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Choosing the right propeller
Your boat's propeller is where your engine's horsepower meets the water and choosing the right propeller is very important in order to get the best performance from your boat.
There are several things to consider.
Pitch
Pitch is a topic you hear a lot when discussing propellers. There are two numbers that are used when describing a propeller one is the diameter of the propeller and the other number is its pitch. For example a propeller may be called a 15 X 17. Although the first number 15, which is the diameter of the propeller is important and does affect the performance, it is the second number 17, the pitch that is the most important factor.
Pitch is a measurment of how far a propeller travels per revolution. For example a 17" pitch prop would travel 17 inches for each revolution. This number is a theoritcal one because it is based on the propeller moving through a solid meduim where no slip would take place. In water though, a propeller does slip and its actual travel is less than 17 inches. With this in mind we can still use the number to understand how pitch affects performance. A prop that has a higher pitch, lets say 19" would travel farther per revolution than the 17" pitch propeller.
We are often asked what is the best pitch for my boat? The best way to answer this is with a tachometer. A tachometer is a guage whether mounted in your dash or a portable one used by our technicians that measures the R.P.M. (revolutions per minute) of an engine. A tachometer is the best tool for determining correct pitch.
All engines whether outboard or sterndrive, two stoke or four stroke have a maximun operating R.P.M. (revolutions per minute) in which the engine develops the most power and is most efficient. The general rule of thumb for outboards is 5000-5500 RPMs and for sterndrives is 4300-4600 RPMs. It is best to consult your owners manual for the specific Maximum RPM Range. What you want is a propeller pitch that allows the engine to run at full throttle within this range when the boat is on plane, trimmed out and with an average load. If your engine is not running in the specified RPM range then a propeller of a different pitch is necessary. If the RPM is low then a propeller of a lower pitch will bring the RPM up. This because a lower pitch is easier for the engine to turn. If the RPM is high then a propeller with a higher pitch will bring the RPM down. This because a propeller with a higher pitch is more difficult for the engine to turn. There is a rule of thumb that is used to determine how much pitch change is required. For each 400 RPM difference that is needed to correct the engines operating RPM-a change in the pitch of the propeller by two inches is required.
Aluminum vs. Stainless Steel
What is better Stainless or Aluminum propellers?
Stainless Steel
Like most things there are advantages and disadvantages to each. In terms of performance, stainless steel propellers perform better. This is because stainless steel is stronger and this allows the blades to be made thinner. A thinner blade creates less drag as it pushes through the water. This reduced effort is transformed into greater speed and performance.
The strength of stainless makes the propeller more durable and less likely to be damaged if you hit something like a sand bar for example. But that strength can also work against you if you hit something really hard like a log or a rock. The stainless steel is less forgiving and many times we see bent prop shafts and internal damage with our customers who run a stainless steel propeller.
Aluminum Propellers
Aluminum is a more economical propeller, its lower cost and ability to shear its blades if you hit something hard can make it a good choice for those who tend to run their boat in hazardous waters.
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