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Saturday, September 14, 2013

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The Difference Between Speed Propellers Power Pitch Propellers

Propeller size is referenced as to diameter and pitch. The diameter of a propeller is twice the distance from the hub to the tip of a blade. The larger the diameter, the more power necessary to turn the prop. Most small recreational boats are limited to a specific diameter because of engine design, weight and hull design.

Pitch and Effects of Pitch

    Pitch is the main concern from a performance perspective. Pitch is defined as the distance a prop travels through the water in inches per one single revolution. A good illustration of pitch can be made with a wood screw. A wood screw with its threads far apart, or coarse threads, threads quickly into the wood but takes considerable effort to turn the screw. Conversely, a screw with the threads closer together or fine threads takes many more turns to install but with much less effort.

    The propellers pitch has a direct effect on the performance of the boat. The original prop generally is the most efficient for overall performance if the engine came with the boat. An engine replacement may need a prop evaluation. Engine horsepower and rpm, the weight of the boat, hull design and intended usage determine prop pitch.

Pitch and RPM

    Outboard engines run between 5,000 and 5,500 rpm at wide-open throttle, whereas inboard or IOs run in the 4,500 too 4,800 rpm range. This is the range where maximum horsepower is made. Horsepower is a product of rpm. A 100 hp motor produces much less at low rpm. The best performance possible is achieved when the engine reaches its wide-open throttle limit rpm but does not exceed it. A prop with too low a pitch generates great acceleration but allows the engine to over-rev, which easily can cause damage. The top speed also suffers. A pitch that is too high causes the acceleration to suffer and create undue manifold pressure within the engine, which also can be damaging.

Choosing the Correct Propeller Pitch

    The diameter and pitch is embossed on the propeller hub for identification. A common pitch for most small boats is in the 28- to 19-inch pitch range. If your engine rpm is too high at wide-open throttle, the pitch must be increased. Conversely, if the rpm is too low, the pitch should be decreased. One inch of pitch change is equal to 200 engine rpm. If your engine wide-open throttle rpm is 5,500 according to the tachometer or owners manual, and it currently is operating at 5,100 rpm, the rpm needs to be increased by 400. Reducing the pitch by two sizes brings the rpm up to its limit. This increases the top end speed and pulling power for skiers. The opposite is true for an engine running higher than recommended.

Considerations

    If pulling skiers is the major concern, a lower pitch prop may be used as long as the rpm is monitored and kept within limits. This brings the skiers up much faster. If speed is the main concern and the boat is not heavily loaded, a higher pitch increases the speed but slows down acceleration and upper rpm. Remember that the higher the pitch, the more load on the engine.

Tips

    Propellers are constructed in three types of material. Composite props are the lightest, offer some flex, and are the cheapest. They are not repairable should the prop strike anything. Aluminum props are the most common. The are heavier than composite, offer some flex, and are a little more expensive and can be repaired cheaply. Stainless steel props are the most expensive and hardest to destroy. They are very difficult to repair. Props are offered that consist of a separate hub with replaceable blades. These are perfect for pitch changes as needed or repair.

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