01 · VEHICLE
Vehicle size and mass
Estimate dry mass, buoyancy in water, payload, and anything that adds drag: cameras, manipulators, tether hardware.
ROV propulsion guide
How to choose an underwater thruster for an ROV.
You choose a thruster for the whole vehicle. Thrust, voltage, rotation, depth rating, control electronics and mounting all affect each other. Use this guide to write a specification before you pick a Blu-Sub thruster with us.
Start with the mission
Decide what the vehicle must do before choosing a model.
Inspection, research, intervention and student ROVs need different propulsion. Write down where the vehicle will work and what it will carry, then compare thrusters against that.
02 · MANOEUVRING
Thrust and layout
Decide whether the vehicle needs horizontal thrust, vertical heave, vectored thrust, or spare motors in case one fails.
03 · ENVIRONMENT
Depth and water conditions
Note the working depth, water temperature and salinity, and how likely weed, debris or silt is to reach the propeller.
Selection criteria
Six checks for a thruster shortlist.
01
Voltage system
Match the thruster to the vehicle's supply. The 12 V and 24 V models use different motors and perform differently.
02
Thrust requirement
Size the number and type of thrusters from the forward, reverse and vertical control you need. Peak thrust alone will mislead you.
03
ESC arrangement
Choose between a separate ESC inside your electronics housing and a thruster with the ESC built in, which cuts the wiring.
04
Rotation direction
On a vehicle with several thrusters, specify clockwise and anticlockwise propellers so their torque cancels out.
05
Depth rating
Check the pressure rating against the deepest point the vehicle will reach. A thruster that runs well in a test tank tells you nothing about depth.
06
Integration details
Check the mounting pattern, cable route, penetrators, connectors and guards against your frame and housings.
Blu-Sub platform comparison
Which Blu-Thrust model to start with.
| Platform | Typically used for | Question to answer |
|---|---|---|
| Blu-Thrust Z60 | Compact ROVs, diver propulsion vehicles and student prototypes. | Is there room for a separate ESC in your electronics? |
| Blu-Thrust Z60E | Compact vehicles where a built-in ESC saves wiring. | Would a built-in ESC help this vehicle? |
| Blu-Thrust Z80-170 | 24 V ROVs, USVs and underwater robots that need more thrust. | Which propeller rotation and controller setup do you need? |
| Blu-Thrust Z80 / Z80E-290 | 12 V vehicles that need high thrust. | Separate ESC or built-in ESC? |
Thruster layout
How many thrusters, and where they point.
The layout decides which ways the vehicle can move before any thruster is chosen. Each extra direction of control costs thrusters, current and space in the frame, so pick the layout from the job the vehicle does. These four are the common ones.
| Layout | What it controls | What it cannot do | Suits |
|---|---|---|---|
| Three: two horizontal, one vertical | Forward and back, turning, up and down | Move sideways, or hold position in a cross-current | Small inspection vehicles in still water, student builds |
| Four: two horizontal, two vertical | The above, plus pitch or roll, depending on where the vertical pair sits | Move sideways | Vehicles that carry a camera or sensor that has to be levelled |
| Six: four horizontal at 45°, two vertical | Forward, sideways, turning and up and down, plus roll or pitch | Control all six directions at once | Most inspection and survey ROVs, and work near structures or in current |
| Eight: four horizontal at 45°, four vertical | All six directions of movement | Nothing in this list; it keeps some control with one thruster lost | Manipulator work, heavier payloads, vehicles that must not be stranded by one failure |
Angling a thruster costs thrust in the direction it no longer faces. At 45°, each horizontal thruster puts cos 45°, about 0.71, of its thrust into forward motion, so four vectored thrusters give about 2.8 times one thruster’s forward thrust, against 2 times for two thrusters pointing straight ahead. The thruster sizing guide works this through from drag and target speed.
Speed controllers
Separate ESC or built into the thruster.
An ESC turns the supply and a control signal into the three-phase drive a brushless motor needs. Where it sits changes the housing, the wiring and what you replace when something fails.
Separate ESC, in your housing
The thruster cable carries the three motor phases. The ESC’s heat goes into your electronics housing, which has to pass it to the water, and an aluminium housing does that far better than an acrylic one. A failed ESC can be changed without touching the thruster, and you can choose the ESC and its firmware yourself.
