Props 101: Basics of Inboard Boat Propellers
Shopping for a new propeller, whether it's simply to replace a damaged one, or because you're looking for better performance, can be a confusing process. Diameter, pitch, cup, rotation, bore, it all ends up sounding a bit Greek. Fear not, we've developed this Basics of Propellers guide to answer all those questions and more so you can be sure you'll find the perfect prop for your specific application.
If you don't care about the specifics, and just want to find the perfect prop, refer to our exclusive PropFinder Guide. WakeMAKERS is the largest prop dealer for watersports boats in the world, and we've used our decades of experience to distill all the options down to a specific recommendation based on the individual year/make/model of boat and how you typically use it.
Understanding the way propellers are measured is paramount to gaining the knowledge needed to choose the right prop. Depending on the brand of prop, most of these measurements will be listed on the prop hub for quick reference.
Diameter: This specification is pretty straight-forward; measure from the middle of the prop's hub out to the end of one blade and double that distance. In general, more diameter means more surface area, which means better transmission of power from the engine to the water. More diameter also means more rotational inertia, which means the prop will not rotate as fast. In general, for slower sports, like surfing and wakeboarding, the larger diameter the better. For higher speed activities like slalom skiing and barefooting, a moderately sized prop will provide better performance. Typically diameter is limited by either clearance between the hull or the trailer, which is important to check.
Pitch: When it comes to props, the question we most often hear from our customers is "what is pitch?". Often times a deceiving measurement, once understood, pitch makes a lot of sense. Basically, pitch is a measurement of the distance the prop would travel with no slippage after making one full revolution. For example, a prop with a pitch of 12" would travel forward one foot after completing one revolution. Obviously we don't live in the ideal world, and slippage does occur, but for the purposes of comparing one prop to another, pitch is instrument. In most cases, pitch is also the most important variable for controlling boat performance. A lower numerical pitch number means higher engine RPMs relative to boat speed, which is translates to better performance for slower speed activities. Therefore, for wakesurfing and wakeboarding typically a lower pitch prop will be run than for slalom, barefooting and higher speed activities.
Cup: Typically not an issue for most of our customers, propeller cup plays a fairly limited role in propellers for watersports boats. Cup, or the curvature of the blade at its edge, primarily translates to the amount of load the prop will put on the motor. Higher pitch means more load on the engine, which will serve to suppress engine RPM vs. boat speed.
Bore: Just like shoe size, the bore of the propeller is specific to the prop shaft that is on your boat. In inboard boats, you'll see 1", 1-1/8" and 1-1/4" tapered bore sizes, as well as spline bore props for use on 2003 and newer Mastercraft boats. Shaft is not interchangeable, you must use the correct size prop for your boat's shaft or the prop simply won't fit.
Rotation: Like bore, rotation is specific to the power train of the boat; if your boat needs a left hand rotation propeller, you must use a left hand rotation propeller.
There are two reasons why you would need a new propeller for your boat. The first reason is also the most common and simply put the prop has been damaged. Most inboard boat manufactures tend to prop from the factory with something that would work well for the majority of users who enjoy all water sports which means this is the perfect time to upgrade. If your current propeller meets all your required performance characteristics there is no reason to replace it with something different. Find the model number or size specs and match it for a perfect fit. Read our Propeller Basics Guide.
The second reason, which we will expand on, would be to increase the boat's performance. This can be the low end pulling power required for a weighted wakesurf boat or the maximum top end speed needed to pull a barefooter behind a waterski boat. Either way this guide will help develop a better understanding of how to find the best wakeboard or ski boat propeller for your application.
To obtain the best performance from your inboard ski or wakeboard boat you will need to select the perfect propeller based on how you spend your days on the water. No matter how much horse power you have if you are not transferring it to the water in the most efficient way your performance and fuel economy will suffer.
Reasons to Upgrade Your Boat's Propeller
- Better Performance: When a boat manufacturer selects the stock propeller that comes on a boat from the factory they don't know how the boat will ultimately be used by the boat owner. Because of that, they select a prop that performs well for all activities, but doesn't necessary excel in any one area. Many of our customers only use their boat for wakesurfing and wakeboarding, and never travel more than 30MPH. For that type of application there are huge performance gains as well as efficiency improvements to be had by upgrading the propeller to one that is better optimized for how the boat will be used the majority of the time. The added benefit is you can use the stock prop that came on your boat as a spare just in case you damage your primary prop.
- Replace Damaged Prop: If your primary prop was damaged due to an impact, replacing it with either the same prop, one that offers similar performance, or one that offers upgraded performance may make sense.
Upgrading a Wake Prop
Wakeboarders and wakesufers that increase the displacement of a wakeboard boat's hull with water ballast find that a middle of the road, stock, propeller will not get the boat to their desired speed quickly enough. As a result the driver must push the engine hard by lengthening the time at wide open throttle on every start. The extreme loads seen when pulling riders out of the water tend to burn a lot more fuel. The answer is to increase the propeller's surface by increasing the overall diameter and decreasing the pitch. When the engine encounters less time at wide open throttle the end result is less fuel burned over the corse of the day. The trade off to the low end gains are decreased top end speeds and comfortable cruising speeds. If you do travel long distances to find calm water it may be wise to look at something in between the boat's current prop and the optimal wakeboard / wakesurf prop.
Upgrading a Ski Prop
Waterskiers and barefooters that have a need for speed also find that the middle of the road, stock, propeller will not produce critical top end maximums. As a result the driver must push the engine to the rev limiter at wide open throttle on every run. Holding the RPM's at their max also burns a significant amount of fuel. Their answer is the opposite of wakeboarders. By decreasing the propeller's overall diameter and increasing the pitch the blade becomes more slippery as it travels through the water. Less drag equals more speed.
3 Blade Prop vs. 4 Blade Prop - For more information regarding the differences between a 3 and 4 blade prop check out the "Do I need a 4 blade propeller for wakeboarding" post here.
High Elevation - Using your boat at higher elevations? Read our high elevation guide to learn more about how much horsepower you will lose based on the elevation.
Boat and Trailer Clearance - In a perfect world every v-drive wakeboard boat would at least accommodate a 14.5” diameter propeller. Unfortunately most boats just a few years old do not. It is critical that you measure the clearance between the bottom of your boat and the blade at the point when it is closest. We strongly suggest that you leave at the very least ¾” of clearance to avoid getting gel-burn. It is wise to also inspect the trailer for any cross members or prop guards that could interfere with a larger diameter as well. Those who don’t find out the hard way when reloading the boat on the trailer.
Use our exclusive prop finder guides with year, make model information and specific recommendations. If you still have questions about your application contact one of our prop experts and we will make sure you get the best solution available for your boat.