A Kort nozzle accelerates flow through the propeller and can lift bollard pull by up to 30%, which makes ducted propellers standard on tugs, trawlers and icebreakers. The duct's drag erodes the gain above roughly 10-12 knots, so current nozzle development targets transit fuel economy with vendor claims of 3.5-10%.
What it is and how it works
A Kort nozzle is an accelerating duct fitted around the propeller. By speeding up the flow through the disc it raises thrust at low vessel speed, and bollard pull can rise by up to 30% versus an open propeller (SetSail; Wartsila encyclopedia). The penalty is drag: as speed builds, the parasitic drag of the duct becomes a handicap, and the advantage over an open propeller is lost at roughly 10-12 knots (NavWeaps).
Two standard profiles cover most of the market. The NSMB 19A is the general-duty duct, while the NSMB 37 profile is used on icebreakers and vessels that need high astern thrust (Becker). Fixed nozzles give very little backing control, so they usually need flanking rudders, and harbour tugs that use them sacrifice some manoeuvrability (Boat Design). On azimuth thrusters the nozzle rotates with the unit, which removes that limitation and is why nozzled thrusters dominate modern tugs.
Why it matters now
Tug and workboat operating profiles are dominated by transit and standby time rather than full-power towing, so transit economy drives fuel bills. CII pressure on workboat fleets has pushed nozzle research away from maximising bollard pull and toward CFD-optimised, compact profiles that keep pull while cutting free-running drag. That is the logic behind Wartsila's HR and High Performance lines, Schottel's VarioDuct SDV45 and SDC40, and Kongsberg's PRO-DUCT. Demand, however, is carried by the tug ordering wave rather than by nozzle innovation.
Key players and products
Wartsila sells the HR nozzle and the High Performance nozzle; Schottel the VarioDuct SDV45, which debuted on the LNG-fuelled tug Dux in 2017, and the SDC40 launched in 2019; Kongsberg the PRO-DUCT, successor to its TK nozzle; Becker and Nautican supply ducts for conventional shaftlines and conversions (Schottel). The SDC40 has a small outer diameter suited to conversions with limited installation space and was specified for US Navy YTL 815-class tugs built at Modutech.
The largest recent reference is Kongsberg's order for US-series azimuth thrusters with nozzles for 16 new ASD tugs at Med Marine in Turkey, 32 thrusters in all, three of the tugs specifying PRO-DUCT; Kongsberg also supplies US-series units to Sanmar tugs (World Ports).
Figures that matter
Every performance figure below is vendor-reported or trade-press reported; no ITTC, MARIN or HSVA comparative test of current proprietary nozzles surfaced in the research (Marine News; Baird Maritime; MarineLink; Kongsberg brochure).
| Metric | Value | Source type | Source |
|---|---|---|---|
| Bollard pull gain, nozzle versus open propeller | Up to 30% at low speed | trade-press | SetSail; Wartsila encyclopedia |
| Speed at which the nozzle advantage is lost | Roughly 10-12 knots | trade-press | NavWeaps; SetSail |
| Tug Hoku Ke'a conversion to Nautican nozzles | 88,853 lb to 132,810 lb bollard pull (about 49%) | trade-press | Marine News, July 2005 |
| Wartsila HR nozzle fuel efficiency versus standard nozzle (claim) | 6-10% higher up to 12 knots; up to 8% more thrust | vendor | Wartsila via Baird Maritime |
| Wartsila High Performance nozzle versus 19A (claim) | Up to 5% more bollard pull at the same power | vendor | Wartsila via MarineLink |
| Kongsberg PRO-DUCT versus TK nozzle (claim) | Up to 3.5% more bollard pull | vendor | Kongsberg brochure |
| Schottel VarioDuct SDV45 at transit speed (claim) | Up to 10% fuel saving | vendor | Schottel |
| Med Marine order with Kongsberg US-series thrusters | 16 ASD tugs, 32 thrusters, three with PRO-DUCT | trade-press | World Ports |
The headline 30% is relative to an open propeller at bollard condition. For a tug that already has a 19A nozzle, the realistic retrofit case is a 3.5-8% gain, and Wartsila's own estimate is 20-30% when converting from an open propeller but 6-10% when converting from a standard nozzle.
Developments 2023-2026
- No new nozzle product launch by Schottel, Kongsberg or Wartsila was found for 2024-2026; the latest confirmed launches remain the Schottel SDC40 (2019) and the Kongsberg PRO-DUCT brochure (October 2022).
- Kongsberg US-series thrusters with nozzles ordered for 16 Med Marine ASD tugs (32 units) and for Sanmar tugs, with PRO-DUCT specified on three Med Marine vessels.
- Schottel SDC40 nozzles specified for US Navy YTL 815-class yard tugs at Modutech.
- Nozzle R&D focus shifts to free-running efficiency as CII pressure reaches workboat fleets.
- Adoption counts for nozzles and current Nautican or Rice speed-nozzle products were not found.
Outlook
Ducted propellers will stay standard on anything that pulls or pushes at low speed. The commercial question is how much transit fuel a compact, CFD-optimised duct can save without giving up bollard pull, and the honest answer from the sources reviewed is a single-digit percentage over an existing nozzle. Buyers should ask for the baseline behind any claim: open propeller, 19A, or the vendor's previous profile.
