Choosing the Right Propeller

Your choices of inboard or outboard motor propellers for your boat will have a strong effect on how the craft performs. The size, shape, and materials used in the prop will determine your boat’s top end speed, the engine RPM limit, acceleration rate, and degree of vibration you experience. Most boat manufacturers and boat dealers will have paired up the boat and propeller for good performance. You may want to make changes, though, to suit the type of boating you prefer. You might be happy to accept a bit more vibration for better acceleration or speed, for example.

One of the choices you may make is about the metal that the prop is made from. Most are either aluminium or stainless steel. Both are corrosion resistant, of course. The greater stiffness of stainless steel provides better acceleration, but also adds to the degree of vibration. Stainless steel will be more durable than aluminium, but is more expensive.

Another choice to be made is the number of blades. The fewer blades on the propeller, the greater the efficiency and less drag, and therefore greater speed. Unfortunately, fewer blades means greater vibration and less comfort. To make the choice, there will be some compromise made in order to get the ride you are looking for.

There are a number of terms used to define the style of a propeller and all will have a technical effect on its performance. Pitch, diameter, cup and rake are all parameters that are engineered into propellers to achieve certain goals. Those parameters must be matched up with the boat’s size, planing needs, engine size and power to achieve the desired ride.

Propeller sizes are given with two dimensions, for example 14 x 18. The first number is the diameter, and the second the pitch. Diameter is the distance across the circle defined by the outer edge of the prop when it has made one full revolution. Pitch is the distance forward the boat would travel with one revolution of the propeller with 100% efficiency (no slippage through the water). A lower pitch will give a lower maximum speed, but also a quicker rate of acceleration. As pitch is increased, the possible RPM’s are reduced. When selecting a prop, you should always check that the RPM’s your engine will achieve are within the specifications for you boat.

Cup refers to the area at the tip of the blade. Increasing the cup will improve the “grip” of your propeller on the water. Increased cup will increase acceleration rates, while decreasing the revolution rate of the propeller.

Rake is a measure of the angle to which the blades are fixed in relation to the axis. An increase in rake will bring the hull higher in the water more quickly. This can provide a faster top speed as the hull being higher provides less drag. This, however, can also decrease control of your boat, and must be managed.

As you can see, there is a lot involved in picking the right inboard or outboard motor propellers. Your craft’s maintenance, warranty, safety and performance are all at stake. Taking into account all the technical aspects is vital, and it is advisable to consult with your manufacturer, dealer or maintenance company to ensure that you are getting the correct product. You’ll want to match up not just the technical parameters that fit your boat, but also communicate the type of boating you will be shooting for. There may be compromises you’ll want to make in terms of speed and acceleration in order to improve comfort and efficiency. But with all the different types available, you are sure to find a prop that suits your needs.

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