Marine Accessories

For long engine life and best overall performance you need exactly the right propeller. But no single propeller is right all the time. If you change the way you use your boats and boat engines, you may have to change the propeller, too. The wrong propeller will not only affect performance, but can destroy your engine in double quick time.

If you bought your engine and boat as a combined package, your dealer will have matched the propeller as part of the pre-delivery rigging procedure. Be sure to verify this with the dealer. But things may change while you own the boat. The dealer will have selected a propeller based on the average load for your boat. But you may be adding heavy items that will alter this important criterion for propeller selection.

Outboard-powered houseboats and pontoon boats are prime examples of craft liable to accumulate heavy gear. Perhaps you’ve just started to tow water-skiers. Perhaps you originally used your engine as auxiliary propulsion for a small sailboat, and are now using it as the primary propulsion for a lightweight skiff. Or maybe you’ve taken to visiting mountain lakes. Extreme changes in altitude can affect the engine’s operating range. If your engine was propped for sea level, operation on a high-altitude lake may require a different propeller to keep the engine revving within its normal range. In fact, all these changes will require re-propping.

The goal of the engine rigger is to ensure that your engine reaches a specified number of revolutions per minute (rpm) at full throttle under the normal circumstances of use for your boat. No more, no less. Typically, this top engine speed falls into a range with a tolerance of 800 to 1,000 rpm. The range is always listed in the workshop manual for your engine, and is generally listed in the owner’s manual as well.

Before continuing with any explanation of propeller theory, let’s define some basic propeller terms.

Boat Propellers Terminology

Pitch. This specification states the theoretical distance in inches that the propeller moves forward or backward in one complete revolution. Think of the propeller as a screw, turning its way into the water.

Diameter. This is the straight-line distance from the center point of the hub to the most distant tip of any of the propeller’s blades, times two.

Cupping. This term describes the “curl” at the trailing edge of a propeller blade. Cupping is used by engineers to increase the blade’s theoretical pitch, giving it a better grip on the water.

Slip. This is related to pitch. The theoretical distance a propeller will move through the water in one revolution (pitch) is affected by the fact that the propeller will actually lose some distance due to slippage through the water. Typically, a standard propeller will have anywhere from 10 percent to 30 percent slip, depending upon its design and use.

Blade thickness. The design of the propeller and the material used determine the thickness of the blades. Ideally, blades are designed as thin as possible to reduce drag. But they also need thickness for strength.

Cavitation. There are many causes of cavitation, including excess loads on the propeller that create vapor bubbles on the blade surface. The effect is to make the propeller lose its grip on the water and revolve at excessive speeds. Cavitation can be caused by some underwater disturbance in front of the propeller, something as simple as a barnacle on your engine’s lower gearcase, or a depth-sounder transducer mounted near the centerline of the boat. Nicks on the leading edge of the propeller blades can also cause this condition. Cavitation can cause extreme damage to the propeller blades, deep pitting, and ultimately, loss of the blade itself.

Ventilation. This is a condition similar to cavitation, but usually involves air drawn down from the surface. Sometimes this comes about through too much “out” trim of the engine, or having the engine mounted too high on the transom. Sometimes exhaust gases are drawn into the area of the blades. Your lower unit has a ventilation plate to prevent air from being sucked down into the propeller from the surface.

Rake. This is the aft-leaning angle between the propeller hub and the blade. High rake angles are used on performance boats to overcome an increased tendency toward ventilation and cavitation.
In addition to the basic definitions, you’ll need to know several other key facts to work your way through the basic propeller selection process.

The method used by manufacturers for basic propeller identification (in addition to the model number of the prop) is to give first the diameter and then the pitch of the prop in inches. They’re usually stamped on the propeller hub. For example, a prop stamped “14 x 17″ has a diameter of 14 inches and a pitch of 17 inches. Any additional markings will be special to the manufacturer, identifying the propeller model and general type.

As for the specific type of propeller to use for your application, you will need to get a copy of your engine maker’s accessories catalog (available free at all dealers) and look at the propeller-recommendation list. It’s set up for specific horsepower and type of use, as well as boat weight or type. Simply find your boat’s category, (pontoon boat, ski boat, runabout, etc.) and the horsepower for your engine, and you will be close to finding the right prop for your boat.

To determine the exact diameter and pitch that’s best for you, study the specification chart more closely. Diameter, for example, will be determined by the approximate normal speed for your boat, and generally the manufacturer will list several possibilities based on this specification.

To determine the exactly correct pitch, you will need to do some testing on the water. I must point out that the only way to perform this test accurately is with a tachometer. If your boat doesn’t have one, you will have to let the dealer set this up for you with his shop instrument.

Here’s the test sequence:

1. Consult your owner’s manual to establish the specified range of revolutions per minute (rpm) your engine is designed to reach at full throttle. This is sometimes referred to as wide-open throttle, or WOT.
2. Using your existing propeller, establish a “benchmark” by making several test runs in fairly calm water. Adjust your engine’s trim angle to achieve maximum speed. Record your maximum engine rpm, and the speed achieved if you have a speedometer.
3. If the full-throttle rpm exceeds the recommended range, you need more pitch. Install the manufacturer’s next-larger-pitch propeller and retest. On average, changing the propeller pitch by one inch will change engine speed by approximately 250 rpm. That is, engine speed will increase by 250 rpm if you decrease the pitch one inch, and decrease by 250 rpm if you increase the pitch by one inch.
4. If the full throttle rpm is below the recommended tolerance, install the next-lower-pitch propeller available and retest.

You will have achieved the optimum adjustment when you can just get the maximum rpm specified for your engine with the boat loaded as it normally would be. This should also give you the best speed.

For installations on non-planing craft, such as heavy workboats and auxiliary sailboats, rpm is the most critical of the specifications in this evaluation process. If the boat is not reaching hull speed, too coarse a pitch may be preventing your motor from developing its maximum rpm, and therefore full horsepower.

Article Source: http://EzineArticles.com/?expert=John_Routledge

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