By: Captain Bill Jennings
Do you know the easiest way to improve the performance, ride, and fuel economy of your boat? The key lies with your propeller, which is a very critical part in your boat's drive train. This article gives you a step-by-step explanation of how to quickly check the efficiency of your propeller and make effective improvements.
The first step is to drive your boat to an area with calm water and little wind. Steer in a direction that provides at least one mile of straight line operation. Throttle up to full power and then trim up to a point where either the boat begins to porpoise, or you feel your propeller beginning to lose its grip with the water. Maintaining full throttle, trim down for about half a second. Run at this setting for about 8 seconds, then make a clear mental note of your engine RPM level and your boat's speed (a GPS speedometer would be preferred). Slow to an idle and write down these two numbers. I know that you probably have no intention of regularly running your boat at such speeds, but obtaining these numbers is the first step in the process of maximizing performance.
The next step is to look up existing data for your motor and propeller. Look in your "owner's manual" to find the maximum RPM level that the manufacturer recommends for your motor. If you don't have an owners manual, you can search online or ask your local dealer. As a guideline, most of today's standard sterndrives set their "red-line" at around 4400 RPM and most larger outboards have a red-line between 5600 and 6000 RPM. You can find the information you need for your propeller (usually just two numbers with an "X" between them) engraved somewhere on its hub, or central section. The number prior to the "X" is your props diameter. The number following the "X is the pitch number. Write down these numbers.
Next, write out how you would like your boat to improve in performance, and/or the purpose for which you mostly use your boat. For example, if you are into tow sports, you may want more pulling power and faster acceleration. If you are into long distance cruising, you may want a higher top speed.
Armed with the above information, you are ready to easily determine if your prop is the best one for your boat. Compare the RPM number that your boat produced with the manufacturers numbers telling what the maximum RPM number should be. The two numbers should match or be very close to the same. If they are, then your prop is working well for most general boating applications. If your actual RPMs are higher or lower than the manufacturer's recommended maximum RPM level, you have the opportunity to make an improvement by changing your propeller.
Knowing what propeller to select is the big question and while it may seem like a stumper, it is not as difficult as you may think. Just follow the logic in these remaining steps. An engine develops a given amount of horsepower. If that given horsepower is unable to bring your RPMs up to the recommended number, you are asking your engine to do too much work. In other words, your propeller is too big. Conversely, if your engine can run your RPMs over the recommended number, you need to give it more work to slow it down by using a bigger propeller. Remember that the first number of a propeller is diameter of the circle carved in the water and the second number is pitch, or how far the prop will move forward with every revolution. As an example, lets pick a typical propeller with a number 15 diameter X 19 pitch.
In the situation where your recorded RPM levels are lower than the recommended RPM number, you know that you need to give the engine less work because the engine cannot turn the prop up to the RPMs that the manufacturer recommends. You need to go to a smaller prop. This step is easy once you know that a propeller part number is a description of its size. You can change the size of a prop by reducing or increasing either of the numbers before or after the "X." Take our example propeller with a part number of 15x19. You can give your engine less work and raise your RPMs by changing this prop to either a 14x19 prop, or a 15x17 prop (or both). If you use your boat for tow sports and/or more acceleration is important to you, reduce the second number which is the 'pitch' and try a 15x17. If higher speeds are most important to you, reduce the first number which is the 'diameter' and try a 14x19. If a dealer stocks propellers, he will usually let you 'test' a different prop size for an afternoon and if it fails to improve performance, he will exchange it.
In a case where your RPM levels are over the manufacturer's recommended maximum, you must give the engine more work in order to lower its RPM levels. You need to go to a larger prop. Using the same example where your prop is a 15x19, you would try a 16x19 prop for tow sports or a 15x21 prop for more speed.
Using this logic you can make custom changes, such as sacrificing diameter to increase pitch, as long as you keep the work load the same by balancing prop factors and RPMs close to what is recommended. Remember that while a bigger prop can give you bigger performance, it can only do this if the engine horsepower is strong enough to turn it up to recommended RPMs. If you change a 15x19 to a 14x21 for more speed, only a test run will tell you for sure that it worked.
Another question that will arise is the choice between aluminum and stainless steel. If your motor is over 75 horsepower and you are not planning to use the boat in your swimming pool, go with stainless. Stainless is seven times stronger than aluminum and because of this they are cast much thinner for more efficiency. They can also be repaired, and the newer hubs will still protect your shaft from strike damage. Some stainless propellers are 'polished' and shiny, while some stainless are 'satin' or dull. Both work equally well, and satin stainless props, which are available through many manufacturers, generally cost less.
When removing your prop for the first time, do it with the boat in very shallow water or on a trailer and lay out the pieces in the same order that you took them off. Through hub propellers for inboard outboards and stern drives usually pull off easily, but if you are removing an inboard prop you may need to borrow a "prop puller" tool.
Following these simple and logical prop change steps can dramatically improve what your boat delivers in performance. In a case where your chosen replacement prop may not give you exactly what you expected, knowing how the replacement prop changed performance will be key to dialing in an accurate assessment of what a third and final choice should be. Try this and put a whole new spin on your boat's performance. #tips #quicktips
