Boat Propeller

The height of outboards in relationship with the bottom of the hull is very important. The higher or “drier” the motor is the faster the boat will go until it reaches the point of ventilation “pulling air into the propeller” which will instantly slow you down. Most motors will be set a little bit lower to take off best. The motor can be trimmed, meaning hydraulics will rotate the engine higher at an angle, this will speed you at higher speeds. This works unless the boat front starts to bounce or “porpose” then the motor trim will need to come back down a little bit. Idealy the motor will have a jacking plate that moves the engine straight up and down instead of on a pivot like the trim. The propeller is very important and has to match the motor and boat to obtain the best performance. A prop that limits the motor speed to max rpm at wide open throttle is ideal. Some people use different pitch props to gain acceleration or top speed. Two numbers describe prop pitch the first number tells the diameter of the prop and the second tells the pitch.

 

The pitch is a theoretical calculation telling how far the prop should move through the water without slipping. Remember if you want to increase your top speed don’t overshoot the peak horsepower or rpm’s recommended for your boat or you can actually slow down. Motor speed should change in the neighborhood of 200 RPM’s per 1″ of change in pitch. If reducing the pitch to gain acceleration be careful as too much change could cause the prop to cavitate. Cavitation is the prop slipping hard instead of moving through the water. This condition creates air behind the prop as the water boils which will damage the prop and slow down the boats acceleration. Many propellers are cupped toward the back of the blade to try to limit cavitation from happening. Multiple blade props can also spread out the load on the blades and stop cavitation from occuring. More or multi blade props (four or five) will limit top speed versus a regular (three) blade prop. Many people also go to stainless propellers over aluminum. There are performance gains with using stainless even at the same diameter and pitch as an aluminum. The stainless prop is narrower creating less drag in the water yet will resist damage better. Stainless will not flex the blades and will hold it’s true shape and diameter when it’s rotating unlike aluminum.