Why a new propeller is needed

Published online: Mar 23, 2012 Feature Brandon Barrus
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Everyone knows that without a propeller, your boat will go nowhere. Yet if you ask new boat owners, few seem to know the details of how a propeller works or how to find the propeller that best works in their situation. Since the Better Boater column is nothing if not a place for newbie boaters to go to find important information, here are the basics on propellers.

For starters, the definition of a propeller. According to Edgar Reyes at www.propellerpages.com, a propeller is "a mechanical device formed by two or more blades that spin around a shaft and produces a propelling force in boats." Technically this includes windmills (though in reverse) and pinwheels, but for this discussion, we'll focus on how they relate to boating.

Terms

The first term that you may hear when discussing propellers is pitch. This is a somewhat confusing term to define simply, but here's a good attempt: the pitch of a propeller is how far, in inches, it will move the boat ahead in one full turn of the blade. So for example, a propeller with a pitch of 19 will move the boat forward 19 inches in one revolution of the blade. It may seem weird that one revolution only moves the boat forward maybe a foot or two, but consider that most boat motors can spin a propeller at upwards of 5000 revolutions per minute and the whole setup makes a lot more sense.

The diameter is exactly what it sounds like: the diameter of the circle made by the blades on the prop. You can measure this by laying the propeller on a piece of paper and tracing the outer edge of the blades. Then measure the resulting circle from end to end.

Potential Problems

If the prop and pitch measurements are  incorrect, you may deal with issues like fuel usage inefficiency or a lack of power. Marine engine manufacturers release recommendations about what the optimal RPM is for a specific motor at a wide open throttle, with an appropriate weight load.

If a propeller is too big, you will not be able to achieve the manufacturer's recommended RPM at the weight load, and can cause frequent spark plug failure, burned pistons, worn gears and black exhaust smoke.

In contrast, if the propeller's diameter is too small, your engine will run beyond the recommended RPM at weight load, and serious engine damage may occur, in addition to issues with bad fuel economy and inability to hit the speeds you should.

Measuring

To see if your boat's propeller is the correct pitch and diameter for your engine, you need a tachometer. This device counts the revolutions per minute and displays this information. Many boats come with a tachometer pre-installed, but if yours does not, you should be able to visit a boat repair shop and have one put in. With a tachometer, you can measure the RPMs and see if they are within the range the manufacturer specifies for your engine. You may want to calibrate your tachometer at a shop, as a misreading even 300 RPMs off may be enough to cause problems.

Once you have a tachometer, take your pontoon or deck boat out on the water and open her up. At top speed, note the RPMs and compare to what it should be. If you are running too many RPMs or too few, you may need to get a new propeller.

Overton's (www.overtons.com) has a great program where you can select your engine's manufacturer and model, and it will give you a list of propellers that will work for you. For example a Honda BF90 at 90hp, 2010 model year, can use a range of aluminum and stainless steel propellers, all around a 17 pitch and 13-inch diameter.

Materials

The three different types of propeller are aluminum, stainless steel and composite. A stainless steel prop is generally regarded as being able to give you the best performance and durability overall, but you will pay for the difference. The majority of boat props are made of aluminum, because aluminum can be used in a wide range of speeds and functions. Composite is a cheaper prop, but also gives an advantage when it comes to performance. Composite propellers cannot be repaired, and broken blades must be replaced. The replacement blades are not cheap, but may present a better alternative than repair.

Stainless steel propellers enjoy the advantage in performance and durability over the other two mainly due to its higher rigidity, which gives stronger propulsion and means the prop can handle stronger strikes and blows.

In contrast, the heavier weight of a stainless steel prop is considered a disadvantage to some, who believe the weight causes additional wear and tear as an engine is repeatedly put in and out of gear.

If you're interested in getting the absolute best performance from your pontoon or deck boat, tweaking your propeller is definitely a good way to go. But even if you aren't interested in attaining the most ideal setup for your boat, keep in mind that gearbox and engine repairs are not inexpensive. If your prop is spinning too fast or too slow, you may see big bills in your future.

For more information on propellers and how they can affect boat performance, visit www.dunnritepropellers.co.nz or www.rbbi.com.

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