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High skew

MAN B&W Controllable pitch propeller. High skew

To suppress cavitation-induced pressure impulses even further, a high skew design can be supplied, fig. 20 (a). By skewing the blade it is possible to reduce the vibration level to less than 30% of an unskewed design. Because skew does not affect the propeller efficiency, it is almost standard design on vessels where low vibration levels are required.

Cavitation and its types

MAN B&W Controllable pitch propeller. Cavitation and its types

Cavitation is associated with generation of bubbles caused by a decrease in the local pressure below the prevailing saturation pressure. The low pressure can be located at different positions on the blade as well as in the trailing wake.
When water passes the surface of the propeller it will experience areas where the pressure is below the saturation pressure eventually leading to generation of air bubbles. Further down stream the bubbles will enter a higher pressure region where the bubbles will collapse and cause noise and vibration to occur, in particular if the collapse of bubbles takes place on the hull surface.

Optimizing propeller equipment. Hydrodynamic design of propeller blades

MAN B&W Controllable pitch propeller. Optimizing propeller equipment. Hydrodynamic design of propeller blades

The propeller blades are computer designed, based on advanced hydrodynamic theories, practical experience and model tests at various hydrodynamic institutes.
The blades are designed specially for each hull and according to the operating conditions of the vessel.
High propulsion efficiency, suppressed noise levels and vibration behaviour are the prime design objectives.
Propeller efficiency is mainly determined by diameter and the corresponding optimum speed. To a lesser, but still important degree, the blade area, the pitch and thickness distribution also have an affect on the overall efficiency.
Blade area is selected according to requirements for minimum cavitation, noise and vibration levels.
To reduce the extent of cavitation on the blades even further, the pitch distribution is often reduced at the hub and tip.
Care must be taken not to make excessive pitch reduction which will effect the efficiency.
Thickness distribution is chosen according to the requirements of the Classification Societies for unskewed propellers.

Propeller blade manufacturing and materials

MAN B&W Controllable pitch propeller. Propeller blade manufacturing and materials

The international standard organization has introduced a series of manufacturing standards in compliance with which propellers have to be manufactured (ISO 484). The accuracy class is normally selected by the customer and the table below describes the range of manufacturing categories.


Class

Manufacturing accuracy

S

Very high accuracy

I

High accuracy

II

Medium accuracy

III

Wide tolerances

Hydraulic bolts and installation

MAN B&W Controllable pitch propeller. Hydraulic bolts and installation

Hydraulic bolts
The propeller equipment can be supplied with hydraulic bolts for easy assembly and disassembly between propeller shaft line, intermediate shafts and main engine flywheel, fig. 15. 

Propeller shaft and coupling flange

MAN B&W Controllable pitch propeller. Propeller shaft and coupling flange

The tail shaft is made of forged steel normalized and stress relieved.
The tail shaft is hollow-bored, housing either a pitch control rod or piping for pitch adjustment.
Coupling flange
A two-parted coupling flange is clamped on to the propeller shaft for flange diameters up to 475 mm. For couplings with flange diameters above 475 mm a special shrink fitted mounting is used, fig. 11. High pressure oil of more than 2,000 bar is injected between the muff and the coupling flange by means of the injectors. By increasing the pressure in the annular space C, with the hydraulic pump, the muff is gradually pushed up the cone. 


Lubricating oil system. VB and VBS

MAN B&W Controllable pitch propeller. Lubricating oil system. VB and VBS

Lubricating oil system, VB
The VB propeller and stern tube have a common lubricating oil system, fig. 10.
In order to prevent sea water penetration the system is kept under static pressure by the gravity tank placed above normal load water line in accordance with the stern tube seal manufacturer’s recommendations.
Because of pumping effect in the propeller hub during pitch changes, oil is circulated through propeller equipment and oil tank. Non return valves in hub and at pitch control rod secure that the oil flow to the hub passes the stern tube journal bearings and further along the chromium steel journal to the blade actuator mechanism.

Servo oil unit - Hydra Pack

MAN B&W Controllable pitch propeller. Servo oil unit - Hydra Pack

The servo oil unit – Hydra Pack (fig. 9), consists of an oil tank with all components top mounted, to facilitate installation at yard.
Two electrically driven pumps draw oil from the oil tank through a suction filter and deliver high pressure oil to the proportional valve. One of the 2 pumps is in service during normal operation. A sudden change of manoeuvre will start up the second pump. A servo oil pressure adjusting valve ensures minimum servo oil pressure at any time hereby minimizing the electrical power consumption. Maximum system pressure is set on the safety valve.
The return oil is led back to the tank through a cooler and a paper filter. The servo oil unit is equipped with alarms according to the Classification Society as well as necessary pressure and temperature indication.

Servo oil system

MAN B&W Controllable pitch propeller. Servo oil system

The principle design of the servo oil systems for VBS (fig. 7) and for VB (fig. 8) is identical.

Mechanical design. Propeller type VB

MAN B&W Controllable pitch propeller. Mechanical design. Propeller type VB

Propeller type VB
The VB propeller equipment is normally used for minor MAN B&W Alpha four-stroke propulsion plants in which the hydraulic servo motor for controlling the propeller pitch is located inside the Alpha gearbox.
The design of the VB propeller is shown in fig. 6.

Mechanical design. Propeller type VBS

MAN B&W Controllable pitch propeller. Mechanical design. Propeller type VBS

Propeller type VBS
In the propeller equipment type VBS the hydraulic servo motor for pitch setting is an integral part of the NiAl-bronze propeller hub. The design of the four-bladed CP propeller is shown in fig 4. The propeller hub is bolted to the flanged end of the tailshaft, which is bored to accommodate the servo oil and pitch feed-back rod.

Propeller equipment

MAN B&W Controllable pitch propeller. Propeller equipment

The standard propeller equipment comprises a four-bladed CP propeller complete with shafting, stern tube, outer and inboard seals, oil distributor and coupling flange.
The location of the hydraulic servo motor for controlling the pitch will depend on size and utilization of the propeller equipment.

General description

MAN B&W Controllable pitch propeller. General description

MAN B&W Alpha have manufactured more than 6000 controllable pitch propellers of which the first was produced in 1902.
The basic design principle is well-proven, having been operated in all types of vessels including ferries, cruise and supply ships ect, many of which comply with high classification requirements.
Today the MAN B&W Alpha controllable pitch propeller equipment types VB and VBS are handling engine output up to 20,000 kW, fig. 1.



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