make sure the the mating surfaces REALLY touch one another for the entire mating surface. Its the friction of the the two mating surfaces (definitely NOT the keyway) that transmits the torque.

Best is get some automotive valve grinding compound and LAP the surfaces to ensure proper fit.
1. 'blue' the tapered shaft then rotate the prop on the blued shaft .... wear patterns or high spots will develop on the blueing - and will show how much is to be removed with the lapping compound. To 'lap', apply compound to the taper and rotate the prop back and forth in the approximate position of the keyway being lined up.
2. DRY fit only. No anti-seize, lanocote, etc.