I will try your suggestions this weekend (the one blade up idea is so simple I didn´t even think of
) and tell you if there was any success. I measured the blades and they are not exactly parallel, i.e. with the blades down and seen from behind the boat they form an inverted "V", beeing the lower ends about 2cms more seperated than the upper ones. I am not shure if this is due to a twisted blade or if the rudder axis are not exactly parallel. How would one check the blades for twist or other deformations?