In short:Subsea motion components must survive pressure first and seawater second: either keep them dry in a one-atmosphere housing rated for the depth, or fill them with oil and pressure-compensate them so they work at any depth. Choose that architecture per component before you choose a motor, slip ring or encoder.
Typical requirements
| Depth rating | 850 m one-atmosphere housings; to 6,000 m and full ocean depth when oil-filled |
|---|---|
| Hydrostatic pressure (rule of thumb) | About 1 bar per 10 m of seawater (600 bar at 6,000 m) |
| Thruster bus voltage | 12–20 VDC (small vehicles) to 48–330 VDC (larger electric vehicles) |
| Thruster input power | About 0.4 kW to 1 kW per thruster on small and mid-size electric vehicles |
| Winch slip ring rating | 1 kV to 7.2 kV AC; 7–125 A per pass; 5–100 rpm |
| FORJ channels | 1 (standalone FORJ) to 52 fibre passes (large hybrid winch units) |
| Housing materials | Hard-anodised 6061-T6 aluminium, 316 stainless steel, 6Al4V titanium, engineering polymers |
Subsea vehicles range from small tethered inspection drones to work-class ROVs and survey AUVs, but they share one design problem: water pressure rises by roughly 1 bar for every 10 m of depth, and it never lets up. Every motor, actuator, encoder and slip ring on the vehicle either has to keep the sea out at that pressure or let a compatible fluid in so that pressure stops mattering. The rest of the specification follows from that choice.
One-atmosphere housing vs pressure-compensated: the first decision
There are three ways to put a motor underwater.
| Approach | How it works | Strengths | Limits |
|---|---|---|---|
| Wet (flooded) motor | Stator potted in epoxy, rotor runs in seawater, water-lubricated bearings | Simple, light, cheap; no seals to fail | Shallow and small vehicles; corrosion and fouling on the rotor |
| One-atmosphere housing | Motor and electronics sealed in a dry pressure vessel, dynamic shaft seal or magnetic coupling | Standard industrial parts inside; electronics stay local | Housing mass rises with depth; seal is the failure point |
| Oil-filled, pressure-compensated | Cavity filled with dielectric oil, compensator keeps oil slightly above ambient | Same unit works from surface to full ocean depth | Oil drag losses, oil maintenance, electronics potted or remote |
Real products show the split. The Blue Robotics T200, common on small ROVs, has a fully potted stator and water-lubricated bearings, runs on 7–20 V and gives 5.25 kgf forward thrust at 16 V for about 390 W. Tecnadyne’s Model 580 uses a one-atmosphere housing rated to 850 m as standard (1,000 m and 1,500 m options), with a magnetic propeller coupling so no shaft seal sees the full pressure. For full ocean depth the same thruster is oil-filled and pressure-balanced, and its electronics move to a remote bottle. maxon’s oil-filled BLDC drives and thrusters are rated pressure-tolerant to 6,000 m, with one fitting for oil filling and one for the compensator.
Rule of thumb: below a few hundred metres, or when you need many actuators, compensation usually wins on mass. A one-atmosphere housing that is thin enough to be light at 100 m becomes heavy at 3,000 m.
How to specify thrusters and propulsion motors
Thrusters are outrunner or inrunner brushless DC motors, normally sensorless on small vehicles and with Hall or encoder feedback where low-speed thrust control matters (dynamic positioning, station-keeping in current). Give a supplier:
- Bollard thrust forward and reverse. Reverse is usually much lower: the T200 gives 4.1 kgf reverse against 5.25 kgf forward at 16 V, and the Tecnadyne 580 gives 10 kgf reverse against 17 kgf forward.
- Bus voltage at the vehicle, not at the surface. Long tethers drop voltage, which is why larger vehicles send power down at high voltage and convert locally. Tecnadyne offers nominal windings from 48 V to 330 VDC.
- Duty profile: transit at full thrust, hover at low thrust, and how long each lasts. Oil-filled motors lose some efficiency to oil shear at speed, and that loss is heat you have to remove.
- Mass in air and in water. Buoyancy budget is as tight as power budget on an AUV.
Size motors with our servo and stepper motor hub and compare drive options on servo drives. Low-voltage drives that accept 24–90 VDC suit most battery-powered AUVs.
Slip rings and FORJs for tethers, winches and rotating sensors
Every tethered system has at least one rotating interface at the surface: the launch and recovery winch or tether management system drum. The drum turns slowly, but it must pass high-voltage power plus data while it does.
Moog’s published winch slip ring range gives a sense of scale: models from 7 A per pass at 1 kV (the compact Model 180, 72.4 mm diameter, 100 rpm) up to 125 A per pass at 7.2 kV AC (Model 129, 50 rpm). Hybrid versions add a fibre optic rotary joint; the Model 180 can carry up to 12 singlemode and 12 multimode fibres. For single-fibre tethers, a standalone FORJ such as the FO197 is often enough. See the wider winch slip ring range.
On the vehicle itself, slip rings appear in continuously rotating sonar heads, pan-and-tilt camera units and rotating tool skids. These are normally oil-filled and compensated with the rest of the vehicle’s oil system. Moog’s downhole slip rings show that oil-filled designs can handle extreme pressure; its product guide rates the 303 series to 1,700 bar and 230 °C.
What to state: voltage per pass, current per pass and total, circuit count split between power and signal, data type (Ethernet, RS-485, coax video), fibre count and type, speed, oil or air environment, and connector standard. Read how to specify a slip ring, the marine slip ring and FORJ pages, and the subsea application page.
