Prop design based on "Ship Form, Resistance and Screw Propulsion" by GS Baker, published in 1920
The best efficiency that Baker shows for a three-blade prop is 0.75, and this would be for a very large slow revving prop in a small fast ship. To get an efficiency as high as this typically needs shaft speeds less than 60 rpm, which is possible, and screw diameters in excess of 28 feet, which is not practical for ships using normal harbors.
He includes a worked example for a twin-shafted ship with 16000 HP at the propellers. The operating speed is assumed to be 20.6 knots. Coincidentally this is roughly the power per prop and speed of the first generation Dreadnoughts. The optimum prop for a given shaft speed is as shown in the following table. He starts with a 0.5 Disc Area Ratio (DAR) (that is, the blades cover half the disc), which cavitates at 200 rpm. To suppress this he switches to a 0.8 DAR design, which is less efficient, but exerts a lower pressure on the water, and so tends to cavitate less.
Optimum prop at 0.5 Disc Area Ratio
Propshaft (RPM) Diameter (ft) Pitch (ft) Efficiency (%)
100 20.6 24.7 72
120 20.6 22.7 70
140 18.0 18.0 69
160 16.5 16.5 68
180 16.0 14.4 66
Optimum prop at 0.8 Disc Area Ratio
Propshaft (RPM) Diameter (ft) Pitch (ft) Efficiency (%)
180 15 15 62
200 14.1 14.1 59
220 13.4 13.4 58
240 12.8 12.8 56
260 12.0 10.8 55
Increasing the DAR lost about 4% and the efficiency drops by 3% for each 20% increase in speed. Not very surprisingly, the slower the prop, the bigger it is, and the longer the pitch. As this example reinforces, it is important to fit a large slow moving prop, if that is practical.
Cheers
Greg Locock
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