1983 Yamaha 30EN Outboards Service Manual, Page 13Get this manual
PROPELLER AND PROPELLER SELECTION The performance of propeller has direct effect on that of both the boat and outboard motorIt is safe to say that the propeller greatly affects the boat speed, acceleration, engine life, fuel economy, and even boating and steering qualitiesTherefore, the selection of the right propeller for the user's boat is of vital significance Movement of the propeller in the water: As the propeller turns in the direction of the arrow, it pushes water backwards which in turn develops reaction force that thrusts the boat forwardThat is, the propeller displaces water in an astern direction and, at the same time, on the forward surface (blade front) of the propeller, negative pressure is produced, thus sucking waterIn this way, the propeller produces the "thrust" to drive the boat
Function of the propeller: As the propeller turns, negative pressure is created on the front surface of the propeller blade and positive pressure on the back of the bladeThis produces the thrust required to move the boat aheadThe point is, however, that the same propeller can never serve well for all types of boatsIn order to bring boat performance into full play, it is necessary to select the right propeller for the specific use being made of the boat, such as water skiing, cruising, fishing, racing, etcWhen boat has to be used for various purposes, several different types of propellers will be neededIn addition, the material the propeller is made of and where the boat is used (e.g rivers, seas, shallows, rock-bound waters, etc) are also importantWhatever the purpose of the boat, always make it habit to always carry spare propellers and hand toolsIn case of heavy propeller damage, you will be able to rely on the spare propeller to enable your boat to cruise to nearby portPropeller pitch: The pitch of propeller is the distance the propeller will advance or move forward in one complete turn[It is the same as the pitch of screw.J