Strength requirements on propellers in backing operations are hardly defined but they are found necessary for highly skewed propellers. The present paper illustrates how the MARIN design procedure copes with this aspect.

Relevant analysis conditions for backing operation and allowable stress levels are discussed. The relevance of strength calculations is demonstrated on the four propellers with systematically varied skew-back angles of 0, 30, 60, and 90 degrees. Finite element stress calculations lead to the conclusion that in backing operation the highest stresses occur at the trailing edge beyond the 0.8 radius and that the maximum stress varies almost linearly with the skew angle. Maximum astern power instead of the minimum required revolution rate, being 70% of the head value, could lead to such stress levels that tip bending happens.

For the derivation of the lift distribution use is the made of the lifting surface theory. Its applicability for backing operation is verified by comparing the calculated open water characteristics with measurements. Boundary layer observations by means of paint tests give information on the extent of separation. For the 90-degree skew propeller, this leads to behavior which is difficult to interpret and with unknown consequences for the detailed lift contribution.

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