The blade edge microscope of the Netherlands Ship Model Basin

For a reliable testing of propellers on cavitation the shape of the first few milimeters of the leading edge of the screw blade, corresponding with 1 to 2% of the chord, is very important. If the model propeller leaves the workshop, its leading edges neither have the right shape nor are they identical. These differences can usually not be seen by the unaided eye but in the tunnel they result in drastic differences in the cavitation inception of the blade. In order to help the workshop to produce model propellers which are dimensionally more accurate, a microscope mounted on a special rig is put into use. With the aid of this new apparatus the actual blade edge shape can be optically compared with the prescribed one. If not correct the blade edge can be filed down. Tunnel tests with a standard Wageningen B-series propeller illustrate the importance of leading edge shape on the inception and extent of cavitation.