They spin, you go – but could you go faster or more efficiently if you had a different prop?
Using the ideal prop for your boat will improve handling, maximize speed and efficiency, and put your powerplant under minimum stress. Run with the wrong one and you’ll burn more fuel while going slower. Worse yet, you could even damage your powerplant. And don’t think that your dealer sold you that new boat with best prop already on it. Most make a best guess, and leave it at that.
No Spin Zone
Before digging into the meat of the matter, let’s make sure everyone understands propeller basics. First, let’s consider the material a prop is made of. Aluminum props are less expensive than stainless steel props, but they flex more and usually top-end is a mph or two less than it would be with a stainless prop. But there’s an up-side to aluminum, because if you strike an object at high speed the soft metal will bend more easily then stainless-steel. That could save you major bucks, in the long run, because if you hit something hard with a stainless prop the first thing to give way is often a part of the drivetrain. Nibral, a nickel/bronze alloy, is only commonly seen on larger props, for boats in the 35’ and over class.
There are a few prop terms you need to be familiar with: DIAMETER is the physical size of your prop, expressed in inches. PITCH is a theoretical measurement, describing how far the prop would move forward through the water with one full revolution, in a perfect world. CUP is the curvature at the edges of the blades. (Adding cup to a prop generally increases a prop’s bite on the water, reducing slippage). BLADE AREA is the surface area of each blade in square inches, multiplied by the number of blades. Standard blade area for a common three-bladed prop is about 50- to 70-percent. If a prop doesn’t have enough blade area, thrust is lost. If it has too much, drag is increased.
When you look at or consider purchasing a prop, you’ll see props identified by diameter and pitch, expressed in inches. A mid-sized outboard prop, for example, might have 17” x 19” stamped on it. The 17” is diameter, and the 19” is pitch. Swapping props to increase pitch by an inch usually results in a drop of about 200 to 250-rpm at wide-open throttle. A cupped propeller of the same pitch and diameter will also draw down rpm by about 200, as compared to a prop with no cup.
How many blades should a prop have? Theoretically, one blade is best—it has the least amount of drag, and no other blades disturbing the water flow. But we don’t live in a theoretical world, and a one-bladed prop has zero balance and would vibrate excessively, to say the least. Two blades need huge blades to create enough blade area for effective thrust, but that creates excessive drag and or vibration, so they’re reserved for applications such as sailboat auxiliary motors and electric trolling motors. Most props used for today’s power plants on boats under 35’ have three blades, which offers the best compromise between balance, efficiency, blade area, and vibration. Four-blade props are popular on boats which often encounter ventilation issues, such as tunnel hulls and powercats, since they usually get a better bite on the water when three-bladers may slip when a little air is introduced. The extra blade can also improve hole shot and reduce vibration, thanks to better balance. Top end, however, is usually cut by a couple of mph, as that extra blade also adds drag. Props with five or more blades are only commonly used on large vessels, or for special applications.
To Swap, or Not?
The most obvious clue that a prop swap would do you some good is evident in your wide-open throttle rpm range. It’s important to make sure rpms meet the manufacturer’s recommendations If you have an outboard rated to turn 5000 to 5500 rpm, for example, and it turns 4900 or 5600 rpm, you have a problem even through you may not know it—and you can solve it with a prop change.
Remember, increasing an inch of pitch will lower rpm by 200 – 250 rpm. Dropping an inch will increase it by the same amount. And once rpms are within the manufacturer’s range you’ll probably discover you get better fuel economy, have a stronger hole shot, and possible a higher top-end.
Now let’s say your engine is right where it belongs, in the middle of the manufacturer’s recommended range. But you take it onto shallow flats quite often, and sometimes getting onto plane during a low tide is challenging. Hole shot is an important feature for you, and if you swap your current prop for one that’s cupped, or one with an inch more pitch, or a four-blader, you’ll probably discover that hole shot is noticeably improved. On the flip side, let’s say top-end speed is the most important feature for the way you run your boat. In this case, dropping an inch of pitch will bring rpm into the upper range of the manufacturer’s recommendation, and your top-end speed should improve by a mph or two. Will hole shot suffer? Yes. As with most things regarding boat performance, propellers offer you a wide range of trade-offs.
Why else might you want to change props? If your motor over-revs often due to ventilation, going from a three to a four-bladed prop will often solve the problem. If you want to increase both top-end and cruising speed immediately, changing an aluminum prop for a stainless one will do the trick. And if you want to reduce vibrations going from a three to a four-blade prop will make a noticeable difference.
Another reason you might want to consider changing a properly sized prop is if you run a dual-use boat. If, for example, the kids want to go water skiing from your center console—which is propped for the best cruising speed, but as a result is slow to get on plane—you may want to swap for a prop that will provide a better hole shot, just for the day.
Your prop may have been perfect the day you bought your boat, but you may also discover that a prop change is desirable after a few years of use. After a few layers of bottom paint and the accumulation of gear, boaters often find that their rpms don’t turn quite as high as they used to (a result of an increase in overall load) and changing props might bring back your boat’s original performance. So consider giving the prop swap a shot—no matter what you find, one thing will remain the same: it spins, and you go.
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