Nozzle are necessary when thrust must be at a maximunm with low hull speeds data. The tug is a good example as well as the trawler while fishing a low speed: they need nozzles. Propeller diameter is reduced by +/- 6% compared ro open water.Nozzles can be in a fixed position with a rudder placed on their aft part. Nozzle can directed: in case they work as a rudder.
