Cylindrical duct around a propeller that increases bollard pull at low speed; common on tugs, fishing vessels, and OSVs.
Regulatory detail & full definition
Cylindrical or aerofoil-section duct fitted around a propeller to increase thrust at low advance speed by accelerating the inflow and reducing tip losses. The nozzle adds 20–35% bollard pull at zero ahead speed compared with an open propeller of the same diameter, which is why it is fitted to tugs, fishing vessels, OSVs, and trawlers operating at the high-thrust low-speed end of their envelope. At free-running speed the nozzle adds drag and reduces top speed, the operational trade-off taken when bollard pull is the main requirement.
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