MAN B&W Controllable pitch propeller. Optimizing propeller equipment. Optimizing the complete propulsion plant
The design of a propeller for a vessel can be categorized in two parts:
- optimizing the complete propulsion plant
- hydrodynamic design of propeller blades.
Optimizing the complete propulsion plant
The design of the propeller, giving regard to the main variables such as diameter, speed, area ratio etc, is determined by the requirements for maximum efficiency and minimum vibrations and noise levels.
The chosen diameter should be as large as the hull can accommodate, allowing the propeller speed to be selected according to optimum efficiency. The optimum propeller speed corresponding to the chosen diameter can be found in fig. 16 for a given reference condition (ship speed 12 knots and wake fraction 0.25).
For ships often sailing in ballast condition, demands of fully immersed propellers may cause limitations in propeller diameter. This aspect must be considered in each individual case.
To reduce emitted pressure impulses and vibrations from the propeller to the hull, MAN B&W Alpha recommends a minimum tip clearance.
Fig. 16. Optimum propeller diameter
The lower values can be used for ships with slender aft body and favourable inflow conditions whereas full after body ships with large variations in wake field require the upper values to be used.
In twin screw ships the blade tip may protrude below the base line.
The operating data for the vessel is essential for optimizing the propeller successfully, therefore it is of great importance that such information is available.
To ensure that all necessary data are known by the propeller designer, the data sheets on page 27 and 28, should be completed.
For propellers operating under varying conditions (service, max or emergency speeds, alternator engaged/disengaged) the operating time spent in each mode should be given.
This will provide the propeller designer with the information necessary to design a propeller capable of delivering the highest overall efficiency.
To assist a customer in selecting the optimum propulsion system, MAN B&W Alpha are able of performing speed prognosis (fig. 17), fuel oil consumption calculations (fig. 17) and towing force calculations (fig. 17). Various additional alternatives may also be investigated (ie different gearboxes, propeller equipment, nozzles against free running propellers, varying draft and trim of vessel, etc).
Fig. 17. Speed prognosis, fuel oil consumption, tow force