The ends of the Kiwiprop propeller blades are strongly beveled for two reasons.
First of all, the profile of the blade section must pass through a flow and activate the feathering even when the propeller is mounted on a shaft that forms an angle not only with the waterline and even more so with the orientation of the water nets parallel to the rear slender lines of the hull that invariably rise in the stern area near the propeller.
The beveled ends make it possible to maintain the stability of the angle of attack of each blade according to their respective position in relation to the water nets whose flow is disturbed by the angle of the propeller shaft and the inclination of the rear slenderness. In addition, since the ratio between the diameter of a propeller and the power required is about 1/5, it is clear that most of the thrust is produced by the ends.
This principle has long been known to aeronautical engineers – it is enough to observe the shape of the most recent helicopter blades or propellers of the new turboprops of short-haul aircraft, which gives a good indication of the current trend.
It is therefore particularly important to give the importance it deserves to the profile of the section of the blades at their end. In propulsion unlike the flag position, the shape of the tip of the blades corresponds to what is usually seen on the best propellers currently available on the market. According to defined criteria, the shape of the profile is comparable for aeronautical and marine applications.
The effect of these design constraints is that a diameter at the ends of the blades must be greater than that of comparable propellers.
By taking the measurement in the middle of the distance between the shorter leading edge and the longer trailing edge, a “medium” diameter is obtained that is roughly equivalent to that of competing propellers.
Our recommendations are therefore based on what we call the “nominal diameter” which is about 13 mm less than the maximum outer diameter measured according to the available space and generally about 13 mm higher than the recommendations applying to competing models.
An appropriate warning is clearly indicated in the quote form. If a simpler system existed, we would use it without hesitation, but when it comes to taking into account the beveled end, these measurement problems are unfortunately inevitable.