Propulsion energy-saving devices are hull appendages such as pre-swirl ducts, stators, boss-cap fins and transom foils that recover losses around the propeller. Independently measured gains run 3-10% per device, and EEXI and CII rules let owners credit them, yet Lloyd's Register found fewer than 2% of the world fleet carries one.
What it is and how it works
Propulsion energy-saving devices (ESDs) are hull appendages that condition the flow into or out of the propeller. The Becker Mewis Duct combines a wake-equalising duct with pre-swirl fins to reduce losses at the propeller inflow, in the slipstream and in the hub vortex (TransNav). Schneekluth's wake-equalising duct, sold since 1984, evens inflow to the upper propeller disc. Wartsila's EnergoFlow is a pre-swirl stator whose connecting ring cuts the peak stress on each fin by about 40%, and its EnergoProFin is a propeller cap with hydrofoil fins that recovers rotational losses (Wartsila). MOL's PBCF is a boss-cap fin, and the Hull Vane is a submerged foil below the transom that generates forward lift on fast displacement hulls (Hull Vane).
Why it matters now
ESDs are among the few retrofits that regulators allow into the EEXI calculation. With verifier approval, the effect of an installed device can be reflected in the reference speed through post-installation sea trials, in-service measurement or dedicated model tests (ABS). CII ratings at C, D or E are the proximate commercial driver. Lloyd's Register reported in June 2025 that demand for propeller retrofits had risen four-fold, yet fewer than 2% of the global fleet carries a propulsion ESD and 8.42% of the orderbook specifies one; in the container segment, rudder bulbs, stator fins and boss cap fins each appear on at least 10% of vessels.
Key players and products
Becker Marine Systems has fitted more than 2,000 ships and keeps a running tally of 1,050 ships operating the Mewis Duct with an estimated 17.5 Mt of CO2 avoided as of 18 December 2025, a company-maintained figure (Becker). Its 2022 retrofit orders for the Mewis Duct and Twisted Fin covered more than 50 vessels from 56,000 dwt bulkers to 400,000 dwt VLOCs and were tied explicitly to EEXI and CII. MOL Techno-Trade's PBCF passed 3,500 vessels in December 2020 and holds a Guinness World Record as the best-selling energy-saving ship appendage (Cyprus Shipping News). Hull Vane has reached its 100th order and has naval references on Dutch Holland-class OPVs (Hull Vane). Wartsila's EnergoFlow retrofit on four International Seaways MR tankers in 2024 targets a 5% saving.
Figures that matter
Vendor claims and independently measured results are kept separate below. The MARIN case is the only independent full-scale verification of a pre-swirl duct found in the research (MARIN).
| Metric | Value | Source type | Source |
|---|---|---|---|
| Share of global fleet with a propulsion ESD | Fewer than 2%; 8.42% of orderbook specifies one | verified | Lloyd's Register, June 2025 |
| Propeller-retrofit demand growth | Four-fold | verified | Lloyd's Register, June 2025 |
| MARIN case: 55,000 dwt bulker, new propeller plus Mewis Duct | 7.6% power saving in ballast, 5.3% at EEXI condition | verified | MARIN Report 138 |
| Mewis Duct on full-form ships (claim) | Up to 10% | vendor | Becker Marine Systems |
| PBCF boss-cap fin (claim) | 3-5% | vendor | MOL Techno-Trade |
| Becker Twisted Fin on 7,100 TEU container ships | Up to 4% in HSVA model tests, 3% average claimed | trade-press | — |
| Hull Vane on FSV Ava J McCall (TotalEnergies) | 11% bunker saving confirmed in service | trade-press | — |
| Becker installed base | More than 2,000 ships; 1,050 running Mewis Duct, 17.5 Mt CO2 avoided (company tally, December 2025) | vendor | Becker Marine Systems |
Hull Vane results depend on hull type: an academic study of a slow container-ship hull found only about 1.8% (LJMU). Utkilen's chemical tankers reported 17% at normal speed and up to 25% in ballast, but that came from a duct plus a redesigned propeller plus a new gear ratio (Brunvoll), and independent researchers warn that manufacturers are reluctant to guarantee claimed savings and that the rudder already reduces the hub vortex (TransNav).
Developments 2023-2026
- 2024: Wartsila EnergoFlow retrofit on four International Seaways MR tankers, estimated 5% fuel saving (Hellenic Shipping News).
- Early 2025: Hull Vane on TotalEnergies' FSV Ava J McCall confirmed at 11% after months in service, 14% at 16 knots and 9% at 21 knots.
- June 2025: Lloyd's Register reports four-fold growth in propeller-retrofit demand against fleet penetration below 2%.
- December 2025: Becker's running tally reaches 1,050 Mewis Duct ships.
- Utkilen's retrofit package rated CII C for 2024 with A projected for 2025.
- No 2025-dated order volumes from Becker, MMG or Nakashima were found.
Outlook
ESDs are the highest-volume, lowest-risk efficiency lever in the propulsion list: proven single-digit gains, retrofittable in a routine docking, and creditable to EEXI. The gap between four-fold demand growth and sub-2% penetration says the market is still early. The buying rule that follows from the evidence is to specify the EEXI loading condition in the guarantee and to separate the device's contribution from any propeller redesign done at the same time.
