Design walkthrough
Designing a 300-metre ROV propulsion system
Propulsion on a 300 m vehicle is four linked decisions: thrust, layout, power delivery and sealing. Take them in the wrong order and you get a vehicle that works in a test tank and disappoints at depth.
Why 300 m is a natural design point
Three hundred metres covers most port, dam, hull, aquaculture and inshore survey work, and it is the depth at which a large amount of well-proven hardware is rated: brushless marine thrusters, aluminium watertight boxes, mid-range gland penetrators. That makes it a sweet spot where a capable vehicle can be built without moving into deep-rated components and their costs.
It also means the pressure boundary is not the hard part. At 300 m in seawater the gauge pressure is about 30.2 bar. Every part on the pressure boundary must exceed that, and the vehicle is rated to the lowest of them. Parts rated to 300 m are easy to find. The hard part at this depth is power delivery down a long tether.
Step one: thrust and layout
Size the horizontal thrusters from the drag at your target speed. Browsing a catalogue comes after. For a typical inspection-class frame of around 0.25 m² frontal area and a drag coefficient near 1.0, holding 2 knots needs roughly 14 kgf of thrust before margin, and about 21 kgf with a 50% allowance for current, tether and fouling.
The usual layout is four horizontal thrusters vectored at 45°, giving surge, sway and yaw from one set, plus one or two vertical units for heave. Remember that a vectored unit gives only cos(45°), about 71%, of its rated thrust to forward motion. Four 4 kgf units at 45° deliver about 11.3 kgf forward, which is short of the 21 kgf target; either move to a larger thruster class or accept a lower speed.
This is the point to decide between non-ESC and integrated-ESC units. Integrated puts the controller in the thruster: fewer parts inside the housing, less wiring through the bulkhead, faster to commission. A thruster without a built-in ESC keeps the controller inside the housing, where it can be replaced, monitored and chosen separately. That suits a vehicle you expect to keep developing.
Step two: bus voltage, decided by the tether
This is the decision that separates a 300 m vehicle from a 30 m one. Thrusters that behave perfectly on the bench misbehave at the end of 300 m of tether, and the cause is nearly always voltage drop. The thruster is usually fine.
Loop resistance is R = ρ × 2L / A. Copper at 1.724 × 10⁻⁸ Ω·m, a 300 m one-way run and a 2.08 mm² (14 AWG) conductor gives about 4.97 Ω round trip. At 10 A that is roughly 50 V of drop, which is more than a 48 V supply has. The vehicle will not run.
There are only three levers: thicker copper, higher voltage, or less current. Thicker copper makes the tether heavy, stiff and negatively buoyant, which then costs foam. Raising the bus voltage is nearly always the right answer, because for the same delivered power, doubling voltage halves the current and quarters the loss. That is why the standard layout sends high voltage down the tether and converts it on the vehicle.
Whatever you choose, size the conductor for the stall current of all the thrusters together. Hover current is far too low a figure. The moment every unit spools up at once is when the bus sags and the controller browns out.
Step three: sealing the propulsion runs
Each thruster cable is a hole in the pressure boundary. At 300 m, aluminium-bodied gland penetrators are appropriate and widely available; stainless bodies take the same threads deeper if the design might grow.
Size the gland on the cable jacket's outer diameter, measured with callipers. The conductor size does not tell you this. This is the single most common subsea leak cause and it is entirely avoidable. Specify FKM for bulkhead seals, even where NBR is used elsewhere. They are handled more than any other seal on the vehicle, and they sit in salt water with regular solvent cleaning.
Count entries early. Six thrusters plus lights, camera, sensors and tether can exhaust an end cap quickly, and discovering it after the housing is machined is expensive. A tube coupling with its own radial ports is usually cheaper than moving up a diameter.
Step four: what propulsion does to buoyancy
Every thruster adds mass and displaced volume, and so does every metre of the heavier tether the power budget just demanded. Six thrusters at 500 g each is 3 kg of added mass before cabling, mounts and penetrators.
Put that in the buoyancy budget from the start, so it does not come as a surprise at trim. The foam that offsets it has to fit on the frame, and the deeper grades a bigger depth requirement calls for lift less per litre. A late change of depth rating can leave a frame with no room for the foam it now needs.
Propulsion, power, sealing and buoyancy are one problem. Any design that treats them as four sequential purchases produces a vehicle that needs rework.
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.
- Thrust, weight, voltage and 300 m depth ratings for the Blu-Thrust range, and penetrator body materials and ratings, as listed in the Vebix Automation shop.
- Copper resistivity 1.724 × 10⁻⁸ Ω·m at 20 °C; conductor areas per ASTM B258.
- Hydrostatic pressure at 300 m calculated as ρgh at 1025 kg/m³ and 9.80665 m/s².
Published 16 August 2026. Last revised 16 August 2026. Corrections to sales@
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