There's no magic in choosing the right propeller for your boat. Here's what you need to know to make the right choice.
Any practical boat owner knows to change the engine oil, swap out the impeller every year or two, and change worn belts before they break. But the propeller is much more likely to be overlooked. Choosing the wrong prop or continuing to use a damaged prop can decrease boat performance, increase fuel consumption, and potentially damage your engine and transmission. Here are five conditions that warrant a closer look at your prop.
With the prop out of the water, tap each blade in turn with block of wood. A different tone may indicate a serious problem.
Is Your Boat Slow To Come Onto Plane?
If your prop has too much pitch (see "A Few Important Definitions" below) like the one above, the engine will have a lousy "holeshot" — the ability to jump up on plane quickly — and will lug (or drag). It's like starting from a stoplight with your car in third gear — not only will it take much longer to accelerate, the engine has to work hard at low speeds, which places a tremendous load on the pistons, crankshaft, and bearings. This, in turn, can cause detonation, piston seizure, and other engine damage.
A Few Important Definitions
Diameter: An important factor in thrust — the larger the diameter of the prop (everything else being equal), the more water that gets moved. As a general rule, when performance suffers, you change pitch and not diameter. Diameter is the first number listed in a prop size, such as 14 x 17.
Pitch and slippage: Pitch is the distance a prop moves through the water in one revolution. A prop with an 18-inch pitch would move 18 inches through a solid medium with each complete rotation. The reason a propeller moves less than 18 inches is because it operates in a liquid medium, which creates slippage. So instead of moving 18 inches, a propeller in water moves maybe 15 inches. Some slippage is essential. Without it, the prop couldn't move the boat. But too much or too little slippage reduces efficiency. The second number stamped on the prop is pitch.
Rotation: Propellers can rotate to the right (clockwise when viewed from astern) or the left (counterclockwise). Most outboard and I/O propellers rotate to the right. Many sterndrives and even some newer outboards have two counter-rotating props.
Number of blades: More blades are smoother, but slightly less efficient. The typical compromise is three blades, although four blades are becoming increasingly more popular.
Thickness: Blades should be as thin as possible because it takes more power to turn a thick blade. Stainless propellers are five times stronger than aluminum, which is why they can be thinner and still retain adequate strength. Hence, stainless steel props are more efficient.
If your engine can’t get to within 100 to 200 rpm of rated wide-open throttle (WOT), you probably have a prop issue, assuming your engine runs fine and minimal growth on the hull. Most props can have the pitch adjusted at a prop shop, which can save you from having to buy a new one. A general rule of thumb is that a 1-inch pitch change will result in a 175 to 225 rpm change at WOT.
Does Your Engine Over-Rev While The Boat Seems Slow?
If your prop has too little pitch, the engine can rev past its red line, which is like driving your car on the interstate in second gear. Continuous running over WOT will soon damage an engine. A prop shop can add more pitch or recommend a new prop. Notice that having either too little or too much pitch can cause engine damage. So why didn’t your boat come with the right prop in the first place?
It's possible it did, but the prop could have been changed later. Also, a boatbuilder doesn't know how or where you will be using your boat, so he or she often uses a compromise prop. Running your boat at high-altitude lakes, for example, requires a different prop than one that will be used at sea level. A boat used to pull skiers or one that is loaded down with weekend gear will require a different prop than a boat used for zooming a couple of fishermen out to a fishing spot.
Did You Run Over A Log, Hit A Sandbar Or A Rock?
A bent blade or nick will severely affect performance and could even damage the engine and transmission. Even worse, a bad prop may look fine to the eye. Larry Carlson at Black Dog Props in Stevensville, Maryland, which uses computers to diagnose and repair props, says that 80% of the propellers on his shop floor look healthy. But those otherwise "healthy"-looking propellers typically have differences in pitch of 1/2inch to 3/4 inch.
Which material is best for a prop? A lot depends on where you do your boating. If your chances of hitting a submerged object are low, an aluminum prop is an all-around inexpensive choice. Aluminum props come standard from most boat and engine manufacturers, but they're less rugged than stainless-steel props. The latter are less likely to be damaged but are more expensive to repair.
Do You Want Your Boat To Be Faster?
You say you want to go faster? The first place to look is the prop. As a rule, a stainless-steel prop (assuming the correct pitch and diameter) will make your boat go faster — up to 2 to 3 knots on some boats. Stainless steel is stronger and can be made thinner, allowing more speed. Stainless steel also flexes less and keeps its shape at higher speeds. The downside to a stainless-steel prop is that it costs substantially more.
Using Too Much Fuel?
It might be your prop. A dinged and bent prop can rob you of 10% of your fuel costs. Prop shops can use a machine to tell how far out of specification your prop might be and repair it like new. They can also advise as to whether you might need a prop of a different diameter. Diameter and pitch are interrelated, and sometimes getting the right prop for the best efficiency is as much art as science and might require trying several different props.