The discussion so far has referred to a situation where there is only a single flex coupling between motor and prop shafts, in which case it is highly desirable to minimise any mis-alignment. However I have one model where such a drive arrangement was not possible, and I ended up having to use double HUCO typ.e couplings and an intermediate shaft between the motor and prop shafts, with offsets both vertically and horizontally. In such drive arrangements vibration is minimised as long as both the motor shaft and prop shaft are parallel, even if not in the same plane. But if there is more angle in one coupling than the other then vibration may be accentuated, with undesirable results.
