Boat Propeller Performance Factors
Design Aspects That Effect Prop Capability
Propeller manufacturers use a number of techniques to improve the performance of boat props.
Once you consider the basic parameters of propeller design, there many other engineering solutions applied to propeller design meant to improve their performance. A key concept for boaters to consider in prop discussions is wide open throttle (WOT). Engine manufacturers specify the optimum revolutions per minute (RPM) that an engine should turn at full throttle. Changing the shape or material used in making a propeller effects its performance at WOT. Two propeller design constraints are ventilation and cavitation.
- Ventilation - The introduction of surface or exhaust gasses into spinning propeller blades is ventilation. This phenomenon raises engine RPM and reduces the thrust generated by a prop. It can be caused by a tight turn, improper trim or a poorly mounted engine.
- Cavitation - The boiling point of water varies with atmospheric pressure. Water boils at a higher temperature at sea level than at high mountainous elevations. Water introduced to the vacuum of space would boil away immediately at very cold temperatures. A turning propeller blade creates an area of low pressure on the backside of the blade. Blades turning at excessive speeds can cause water to boil on their surfaces. The incorrect choice of propellers, damage caused to a prop, and vessel design faults are other factors that influence potential cavitation. Excessive cavitation mimics performance problems found with ventilation and can physically damage the propeller.
Manufacturers can change the blade shape, diameter, materials, and number of blades used to build a propeller.
- Propeller Cupping - Placing a cup in trailing edge of a propeller offers a few performance benefits. A cupped prop will bite into the water better offering less ventilation and a better "hole shot" out of the water to reach planning speeds. A cupped propeller trimmed near the surface of the water is more efficient and can improve speeds at WOT. Switching to a cupped prop of the same diameter and pitch will potentially cause a reduction of 200 RPM.
- Propeller Rake - Propeller blades can be tilted fore and aft on the hub in relation to the boat. Rake affects the trim of a vessel as it changes the flow patterns of water moving through the prop. Raking a prop aft will improve top speeds of boats at WOT by trimming the bow up and reducing the hull area in the water. Negative or forward rake trims the bow down, improving performance of heavier boats at slower speeds.
- Counter-rotating Propellers - The descending blade of a propeller generates more thrust than the other blades. This asymmetric thrust has negative effects on boat handling. Single engine boats can have the turning torque transmitted to the steering wheel. Boaters will notice a backing boat will always pull to one side due to asymmetric prop thrust. The Mercruiser Bravo Sterndrive is on attempt to solve the problem in single engine boats. In twin engine boats, using opposite turning propellers will improve handling, eliminate steering torque issues, and make backing in a straight line feasible.
- Propeller Materials - The two common building materials used in recreational props are stainless steel and aluminum. Stainless steel offers some hydrodynamic efficiency other aluminum and is much more durable. Stainless steel is harder than aluminum and offers less protection to a boat's drive system if the prop is run aground or into a rock. Aluminum offers good overall performance for its cost. The blade of aluminum is more likely to fail after a rock strike than the engine's prop shaft. Due to cost, many boaters carry a spare aluminum prop for emergencies.
Finding the optimum propeller is always going to be a compromise between low and high speed performance. The best prop for a lightly loaded boat operating at high speed will not be efficient for the same boat carrying a heavier load. The goal is to find a prop configuration that produces the manufacturers recommended RPM at WOT. A cup bladed prop will reduce RPM by 200. Increasing prop pitch will reduce RPM by up to 400 and decreasing pitch will increase RPM by the same values. Engine and propeller manufacturers offer detailed information on selecting an optimum propeller for your boat.
