Choosing the right propeller has more to do with matching it to your boat’s hull and engine than the speed you want. The goal of any sailboat propeller is to turn a ship in water so that the craft moves forward. Regardless of size, shape, or style, boats move through water when shifting from one direction. Turning faster and reaching higher speeds requires more energy through heavier engines, shorter props blades, or wider rudders. Speedboats get their acceleration from long blades that are low on the water’s surface for better hydrodynamics.
Fixed or Vane Propellers
Casting propellers with cast vanes are created for speed through the efficiency of thrust and lift. Vane blades imitate the action of a jet engine by transferring energy from forwarding motion to rotational motion in a smooth, efficient manner. These work best when leaning on the throttle. Fixed propellers are lighter and less expensive than vanes. Fixed props are great for cruising at a constant speed or trolling in an area where the boat will not be reefed. The blades of a fixed prop are set in place and can’t be adjusted like some vanes.
There are two styles of blade design used by casting propellers: straight blades and curved blades with cupped ends (skeg-style). Straight edges work best in calm, smooth, and without current water. They are helpful when anchoring but don’t work well in a strong current or rough water. Curved blades with cupped ends ‘bite’ more efficiently into the water than straight blades are susceptible to cavitation, noise, and vibration. Cupped-end blades also have less drag and create less noise than straight blades of similar pitch. They are usually found on high-performance boats that need to make a sharp turn at high speed, such as outboards and small gasoline engines.
Depending on the manufacturer, the propeller’s pitch is measured from the hub to the tip of all the blades in feet or inches. Higher pitch means more edges. A higher pitch propeller has less drag, gives more extraordinary power, and allows the boat to turn or maneuver easier in the water. A higher pitch propeller also helps reduce cavitation noise and improve water flow around the blades. For example, a 9:1-ratio blade pitches more than a 5:1 blade for a given boat model. The most popular ratio is 6:1, which works well for boats that need to be able to turn quickly on a dime with nothing more than full throttle and no headwinds or crosswinds.
A propeller with smaller, more numerous vanes has a lower pitch, which provides greater efficiency and reduced cavitation at low speeds. It creates more drag and makes the boat turn more slowly than other propellers with a higher pitch and fewer vanes. The primary benefit of a high-pitch vane is fuel flow to the engine. It can help increase fuel efficiency by reducing energy loss from resistance in water travel.
Propellers are sized by blade diameter in inches or centimeters (cm). The lower the number, the larger the prop’s diameter. For example, a 12-inch prop has a smaller surface area than a 15-inch diameter prop. The boat will go faster with the same engine on the smaller diameter props. A larger blade also creates more lift and drag than a small blade. Larger blades ‘bite’ better into the water and produce more thrust.
Pitch and Hub
To be efficient, propellers must be matched to their engines’ pitch (blade width), design, and type of hub they were manufactured from. The hub of a prop can be bronze, stainless steel, plastic, or brass. There are two types of hubs: One is cast into the propeller, and the other is a removable component that fits onto the propeller shaft. The difference in going from a removable to a cast-in hub is nothing more than fastening a nut on each side of the prop shaft.
With all of the above considerations, you should better understand what a propeller is and how it works. If you want to be safe and sound, choose a propeller compatible with your boat’s engine. Get someone to help you if you are unsure about how it fits. There are a lot of options when it comes to choosing a propeller.