1992 Johnson Evinrude "EN" 40 thru 55 Service Manual, P/N 508143, Page 32Get this manual
Weather, Altitude, and Engine Performance The horsepower ratings of all engines are those that would be obtained under standard conditionsHowever, changes in weather and altitude will affect the performance of any outboardSome factors that will decrease the output of an engine are: Low barometric pressure High temperature High humidity High altitude
Performance can decrease noticeably when an engine is run on hot, humid daySometimes the weather or altitude alone can cause the engine RPM to drop below the recommended operating rangeIn such case, lower pitch propeller can be used to bring RPM back into the operating rangeIn any case, top boat speed will not be as high as on cool, dry day