Application of a thrust-delivering nozzle around the propeller allows 350 kW per unit disc area and even 400 W/m2 when skewed blades are applied. Although the diameter of the propeller slightly reduces because of the presence of the nozzle, more engine power can be absorbed by the propeller without suffering from cavitation. Also the delivered thrust per unit area increases by application of the nozzle and skewed blades. The required thrust however remains the same. So an increase in efficiency is obtained as the nozzle propellers deliver the required thrust at a lower power level. The gain in propulsion efficiency is about 20-25% if a naked propeller is replaced by a nozzle propeller with skewed blades. If a nozzle propeller (e.g. Kaplan type) is considered as a basis, the efficiency gain is about 6-10%.

To reach the above described propeller optimisation a new propeller and nozzle have to be designed and installed. In case this is financially viable, there are several ways to upgrade a propeller to and reduce emissions: e.g. blade straightening, pitch adjustment or replacement of the underperforming propeller by a suitable refurbished one.

Further notes to propeller replacement: It can be necessary to modify or replace the propeller of your ship for following reasons:

• replacement of engines and/or reduction gearboxes;
• different usage of the ship e.g. reduced draft or operation limited water depth;
• frequently pushing one or more barges;
• under-performance of the ship in current situation