Rim-driven thrusters carry the propeller blades on a rotating ring driven by a permanent-magnet motor in the duct, so there is no gearbox, shaft or blade tip. Their decisive selling point is low underwater noise, which won Kongsberg a DNV Silent-E notation in 2024, while suppliers now scale units toward 1.6-3.5 MW.
What it is and how it works
A rim-driven thruster mounts a permanent-magnet motor in the rim of the duct, so the propeller blades are carried by a rotating ring rather than a hub. There is no gearbox, no vertical shaft and, on smaller units, no hub at all. Because the ring propeller has no blade tips it avoids tip-vortex cavitation, which Kongsberg describes as a major noise source (Kongsberg; Naval News). Larger units are not fully shaftless: Brunvoll's RDT2100 keeps a central hub support, while its smaller RDTs do without one (Brunvoll). A 2025 academic review notes that Kongsberg, Schottel, Brunvoll and Voith have all brought rim drives into commercial production and keep extending their power ranges (JMSE).
Why it matters now
Underwater radiated noise has become a purchase criterion. DNV's Silent-E and Silent-R notations are now specified for offshore wind service vessels, research vessels and yachts, and rim drives are a class-certified route to them. Compactness is the second argument, since the motor is integrated in the duct, and efficiency is third: no percentage efficiency comparison with geared thrusters was found. The main R&D direction is scaling power from the sub-megawatt class toward 1.6-3.5 MW so that rim drives can take on DP duty on larger offshore vessels.
Key players and products
Kongsberg Maritime launched commercial rim-drive thrusters in 2015 with the RD-AZ azimuth and RD-TT tunnel families, marked ten years in October 2025 with almost 100 orders, and passed 100 units delivered by January 2026 (Kongsberg; Clean Shipping International). Brunvoll, which has worked on rim drives since 2002, launched the RDT2100 in 2024 with a 2.1 m propeller and up to 1,600 kW, with first units going to an SOV project (Brunvoll). Schottel's SRT RimThruster, added in 2012 after acquiring the technology from Van der Velden, spans 200-800 kW with a retractable SRT-R up to 500 kW (Schottel). Voith's Inline Thruster has been in series production since 2008, with a 1,500 kW VIT 2300-1500 on an A2SEA wind-installation vessel (Offshore Wind.biz). The eVSP uses the same Voith motor technology.
Figures that matter
Order counts and unit ratings are vendor figures; the adjacent non-rim permanent-magnet tunnel thrusters are included because they compete for the same slots (Kongsberg).
| Metric | Value | Source type | Source |
|---|---|---|---|
| Kongsberg rim-drive units in ten years | Almost 100 orders by October 2025; more than 100 delivered by January 2026 | vendor | Kongsberg; Clean Shipping International |
| Brunvoll RDT2100 (2024) | 2.1 m propeller, up to 1,600 kW | vendor | Brunvoll |
| Schottel SRT RimThruster range | 200-800 kW in four sizes | vendor | Schottel |
| Largest Voith Inline Thruster | 1,500 kW on a wind-installation vessel | trade-press | Offshore Wind.biz |
| Kongsberg RD-AZ 3300 under study | 3,300 mm, up to 3.5 MW | trade-press | — |
| Kongsberg RD-TT range extension (August 2026) | 2,300 mm and 2,600 mm models | trade-press | — |
| Adjacent PM tunnel thrusters TT-PM 1600/2000 | 500-1,000 kW and 1,000-1,600 kW | vendor | Kongsberg |
Kongsberg's RD-AZ-2300 is claimed to bring noise below DNV Silent-R levels. The market is still small: roughly 100 Kongsberg units over a decade, with no current order counts available from Brunvoll or Schottel.
Developments 2023-2026
- 27 August 2024: Kongsberg's rim-drive azimuth thruster becomes, by its account, the first thruster to achieve DNV Silent-E, on seabed hydrophone evidence from the CSOV Norwind Hurricane and sisters (Offshore Magazine).
- 2024: Brunvoll launches the RDT2100, its largest rim-driven thruster.
- October 2025: Kongsberg marks ten years of rim-drive production with almost 100 orders.
- January 2026: Kongsberg wins two RD-AZ and two RD-TT units for DRDO/NPOL's acoustic research vessel at GRSE, India, selected for exceptionally strict noise limits (Kongsberg).
- August 2026: Kongsberg adds 2,300 mm and 2,600 mm RD-TT models for offshore DP.
- No rim-driven main propulsor on a submarine or surface combatant was found; the sourced naval angle is quiet research vessels.
Outlook
Rim drives will grow with the vessel classes that pay for silence: CSOVs and SOVs in marine-mammal sensitive waters, acoustic research ships and yachts. The 2026 RD-TT extensions and the studied 3.5 MW RD-AZ 3300 show the suppliers chasing DP tunnel and azimuth slots on larger hulls. Until an independent efficiency comparison appears, the business case should be written around noise, vibration and space rather than fuel.
