The propeller is one of the most important items on your boat. The choice of a propeller can significantly affect the performance of your boat in terms of top speed, acceleration, fuel economy, and engine life, particularly in higher horsepower and higher potential top speed applications.
Although there are basic propeller selection guidelines, the best propeller for your boat can only be determined after trying various propellers. Your dealer or the boat manufacturer may have already tested different propellers with your boat/motor combination, or with a similar one, which determined the "best" propeller. These tests provide you with an excellent starting point. Remember, however, that your operating conditions and requirements could be different than the conditions during testing, so a different propeller may actually be better for you.
To pick a propeller for your boat follow the procedures below (also see Propeller Basics):
Propeller Selection - Propeller Series
Choose a propeller from the right Propeller Series for your engine. A propeller designed to fit a large V6 will not physically fit a mid-size engine, The propeller selection charts in the following pages are grouped by the engine groups that can physically use the same propeller.
Choose the type of propeller material and construction which meets your needs. Often this means finding the best balance between cost and the performance level desired. Aluminum propellers are less expensive than Steel propellers, but generally deliver lower performance and aren't as durable. Propellers designed for specific applications, like the Saltwater Series props, are often highly polished stainless steel and usually cost more and produce even better performance than conventional steel types. See Propeller Types.
Expected Boat Top Speed
The expected top speed for your boat can usually be determined by asking the dealer or boat manufacturer. Note that some boat speedometers produce optimistic (higher than actual) speed readings. The Yamaha multifunction digital speedometer is one of the most accurate speedometers available and is a good source as a speed reference.
Choose a propeller from the chart that lists your boat's expected top speed near the maximum of the speed range. The propeller you select from the charts using these guidelines should be a good starting point for further testing. Generally the propeller selected should produce engine speeds near or at the maximum recommended rpm range. This will produce less strain on the engine and better performance when additional load is added to the boat, especially as the boat ages.
Referred to by a series of two numbers, such as 14 x 17. These numbers correspond to the diameter and to the pitch, respectively. Generally the second number, the pitch, is the most important number.
Pitch is described in terms of inches and is the distance the propeller would travel during one full revolution without slippage. Most pitches fall between 5 and 35 inches.
To understand pitch, visualize the outboard mounted on the stern of the boat, looking at the propeller from the port or starboard side. The angle of the blade in relation to the hub determines how much pitch the propeller has. A blade that was exactly perpendicular to the hub (90° from the prop shaft) would have zero inches of pitch. A shallow angle produces a low number of inches of pitch. The effect of low pitch is similar to low gears in a car's transmission: greater low speed pulling power and acceleration, with low top end speed. A propeller with high pitch provides a greater top-end speed but reduced low-end performance, similar to the car's higher gears. Typically, increasing the pitch by 2" with the same type propeller will drop engine speed by approximately 400 rpm. The reverse is also true.
Even though a boat/engine combination may be able to use a high-pitch propeller, choose a lower pitch if a heavy load use is expected, such as pulling a water skier
Diameter is twice the distance from the centerline of the hub to the tip of blade as the propeller rotates, or the width of the circle described by the blades. Here are some points to consider when choosing the propeller diameter:
- Smaller diameter propellers have reduced drag and perform better at high speed.
- The propeller must physically fit the specified outboard.
Larger diameter demands a higher horsepower.
- Typically, a larger diameter propeller allows for higher mounting heights because the propeller reaches deeper into the water. Therefore, if you have a 225 hp outboard on a large, heavy boat, a good choice would be a 15-3/4 x 13 prop. The same engine on a light, fast bass boat would be better fitted with a 14-1/2 x 25 prop.
The small curved lip on the blade tip and trailing edge is the cup. Cupping permits the prop to get a better bite. It reduces ventilation (propeller slip caused by air on the surface of the blade) and slippage, which allows you to mount your motor higher on the transom for more speed and less drag. You can also trim out farther for greater bow lift.
The rake is the angle the propeller blades are mounted at in relation to the centerline of the hub and 90 degrees from the pitch angle. During operation a propeller with a high rake angle will tend to hold the water on the blade at high mounting heights and therefore be less likely to ventilate. A high rake angle will also produce thrust that tends to pull the stern down and lift the bow of the boat.
Number of Blades
Some propellers have three blades, while others have four. Which should you chose? Generally, a 3-blade propeller provides good performance in most circumstances. A 4-blade propeller, however, can provide better performance in rough water or with high mounting heights (such as Flats boats). This is because a 4-blade propeller has more blades to hold the water when air may be present in the water, which is more common in rough water or with a high engine mounting height.
Keep in mind that the load on the engine is determined by the total blade area and the pitch of the blade. A 4-blade propeller will increase the load on the engine compared to a 3-blade propeller. To compensate for this, a 4-blade will require a slightly smaller diameter or a higher engine mounting height than a comparable three-blade propeller, or special design. For example, the Yamaha Performance Series 4-Blade propellers for the 150hp and up have a special design using a smaller exhaust tube than normal for a V6. This is done to allow exhaust gases to pass between the propeller hub and the gear case during low speed acceleration, reducing the effective blade area and improving acceleration.