ESC built into the thruster
The thruster takes DC power and a control signal. The ESC’s heat stays out of your housing, and the housing holds fewer boards and less wiring. The ESC comes as part of the thruster, so a fault in it is a thruster repair, and its control input has to match what your flight controller or motor board sends.
Either way, count the conductors each thruster brings to the housing, because every one has to pass through a penetrator or connector rated for the vehicle’s depth. The penetrator and connector guide covers that choice.
Supply voltage
The voltage sets the current, and the current sets the cable.
Power is voltage times current, so the same thruster power at twice the voltage needs half the current. Take a thruster drawing 300 W: that is 25 A at 12 V and 12.5 A at 24 V.
The current matters because every cable has resistance. Run that thruster at the end of a 5 m cable with 0.01 Ω in each metre of each conductor. The loop, out and back, is 10 m, or 0.1 Ω. At 25 A the cable drops 2.5 V, which is a fifth of a 12 V supply, and turns 62.5 W into heat. At 12.5 A it drops 1.25 V, about a twentieth of 24 V, and wastes 15.6 W. Halving the current cuts the loss in the same cable to a quarter.
That is a first approximation, because a motor on a lower voltage also draws differently. It is enough to show why tethered and larger vehicles move to higher supply voltages. A thruster’s thrust is quoted at a stated voltage and falls when the voltage at its terminals falls, so check the drop on your own cable with the cable voltage-drop calculator before comparing thrust figures.
Direction
Forward thrust, reverse thrust and propeller rotation.
Many thrusters produce less thrust in reverse than forward, because the propeller blades are shaped to work one way. Where the datasheet gives both figures, size from the direction the vehicle depends on. A vertical thruster that has to pull a slightly buoyant vehicle down is working in whichever direction that is, for the whole dive.
A spinning propeller also twists the vehicle the other way. Order clockwise and anticlockwise propellers in pairs and mount each pair symmetrically, so that the twisting cancels. With every propeller turning the same way, the vehicle yaws whenever the thrusters run together, and the controller spends thrust holding the heading.
Before the first dive
Test the thrusters on the bench and in a tank.
A datasheet figure is measured on the maker’s rig at the maker’s voltage. Measure yours on your supply before the vehicle is built round them.
- Fix the thruster to a load cell or a lever arm with a spring balance at the edge of a tank, clear of the walls and the bottom.
- Run it from the supply the vehicle will use, through the cable length the vehicle will have.
- Step the throttle in both directions and record thrust, current and the voltage at the thruster at each step.
- Hold it at the thrust you expect to use most for several minutes, and check the thruster, the ESC and the cable for heat.
- Compare the results with the datasheet at the same voltage. A large shortfall usually points to the cable, the supply or the ESC settings before the thruster.
Four mistakes we see in thruster enquiries
- Sizing from peak thrust, when the vehicle spends its time at cruise and needs margin above that.
- Mixing thrusters, housings and penetrators with different depth ratings. The vehicle is rated to its weakest part, as the 300 m propulsion guide explains.
- Leaving the tether out of the drag and the voltage drop.
- Ordering every propeller in the same rotation.
Prepare your enquiry
What to send us for a quick answer.
- Vehicle type, use case and expected operating depth
- Number of thrusters and how you plan to mount them
- Battery voltage, current available and preferred controller
- Target thrust, or the vehicle mass and payload so we can work it out
- Quantity and delivery location in India
Sources & basis
What this is based on.
- Thrust, voltage, depth rating and ESC figures for the Blu-Thrust range, as listed in the Vebix Automation shop.
- Thrust rises with the square of propeller speed and is quoted at a stated voltage, so a figure at one voltage does not hold at another.
- Layout, vectoring, voltage-drop and power figures are worked from P = VI, P = I²R and cos 45°, and the example cable is illustrative: use your own cable’s resistance.
Published 6 August 2026. Last revised 25 September 2026. Corrections to sales@
Need help choosing?
Talk to us about your ROV thrusters.
We are Blu-Sub's authorised distributor in India. We can compare the options for your vehicle and quote for the quantity you need.