The two most important things to remember in choosing a propeller are that it meets your individual needs for your individual application and that it allows the engine(s) to run within the specified RPM range at full throttle.
What’s important to you... Top Speed, Cruising Speed, Hole Shot, Load Carrying, Slow-Speed Handling, Slow Trolling, Fast Trolling? Two identical boats with identical engines could be propped quite differently, depending on the usage, water conditions, and load. There is no such thing as the best or ideal prop for all applications of a similar nature. Acceleration may be compromised for top speed and fuel economy, and visa versa. Often times, there may be 6 or 8 different props that seem to run about the same, with differences so subtle that any of them could be considered satisfactory by most standards. This just makes it that much harder to make a decision and choose the right prop.
In most cases, right hand propellers are used for single engine applications and one left hand propellers are used for twin engine applications to achieve better steering control.
Propellers are sized and described by their diameter and pitch. A propeller listed as a 15 x 17 x 3 would indicate a 17 inch pitch, 3 blade propeller having a diameter of 15 inches. Pitch is the theoretical distance that the boat will move forward with each revolution of the prop shaft, minus the slippage. The pitch ultimately is responsible for the top speed of the boat, much like the main jet in a carburetor is responsible for the ultimate power and speed of an engine. Diameter is the distance across a circle encompassing all of the propeller blades.
The pitch must be matched to the engine’s recommended rpm range for full throttle. For most engines, this top range is about 500 to 1,000 rpm (typically 5,000 to 5,500 for 2-strokes, 5,000 to 6,000 for 4-strokes). A light boat and load will pull a high numerical pitch prop, whereas a heavy boat and load would have to run a smaller numerical pitch to load the engine less and allow the engine to reach recommended full throttle rpm. Keep in mind that most propeller manufacturers design their pitch in a progressive manner, to the point that the actual pitch will vary across the blade surface. Also, keep in mind that different propeller manufacturers each measure their pitch in slightly different ways with different tolerances. This means that two propellers of the same diameter and pitch from two different companies can yield different performance data.
For anglers slow-trolling for species like rockfish and flounder, a propeller with lower pitch (less distance per turn) that still allows the engine to rev to the top of its range will offer lower trolling speeds. It will also push loads easier and make maneuvering around a dock easier. On the other hand, a prop with more pitch that lets the engine turn to the lower end of its range may yield higher top speed
Pitch is the theoretical distance a propeller will move through the water for each revolution. (i.e. a 21”pitch propeller should move a boat forward 21" for each revolution, assuming there is no slippage).
There are other dynamics that come into play as the boat accelerates to its top speed. When it’s sitting still in the water and the skipper advances the throttle(s), the diameter and surface area of the prop develop the initial static thrust and launch the boat. As the hull gains momentum and speed, the dynamic thrust now is largely influenced by the prop’s ability to connect itself to the water and hook-up without cavitating or ventilating.
Cavitation is loss of hook-up due to the water literally boiling, caused by extreme low pressure near or at the blade surface or blade edge. Ventilation is a loss of hook-up due to the introduction of air or exhaust gases around the propeller. Basic blade design and diameter can affect these problems. If the diameter is too small for instance, it can cause cavitation. If the engine is mounted too high, it can cause ventilation. Both of these phenomena can be minimized by installing the correct prop. Going to a larger diameter or switching to a 4 blade can sometimes accomplish better hook-up. Larger diameter propellers usually yield better maneuverability as they push a larger volume of water on initial rotation, especially at slow speed. They also grab more water for better control when reversing. Matching the diameter and pitch for a given load and application gives the best performance for a specific boat.
Rake is a special design feature. Positive rake benefits from normal bow lift to permit the use of a larger diameter propeller, while negative rake can provide extra blade strength for use in very high speed applications.
This is a technique used to enhance the performance of propellers under certain conditions. A cupped propeller will exhibit very high efficiency at high speeds as well as minimize cavitation under heavy loads.
Vented props are available with an exhaust relief hole at the base of each blade. These holes can range from ? inch to approximately 3/8 inches in diameter. They allow exhaust gases to escape around the propeller as it begins to spin up on acceleration. The engine gains rpm more quickly and reaches its ideal power curve sooner to improve overall acceleration. These props, however, do not work well for fishermen who do a lot of slow trolling, as the boat never gains enough speed to leave the ventilated water, causing the prop to catch-and-release, making the boat surge. Vented props also do not work well with cat hulls.
Matching style, blade design, pitch, and diameter is just as important for boat handling and safety as for maximizing cruising speed and fuel efficiencies. Large diameter props with lots of surface area help a boat climb a big wave and allow the operator to maintain good control in offshore conditions with rough water. Again, good-hook up is essential without overloading the engine and prop to the point of causing cavitation.