With some practical assumptions, the critical one of which is a propeller weight of 2.2 lbs for an 8.0-inch diameter marine propeller, the critical speed for a 5/8-inch diameter shaft, 28 inches long is about 580 rpm.  With that critical speed, the mixer should not be operated over 490 rpm.  It the prop is heavier than 2.2 lbs, the critical speed will be reduced, along with the maximum operating speed.  With considerable care, operating the mixer in air (no liquid in the vessel) and gradually increasing the operating speed should identify a speed at which vibrations begin to occur.  Operating at or above that speed should be avoided.