Actuators for manipulators, tooling and control surfaces
Work-class ROV manipulators have traditionally been hydraulic, because hydraulics give high force density and the fluid is already pressure-balanced. Electric manipulators and tooling are growing on electric vehicles, where removing the hydraulic power unit saves mass and leak risk. Typical electric subsea actuators combine a BLDC motor, planetary gearhead or screw, and an output encoder in one compensated housing; maxon describes using its screw drives in this way for grippers, manipulators and fin actuation.
For AUV fins and buoyancy-engine pistons, the key figures are holding torque or force with power off, backdrivability (do you want the fin to hold under hydrodynamic load?), and stroke. Ball screws and roller screws both work in oil; see ball screw vs roller screw and how to size a linear actuator. If you are replacing hydraulic tooling, our hydraulic to electric actuator guide covers the force and duty questions.
Encoders and feedback underwater
Optical encoders dislike oil films and contamination. Inside compensated actuators, non-contact magnetic, inductive and capacitive encoders are the safer choice. Inductive absolute encoders such as the Novanta (Celera Motion) Mini Ultra IncOder and capacitive types such as the Netzer VLP have no optical path to foul. Check that the electronics are rated for the oil, that any potting compound is compatible with it, and that the interface (SSI, BiSS-C) survives the cable run to the electronics bottle. More on choosing between technologies in absolute vs incremental encoders and the encoders hub.
Environment and compliance
- IP ratings do not equal depth ratings. IEC 60529 leaves IPX8 conditions to manufacturer–user agreement, so treat IP68 as a minimum for splash zones and deck equipment, not as a depth rating. Ask for the rated depth and the test pressure.
- Materials. Tecnadyne’s datasheet lists hard-anodised 6061-T6 aluminium, 316 stainless and 6Al4V titanium housings. Keep dissimilar metals isolated and fasteners compatible.
- Subsea production interfaces. If the vehicle operates tools on subsea production equipment, API RP 17H covers the interface design.
- Deck equipment. Launch and recovery winches are machinery; IEC 60204-1 applies to their electrical equipment.
Common pitfalls
- Specifying depth without a test margin. Agree the proof test pressure, not only the operating depth.
- Forgetting the compensator volume. Oil contracts with cold and pressure. The compensator must cover that change plus any slow leak over the service interval.
- Mixing oil-filled and dry parts on one oil circuit without checking that the oil suits every elastomer, adhesive and cable jacket in contact.
- Under-rating the tether voltage drop. Thrust collapses if the drive sees much less voltage than the motor was wound for.
- Treating the surface winch as an afterthought. The slip ring and FORJ on the drum are single points of failure for the whole system.
If you share your depth, voltage, thrust and tether details, our specialist engineering and supply partners can propose a matched set of motors, actuators, slip rings and feedback.
Standards to have in view
- IEC 60529 (IP codes)
- API RP 17H (remotely operated tools and interfaces on subsea production systems)
- IEC 60204-1 (electrical equipment of machines, for launch and recovery winches)
Which standards apply depends on the end product and its market. Your engineer will confirm what the component itself needs to meet.
Frequently asked questions
Is an IP68 motor suitable for an ROV?
Usually not on its own. IEC 60529 leaves the IPX8 immersion depth and duration to agreement between manufacturer and user, so an IP68 rating may mean a few metres for a few hours. Seawater at 300 m pushes on seals at about 30 bar continuously. For real depth you need either a pressure housing designed and tested for the rated depth plus a safety margin, or an oil-filled, pressure-compensated motor.
What is a pressure-compensated, oil-filled motor?
The motor cavity is filled with a dielectric oil and connected to a compensator, a bladder, bellows or spring-loaded piston that lets the oil pressure follow the surrounding water pressure with a small positive offset. Because the pressure difference across the seals stays near zero, the same unit works at 100 m or 6,000 m. The trade-offs are viscous drag losses in the oil, oil maintenance and keeping electronics either potted or remote.
Do I need a fibre optic rotary joint on an ROV winch?
If the umbilical or tether carries optical fibre, yes. Modern work-class and many inspection ROVs send video, sonar and control data over fibre, and a winch drum must rotate while staying connected. A FORJ passes one or more fibres across the rotation, and hybrid units combine it with the electrical slip ring for the power conductors. State fibre type, channel count, insertion loss budget and wavelength.
How do I protect subsea actuators from corrosion?
Avoid mixing dissimilar metals in contact, or isolate them, because seawater is an effective electrolyte. Common choices are hard-anodised aluminium for light housings, 316 stainless for general parts and titanium where weight, depth and life justify the cost. Add sacrificial anodes where the vehicle design calls for them, use compatible fasteners, and specify the seal elastomer against the oil and any hydraulic fluid used.
Which encoder works best inside an oil-filled actuator?
Non-contact magnetic, inductive and capacitive encoders cope well with oil because nothing optical has to stay clean. Inductive designs tolerate contamination and some misalignment, and sealed versions exist. Check that the encoder electronics are rated for the oil and pressure, or are potted, and that the output protocol survives the cable length back to the electronics bottle.
What information do I need to send for a subsea motion enquiry?
Send the maximum operating depth and test depth, whether the part sits in a housing or is compensated, the oil type, water temperature range, duty cycle, supply voltage, required thrust, torque or force, feedback and interface, mass limits in air and in water, connector standard and any approval body involved. For winches, add tether construction, fibre count, voltage, current and drum speed.
Sources
- Blue Robotics: T200 thruster specifications
- Tecnadyne: Model 580 DC brushless thruster datasheet
- Eureka: compact underwater DC drives for ROVs and AUVs (maxon)
- maxon: subsea pressure-compensated actuators
- Moog: winch slip rings model comparison
- Moog: slip ring product guide (marine, hybrid FORJ and oil-filled downhole units)
- API RP 17H:2026 remotely operated tools and interfaces on subsea production systems
Figures are summarised from the sources above and from manufacturers' published data on the date shown. Always confirm against the current datasheet for your exact part.