What you need to know about your Propeller

Ok so props seem to be a simple part of your boat. They spin around and you go! Too bad that’s not the extent of them. Unfortunately, there is a lot more to props than what meets the eye. You need to make sure that you have the best prop for your boat because it will improve handling, maximize speed and efficiency, and put your power plant under minimum stress. If you are running the wrong prop, more than likely you are burning more fuel while going slower, and possibly even taking years off the longevity of your motor.

Now here is everyone’s theory… “the manufacturers put the best prop on your motor when you bought your boat.”  Don't count on it! Most manufacturers/dealers usually go with the prop that's best for the most diverse applications, because they never know if that 250-hp mill will be hung on the transom of an offshore center console, pontoon boat, or a bass boat. Funny thing is, some manufacturers don’t even supply a prop, so the dealership has to make the decision. Most likely if you are running with the original screw, chances are you are a prime candidate for a prop exchange.

Here are some prop basics for you to know before choosing your next prop.

DIAMETER is the size of your prop, expressed in inches. Diameter of propellers is measured by radius times two, with propellers you can’t take a measurement across the entire propeller, just take a measurement from the center of the propeller to the outside of one blade tip and double it.

PITCH is a hypothetical measurement that describes how far the prop will move forward through the water with one full rotation. .   It can be measured, but only by a pitchometer, basically it is a measurement of drop from one point of the blade to another.  The pitch of propeller means that for one revolution, the boat should advance the amount of pitch in inches, but remember you have to calculate out slip.

CUP is the curvature at the trailing edge of the blades. Adding cup to a prop usually increases its bite on the water, reducing slippage.

BLADE AREA is the surface area of each blade, expressed in square inches, multiplied by the number of blades. The blade area can be looked at from a bird’s eye view.  If you set the propeller on the ground and draw a circle around the propeller, the blade area is the amount of the circle that the propeller covers.  Average blade area for a common three-blade prop ranges from t 50 to 70 percent. If a prop does not have sufficient blade area, propulsion is lost.

When it’s time to shop for props, you will usually see them branded by pitch and diameter, expressed in inches.  Increasing pitch by an inch typically results in a drop of about 200 to 250 rpm at full throttle. A cupped prop of the similar pitch and diameter will also drag down rpm compared to a prop with no cup.

Now how many blades should you go with? The majority of common props on boats under 35 feet have three blades, which offers the best compromise between balance, efficiency, blade area, and vibration. Four-blade props are prevalent on boats that frequently encounter ventilation issues, such as tunnel hulls and powercats. They get an enhanced bite on the water when three-bladers might slip too much. The additional blade can also improve hole shot and decrease vibration. Top end, is typically cut by a couple of miles per hour, as that additional blade also adds drag. Props with less than three blades are nearly all for applications such as sailboat auxiliary motors and electric trolling motors.  As a rule, five or more blades are on large vessels or special applications.

The material a prop is made of also affects performance. Aluminum props are not nearly as expensive as the stainless steel props, but they usually flex more and are a little slower at top-end by a mile-per-hour or two. The advantage to aluminum is that if you hit an object at high speed, the softer metal bends more easily. Whack something hard with a stainless prop, and typically the first thing to give away is part of the drive train

The most efficient technique to determine if you need a prop exchange is to look at maximum throttle rpm, and make sure it is in the middle of the manufacturer's suggested range. If you have an outboard rated to turn 5000 to 5500 rpm and it turns 4900 or 5600 rpm, you have a problem.

So let’s see how to fix your problem:

How do I find the proper pitch for an RPM range?

Each engine is geared to operate within a specific range specified by the manufacturer. Taking your current propeller pitch, and the RPM's that are being generated, you can calculate which pitch to move to by the following equation:

(Current RPM / Desired RPM) x Current Pitch = New Pitch to achieve desired RPM.

Example: If you have a 21 pitch propeller, and are turning 4900 RPM, and want to go to 5350 RPM, then

(4900 / 5350) x 21 = 19.25, which is the new pitch. 

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However, let's say top-end speed is the most important feature for the way you run your boat. In this case, reducing an inch of pitch will cause the rpm to the upper range of the manufacturer's recommendation, and your top-end speed ought to improve by a mph or two. However, hole shot will suffer. As with most things regarding boat performance, this is the trade-off.

If your engine over-revs from ventilating (cavitations), going from a three- to a four-blade prop frequently solves the problem. If you want to increase both top-end and cruising speed instantly, swapping an aluminum prop for stainless steel will do the trick. To decrease vibration, go from a three- to a four-blade prop.

Another cause to contemplate changing a correctly sized prop is if you run a dual-use boat. If the kids want to go water skiing from your center console (which is propped for finest cruising speed, but as an end result slow to get on plane), you might want to exchange for a prop that provides a better hole shot when you plan to spend the day water skiing.

Our point of this information is that any time you purchase a new boat, plan on trying some different props to find the one that best fits your boating needs.