critical speeds
When aboard ship in the Navy we had posted critical speeds that we had to be avoided to be at all costs because they tended to rip the ship apart. Critical speeds and rpm's exist in most mechanical systems: airplanes, trains, aoutomobiles and boats. Thos vibrations are harmonics greated by just the perfect resonance of the engine vibrations caused by the turning of the prop the shaft, engine bed and hull. The vibrations you feel smooth out when the shaft vibrations cancell out the other harmonically balanced structures acted on. Other vibrations that you feel when turning are probably an increase in prop wash contacting the increasinly larger rudder area as you move the rudder across the wash. JB
