Sizing method
How to size an underwater thruster for an ROV
Thruster selection guides tell you how to compare one unit against another. This is the step before that: working out how much thrust the vehicle needs, and so how many units to compare in the first place.
Start from drag
The question "how big a thruster do I need" has no answer until you have decided how fast the vehicle must move and how much water it has to push out of the way to do it. Everything else follows from that.
Hydrodynamic drag on a bluff body is F = ½ × ρ × Cd × A × v²: half the water density, times a drag coefficient, times the frontal area presented in the direction of travel, times velocity squared. At a steady speed, thrust equals drag, so the drag at your target speed is the thrust you need before margin.
Two of those terms are usually estimated. Cd for an open-frame inspection ROV is typically between 0.8 and 1.2; a boxy frame with exposed cross-members sits at the top of that band, a partially faired vehicle below it. Frontal area is the projected area of the whole vehicle, including thrusters, floats and anything bolted to the outside. It is much larger than the cross-section of the main housing.
The square law is the part that catches people out
Drag rises with the square of velocity. Doubling the target speed does not require twice the thrust; it requires roughly four times. A vehicle specified for a comfortable 1 knot and later asked to hold station in a 2 knot current has a quarter of the thrust it needs.
This is why speed is the most expensive requirement on an ROV specification and the one worth interrogating hardest. A survey vehicle that needs 1.5 knots in transit and 1 knot to hold station is a very different machine from one specified at 3 knots because the number sounded safe.
It also means a modest speed reduction buys a great deal. Lowering a requirement from 2 knots to 1.5 cuts the drag, and with it the thrust, current and battery, by about 44%.
Vectoring angle: what reaches the direction of travel
Horizontal thrusters on an inspection-class ROV are usually mounted at an angle, so the same four units give control in surge, sway and yaw. The cost is that each contributes only its forward component: a 4 kgf thruster at 45° pushes 2.83 kgf in the direction of travel.
That is not waste. Lateral authority is what lets a vehicle hold a line against cross-current while a camera stays pointed at the structure being inspected, and in real survey work that matters more than top speed. But it has to go in the sums. Size four thrusters for a straight-line requirement, mount them at 45°, and the vehicle ends up 30% short.
Vertical thrusters for heave sit outside this calculation. Size them for how far the vehicle is out of trim and the vertical speed you want. Horizontal drag does not come into it.
Margin, and what it covers
The drag equation describes a clean vehicle moving in a straight line through still water. A working ROV is none of those things. A margin of 50% over the calculated figure is a reasonable default, and it is covering four specific things.
Current. Thrust required scales with speed through the water, not over the ground. Holding position in a 1 knot current is the same hydrodynamic problem as transiting at 1 knot.
Tether drag. On a working ROV this is frequently the largest single term, and it grows with deployed length and with current across the tether. A long tether in a cross-current can produce more drag than the vehicle itself.
Fouling and condition. Marine growth, a damaged propeller or a partially blocked duct all reduce delivered thrust over a season.
Voltage. A thruster quoted at 4 kgf produces that at its rated voltage. Under load at the end of a long tether, the thruster sees less voltage than the surface supply. That is a cable sizing problem, but it shows up as missing thrust.
Working it through
For a vehicle with 0.25 m² frontal area and Cd of 1.0, targeting 2 knots in seawater: v = 1.03 m/s, so F = 0.5 × 1025 × 1.0 × 0.25 × 1.03² ≈ 136 N, or about 13.8 kgf of drag. With 50% margin that is 20.7 kgf. At 4 kgf per thruster mounted at 45°, each gives 2.83 kgf, so the vehicle needs eight horizontal units, or four larger ones.
That last observation is usually where the design converges. Eight small thrusters is a lot of penetrators, a lot of ESCs and a lot of frame; four units of roughly twice the thrust does the same job with half the plumbing. The calculation is most useful for telling you which size of thruster to look at. Fix the exact count later.
Run the numbers. The method above is built into our free ROV thruster sizing calculator, which lists its assumptions and what it leaves out.
Sources & basis
What this is based on.
- Drag relation F = ½ρCdAv², from standard bluff-body hydrodynamics. Density taken as 1025 kg/m³ for seawater and 1000 kg/m³ for fresh water; gravity 9.80665 m/s².
- Thrust, weight and depth figures for the Blu-Thrust range as listed in the Vebix Automation shop.
Published 16 August 2026. Last revised 16 August 2026. Corrections to sales@
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