Propellers are a mystery to many boaters. While many mariners claim selection of the perfect prop depends on black magic, boaters can still narrow their choices through a better understanding of propeller design.
Boat propellers can be described by a set of three basic numbers. They are outside diameter, pitch, and number of blades. The first two values detailing size are usually found on the propeller hub, an example being 15 X 19. The first number is diameter, and the second is propeller pitch.
Diameter - Measuring the diameter of a circle that encompasses the blades of a propeller describes prop diameter. Diameter can be easily determined by visual examination and measurement of the prop. As prop diameter increases, the torque required to turn it at a specific speed will increase. In smaller vessels that use outboard drives (outboard or inboard/outboard systems), the physical dimensions of the lower unit will limit options for use of varied prop diameters.
Pitch - Pitch is an obscure propeller parameter that can be difficult to grasp. Propellers provide thrust to drive a boat forward in the water through use of angled blades that dig into the water ahead of it. Props with flat blade angles are easy to turn and spin faster at a given level of thrust, compared to propeller blades set at steeper angles. Using the same comparison, a prop with a steep blade angle is harder to turn and will spin at a slower speed for the given thrust.
Measuring Pitch - Pitch is the measurement of the propeller blade angle. It describes the forward movement caused by making one complete revolution of the propeller. A close analogy is a wood screw. The threads of a wood screw have a fixed pitch. Measuring the distance a screw bores into wood from one complete turn would describe its pitch. Water, unlike wood, is fluid medium and propellers slip as they pull themselves through the water. Propeller manufacturers often twist the blade angle from the hub root to the blade tip making measurement of pitch difficult for boaters. Fortunately pitch is normally stamped on the propeller hub. Since there are limited diameter options available for smaller boats, pitch is often changed to adjust performance for factors like cargo weight, available horsepower, and hull shape.
Blades - The number of blades installed on a propeller effects its performance. Most recreational vessels use a three bladed propeller. Four and five bladed propellers are commonly used in situations were there are greater horsepower, speed, and load requirements made of a vessel. The optimum number of prop blades will increase with an increase in horsepower and vessel weight.
Performance - Since the parameter of diameter, pitch, and number of blades is fixed for most recreational boat propellers, any design used will constitute a compromise. Some operators will need a prop that will pull their boat on plane under heavy loads, while others will be looking for the best performance at high cruise speeds. In general a steeper blade angle will favor slow speed performance of boats under heavy loads, while flatter blade angles are more efficient at higher boat and engine speeds. While the choice of propellers available in the marketplace is staggering, your vessel and engine manufacturer will have specific recommendations available for selecting a prop that is right for your boat.
 |
 |
 |
 |
