for 1 knot and later asked to work at 2 knots in a current has a quarter of the thrust it needs.
How it is calculated
Bluff-body drag and vectoring.
Drag on a blunt body in water is F = ½ × ρ × Cd × A × v². To hold a steady speed in still water, thrust has to equal that drag. The margin covers what the equation leaves out: current, tether drag, marine growth, and a vehicle that is seldom pointed straight where it is going.
A thruster mounted at an angle only contributes its forward component, so a 4 kgf unit at 45° gives 2.83 kgf forward. The angle buys control in sway and yaw, which matters more than top speed in survey work, but the loss still has to go in the sum.
This covers horizontal thrust only. Vertical thrusters for heave and holding depth come on top, and are usually sized for how far the vehicle is out of trim.
Related
Other calculators and guides.
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