Azimuth thruster

01

Size the power, drive input and propeller

Wärtsilä's steerable thruster (WST) family covers 700 to 4,900 kW for offshore vessels, tugs, double-ended ferries and river vessels, with electric drive inputs or mechanical inputs including high-speed engines up to 1,800 rpm, and options for propeller, nozzle, drive layout and auxiliaries; it lists monitored propeller-shaft and steering seals and fieldbus controls. [1] Its WST-18E has an embedded permanent-magnet or asynchronous motor to reduce height, so it fits under a working deck, and is rated 1,925 kW at 1,000 rpm input with a fixed-pitch propeller of 2,200 or 2,400 mm, a nozzle optimised for bollard pull or free sailing, a straight or 8° tilted gearbox, and a weight of about 20,000–21,500 kg without motor and oil. [2]

The WST-18E page also lists VGP2013-compliant or optional zero-pollution sealing and an option for fast electric steering and fast power ramp-up for DP and station keeping. [2]

Editorial note: tell the supplier whether bollard pull or free-running speed matters more, because the nozzle and propeller are optimised for one or the other.

Give the supplier the vessel type and duty profile (transit, DP, towing), required bollard pull and speed, available power and drive (diesel mechanical, diesel-electric, battery), input speed, installation space and height, hull lines near the thruster, DP class and redundancy, ice class if any, environmental sealing requirements, classification society, and control system interface.

02

Limit thruster–hull interaction losses

In an article on retractable thrusters for offshore station keeping, Wärtsilä states that interaction losses between retractable thrusters and the hull can be as much as 15–20%, depending on hull shape and thruster location, and that its 8° downward-tilted propeller shaft, introduced in 2018, increases available thrust by 15–20% compared with non-tilted thrusters, can improve bollard pull of cable and pipe-laying vessels by 15% or more, and can save up to 10% fuel at the same power. It recommends sizing main, retractable and tunnel thrusters together early in design with redundancy for thruster failure. [3]

Editorial calculation: a thruster rated for 300 kN of thrust that loses 15–20% to hull interaction delivers about 300 × 0.85 = 255 kN to 300 × 0.80 = 240 kN in practice. Recovering that loss with a tilted shaft can avoid installing about one-fifth more power for the same DP capability. The 300 kN figure is an assumed example; ask the supplier for thrust predictions that include interaction for your hull and thruster layout.

03

Compare frame sizes across a second maker's range

SCHOTTEL, the German propulsion maker, publishes a technical data table for its RudderPropeller (SRP), a 360° steerable Z- or L-drive first built in 1950 by the company's founder Josef Becker; it states more than 17,000 units installed. The table runs from the SRP 100 at 190–225 kW with an 850 mm propeller to the SRP 800 at 4,800–6,200 kW with a 4,200 mm propeller, with fixed-pitch (FPP) propellers across the range and controllable-pitch (CPP) from the SRP 340 upward. Variants include the SRP-U, which is mounted and dismounted underwater without dry-docking, and the retractable SRP-R; both are offered with an eight-degree inclined propeller shaft, which SCHOTTEL says reduces interaction between units and with the hull. Ice class up to FSIC 1AS/PC6 is listed. [4]

SCHOTTEL states that the table is for guidance only, does not apply to ice-classified ships, and that it determines the final unit from the vessel's operating profile. [4]

Editorial comparison: Wärtsilä's WST-18E at 1,925 kW falls inside the 1,560–2,000 kW band of SCHOTTEL's SRP 430, which SCHOTTEL lists with a 2,500 mm propeller against 2,200 or 2,400 mm for the WST-18E; compare thrust data at your duty point rather than frame names. [2] [4]

04

Three thruster listings to set beside Wärtsilä and SCHOTTEL

Dutch Thrustleader Marine Propulsion B.V. lists azimuth thrusters among other thrusters, ACIR Marine lists azimuth thrusters with deck and mooring equipment, and Fountom Marine lists hydraulic azimuth thrusters with its propeller products. Ask each for power, input, propeller diameter, thrust data, classification and service network.

05

Which part sets thrust, tilt, steering and sealing

06

WST-18E data and the thrust figure to insist on

The example values come from Wärtsilä's WST-18E and WST family. Maximum thrust is left open because Wärtsilä's article says hull interaction losses can be as much as 15–20%, so ask for a thrust figure that already includes them. [3]

07

Send the vessel, duty and drive data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Vessel: [type; hull lines; space]
Duty: [transit; DP; towing; bollard pull]
Power: [kW; drive; input speed]
Propeller: [diameter; nozzle; pitch]
Layout: [main; retractable; tunnel thrusters]
Class: [society; DP class; ice class]
Controls: [DP interface]

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