1991 Yamaha Outboard Factory Service Manual 9.9 and 15 HP, 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 tha 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 significanceFunction 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 carry spare propellers and hand toolsI case of heavy propeller damage, you will be able to rely on the spare propeller to enable your boat to cruise to nearby port 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