Johnson Evinrude Outboard Motors 1956-1970 1.5-40hp repair manual., Page 9Get this manual
PROPELLERS Ventilation Ventilation is the forming of voids in the water just ahead of the propeller bladesMarine propulsion designers are constantly fighting the battle against the formation of these voids due to excessive blade tip speed and engine wearThe voids may be filled with air or water vapor, or they may actually be partial vacuumVentilation may be caused by installing piece of equipment too close to the lower unit, such as the knot indicator pickup, depth sounder, or bait tank pickupVibration Your propeller should be checked regularly to be sure all blades are in good conditionIf any of the blades become bent or nicked, this condition will set up vibrations in the drive unit and the motorIf the vibration becomes very serious it will cause loss of power, efficiency, and boat performanceIf the vibration is allowed to continue over period of time it can have damaging effect on many of the operating partsVibration in boats can never be completely eliminated, but it can be reduced by keeping all parts in good working condition and through proper maintenance and lubr icationVibration can also be reduced in some cases by increasing the number of bladesFor this reason, many racers use two-blade props and luxury cruisers four- and five-blade props installed 1-5 have