Propeller and Engine

Check of blade area

If the propeller is very highly loaded the pressure on the suction side may get so low that the water evaporates, ie bubbles of vapour are created. If these are large the thrust is influenced and noise and erosion of the propeller blades occur. This is called cavitation. To avoid this problem the area of the blades carrying the thrust must be large enough. A simple check may be made using a method proposed by Burrill. We will describe this in the following discussion.

Fig 9.16 Engine and propeller power

Fig 9.17 Burrill diagram po [kW]

Rough weather

Design point optima/ propeller

Max engine output

7 knots

7 knots

S/Max rps

Calm weather

S/Max rps

Calm weather

O 10 1500

15 2250

20 3000

25 Propeller rps 3750 Engine rpm

Pressure at propeiier shaft: = Patm+

: Atmospheric pressure h : Depth at propeller shaft

Cavitation number :

PP~PV

Vaporization pressure

Risk of cavitation

Risk of cavitation

No cavitation

Blade area

Ad : Developed blade area Aq : Area of propeller disk

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