Welcome

 

Introduction

 

Abstract

 

In this project we were to design a variable pitch propeller to be used in the marine market with specific initial statements of requirements to fulfil the market requirements both financially and practically. The propeller designed it to be used for small craft and inshore fishing boats. The design must achieve high reliability, robustness, easy building and assembly, and must easy to be maintained. However, unit cost is one of the main objectives for this design.

 

Requirements

 

The Following were the requirements:

 

1. A work boat is to be propeller driven through a 2:1 reduction gearbox, the design speed in still water is 10 knots.

2. The propeller drive shaft is enclosed in a stern-tube and extends forward into the boat from the rear of the keel, which is bonded into the hull just forward of the propeller.

3. The propeller shaft is supported in two bearings at either end of the stern tube, 2.0m apart. One bearing capable of taking the thrust from the propeller is mounted in the stern-tube at the aft (rear) of the boat and the second bearing is a cylindrical roller bearing mounted at the front.

4. The engine chosen for the first application is a twin cylinder diesel which is capable of delivering 20.8kW at 3600 rpm. The maximum torque is developed at 1700rpm which can then be considered essentially constant over the speed range 1700-3600rpm, the Specific Fuel Consumption is 0.264 kg/kWh over the same range.

5. There is an electronic limiter on the engine which can be adjusted to any speed less than the maximum rating of the engine. This will be used to ensure the propeller shaft speed is limited to 1300 rpm +/- 5% with rated power of 15 kW.

6. In the event of a fouled propeller, with the throttle open, then the engine will stall and the torque will reach 150% of its rated value. The propeller shaft shall not suffer permanent set or damage under these conditions.

 

7. The overall diameter of the propeller shall not exceed 500 mm. In the interests of efficiency, the diameter of the hub should be as small as is consistent with providing a reliable working system: typical propeller tip to hub diameter ratios are around 3:1.

8. In order to obviate the need to provide a reversing gear and a clutch, the pitch of the propeller blades is variable. Thus by changing the angle of the propeller blades the boat can be made to move forwards, backwards or remain stationary.

9. Since the boat is not required to travel quickly in reverse analysis has shown that the optimum angles of attack for the propeller are from 30 degrees pitch for forward movement to 15 degrees pitch for reverse travel.

10. The pitch control is to be actuated hydraulically using an existing hydraulic source from a gear pump of 100 bars. It is expected that the forward/reverse selection will be connected with throttle operation so that the craft may be controlled by a single lever, which does not form part of this design, see Fig 2

11. The hydraulic fluid is directed to the propeller by two concentric tubes. One for forward pitch and the other for reverse pitch, there is design freedom of choice which does which, Fig 3. These tubes rotate with the propeller shaft. And are supported along their length by spiders so you do not need to worry about their whirling behavior.

12. The inner tube has an additional function. It is connected at the servo-valve to indicate the pitch sense and magnitude. Therefore it must be capable of axial movement and be connected to the hydraulic piston in the pitch change mechanism in the propeller, only this part needs be designed, ignore the servo connection.

13. All parts shall be designed for a maximum life of 106 cycles.

14. The first whirling speed of the shaft shall be at least 120% of the maximum speed. (Consider only the part of the shaft between the bearings, ignore the overhung mass)

15. When operating in shallow water the propeller will stir up mud and sand from the bottom. Reliability is of paramount importance and it must not be possible for the propeller to be lost nor for the bearings to be damaged by contaminants.

16. The annual production will be 10000 units.

17. The propeller arrangement shall remain water and oil tight at all conditions; all joints shall have suitable sealing.

18. As the unit is directly in contact with sea water consideration shall be given for corrosion protection.