Boat propellers have always been a bit mysterious to many people, yet they’re a critical component of your boat and swapping props could provide a performance upgrade, improve basic boat handling, and more. A lot of people swap one for another and back again, change from aluminum to stainless-steel or vice versa, try to tweak pitch, and on and on. But many of them may not ever recognize just what sort of effects different props can have on a boat. The major factors to consider include:
Boat Propeller Material
In the case of outboard, stern drive, and relatively small boats, propellers will usually be either aluminum or stainless-steel. If you want to go fast, go stainless. It’s stronger than aluminum, which means manufacturers can make thinner blades, increase efficiency, and reduce drag, according to John Scherrer, manager of propeller and hydrodynamics engineering for Mercury Racing. It also increases the variety of propellers you can buy.
“Once you go to stainless, you get all these different model lines,” Scherrer said. “There’s an ability to tune your boat for what you want to do.”
Just how big a difference will going from aluminum to stainless-steel make? On most boats, you’ll see an extra mile or two per hour at cruise and sometimes as many as three additional mph at wide-open throttle. The larger the boat is, the more dramatic a difference you’re likely to see. But there is one potential drawback: since aluminum flexes more than stainless-steel, if you strike a hard object the blade(s) of an aluminum prop generally take the brunt of the damage. If you have a stainless-steel prop, however, the shock of the impact is more likely to damage the lower unit. For this reason, many boaters who spend their time in waters where impacts are likely, such as boulder-strewn rivers, opt for aluminum props regardless of the performance loss. We should also note for the record that stainless-steel propellers do cost a bit more than those made from aluminum.
On small, portable motors, you may also see composite propellers. These work well for low-power applications, and many electric boats with low-horsepower applications use plastic props.
At the opposite end of the spectrum are Nibral (nickel-bronze-aluminum) and bronze propellers. These are strong, durable materials, and are generally used for inboard boats. They’re softer than stainless-steel, a critical factor in case of striking an object in a large, expensive inboard boat, where very serious, expensive damage could occur.
Propeller Pitch
Propeller pitch is one of two major measurements (diameter being the other), both expressed in inches, which come into play when you’re choosing a propeller. But pitch confuses a lot of people, and it’s hard to get a handle on exactly what pitch is because it’s theoretical. The dictionary definition: pitch is the theoretical distance a prop will travel forward with one rotation, if there isn’t any “slip.” Slip is a loss in this theoretical distance due to a number of factors like turbulence, hull design or condition, engine trim angle, and more.
Clear as mud? Okay, here’s a much easier way to visualize it. Think of taking a screw, placing it against a piece of soft wood, and turning it one full rotation. How far it penetrates the wood is, essentially, its “pitch.” Propeller pitch is no different, except that since the prop is turning through a liquid and there are a number of variables in play, it will always have some level of slip.
So, what pitch is best for you? Pitch will act more or less like gears on a car or bicycle, delivering faster acceleration but less top-end speed, or conversely, faster top-end speed but less acceleration (and in the case of a boat possibly problems getting onto plane). The best way to figure out what pitch you need is to look at the engine manufacturer’s recommended wide-open throttle (WOT) rpm range. If your engine tops out several hundred rpm lower than that range you probably need a lower pitch propeller. But if it exceeds the recommended range, you need a higher pitch propeller. As a general rule of thumb increasing pitch by an inch will lower engine rpm by about 200 rpm, and lowering pitch by an inch will increase it by the same amount. And, since most manufacturers have WOT ranges anywhere from 500 to 1000 rpm, there’s often some room for adjustment.
Propeller Diameter
Diameter is the next critical measurement, and this one’s easy to understand because it’s exactly what it sounds like. Propeller diameter is simply the diameter from blade tip to blade tip. Larger diameter props create more thrust and enhance slow-speed handling and maneuvering in reverse, but they also create greater resistance and drag.
Number of Blades
Depending on the type of boat you’re talking about you may see two, three, four, or even five or six blade propellers. More blades means more power and better balance (which translates into less vibration), but also more drag. As a general rule of thumb, two blades are only seen on very small, low-power boats where vibrations aren’t an issue, and three blades are most common. Four bladed propellers may be used on boats that need a little more low-end grunt to climb onto plane (at the cost of a mph or two at top-end) or for boats that may have issues with ventilation, like some planing power catamarans or boats with stepped hulls. Props with additional blades will usually be seen only on very large boats and yachts.
Single Props vs. Twin Propellers
There are a few outboards and stern drive rigs with twin propellers. These are generally contra-rotating, meaning that the two props spin in opposite directions on the same shaft. (Two propellers on different shafts swinging in opposite directions would be counter-rotating). This counters prop torque, improves grip on the water for enhanced maneuverability, and increases efficiency. However, there aren’t many options available. Generally speaking either a motor will have contra-rotating props or not, and there won’t be numerous differing propsets with a wide range of pitches and diameters.
Specialized Propellers
While we’ve covered the main options here, there are a few specialized propellers that you may need to know about at one time or another. The newest is the “loop” propeller, a prop that has loops instead of blades, manufactured by Sharrow Marine. Sharrow claims a reduction in vibration and a boost in efficiency and speed. Another is the variable-pitch propeller, which as the name indicates can modify pitch. On sailboats, you may also see folding propellers. These have blades that fold back to reduce water resistance when traveling under sail. And there are surface drive props for some high performance speed boat applications, which are designed to spin with the blade(s) at the top of the rotation coming out of the water to reduce drag.
Propeller Terms
In addition to those we’ve already used, there are a few additional propeller terms every boater should know.
- Cavitation – Cavitation is water vaporizing due to an extreme pressure drop on the leading edge of the propeller. It’s most commonly caused by blade damage or mismatching the propeller to the boat and engine.
- Cupped – Cupped propeller blades have a slight curvature in the leading edge of the blades, which can help reduce slip.
- Hub – The hub is the center of the propeller where it attaches to the drive shaft.
- Rake – Raked blades are slanted slightly aft from the propeller hub.
- Right Hand/Left Hand – Right handed propellers turn clockwise when faced from the rear, and left handed propellers turn counter-clockwise.
- Ventilation – Air or exhaust gas being drawn down to the propeller blades as they spin. This often happens if a drive is trimmed too high, and is commonly mistaken for cavitation.
Okay—we covered a lot of ground there, boaters. But understanding propellers, how they work, and how they effect your boat’s performance is terrifically important. And hopefully, now all of those prop mysteries have been solved.