Without the right boat propeller, it will be tough for your boat to go anywhere. Fortunately, your boat probably came with a propeller when you initially bought it. But what happens if you need to replace or upgrade your boat propeller?
Why Would You Replace Your Boat Propeller?
If you’re a safe boat driver and you keep up with routine maintenance, your propeller could potentially last forever. But the odds suggest you’ll need to replace your propeller at least once over the life of your boat.
Here are some reasons you might need to look for a replacement boat propeller:
- You hit something. Your propeller is bent or broken, compromising your boat’s performance. If you hit something, bring your boat to a repair specialist to check for other structural damage.
- Decreasing fuel efficiency. If you notice you’re filling up more than usual, it could be due to a damaged or inefficient propeller.
- Your boat feels sluggish. Even if your prop isn’t damaged, upgrading to a better propeller can improve your boat’s acceleration and top-end speed.
- Blades are worn. Over time, some propeller blades wear down from normal use. This is more likely with aluminum propellers, which are fine for boats that are only used a few times a year. But if the blades get too thin over time, it’ll compromise your boat’s performance.
- Corrosion has developed. This is less likely with stainless steel propellers, but a corroded prop should be replaced.
How to Choose the Right Boat Propeller
If you’re experiencing any of those issues, or you simply want to upgrade your boat’s propeller, consider these selection criteria:
Check Your Owner’s Manual
To get started, your boat’s owner’s manual should offer a range of acceptable propeller diameters and pitches based on your boat engine. Installing a prop that’s incompatible with your boat’s specifications will lead to decreased performance.
If you’ve misplaced your manual, you can contact your boat manufacturer directly. Or, you can utilize our Boat Propeller Tool to search for propellers based on your boat’s engine or an OEM part number.
Understanding Propeller Diameter and Pitch
Propellers are usually advertised by two dimensions: diameter and pitch. Diameter is the measurement from the tip of one blade to the tip of the blade directly across from it. Or, you can measure from the center of the prop out to the tip of one blade and double that measurement.
Pitch refers to the forward movement of the propeller in a single revolution. Unfortunately, all propellers experience slippage, which is when the prop doesn’t move as much as it was designed to. Even the best propellers can have up to 30% slippage, depending largely on size, shape, and blade configuration.
Pitch and engine revolutions per minute (RPMs) have an inverse relationship. When you increase the pitch of your prop, engine RPMs decrease, and vice versa. As a rule of thumb, a two-inch change in pitch translates to a 300-400 RPM alteration (Pitch ↥ = RPM ↧; Pitch ↧ = RPM ↥).
Most propeller manufacturers make their props in two-inch pitch increments. Some offer props in one-inch increments, but it’s less common.
The important thing to remember here is the relationship between pitch and RPMs. If your boat is spinning too many RPMs (i.e. under-propped), you’ll need to increase propeller pitch. And the reverse is true if it’s spinning too few RPMs (i.e. over-propped).
Consult our Boat Propeller Tool to search for a new propeller with the pitch you’re looking for.
What are Boat Propellers Made Of?
Composite, aluminum, and stainless steel are the three most common materials, but you’ll also find brass and NiBrAl boat propellers on the market. Here’s a quick overview:
- Composite propellers are the most affordable option. Popular with boaters that stick to shallow water, they are designed to break off the hub at impact to protect the rest of your boat’s drive components. The downside is that this leaves you with a disabled boat, so you’ll benefit from keeping multiple spares on board if you choose this prop type.
- Aluminum propellers are the most common. They are more commonly found on outboard and sterndrive boats that travel at slower speeds. While they dent more easily than stainless steel and brass props, they offer an excellent combination of performance and affordability.
- Stainless steel propellers are more expensive, but they are also five times more durable than aluminum. They can withstand minor impacts without bending or cracking. But the downside to their durability is an increased likelihood of damage to the propeller shaft, which can be more expensive to repair than replacing just the propeller itself.
- NiBrAl propellers are made with a combination of nickel, bronze, and aluminum. Typically found on wakeboard, wake surf, and ski boats, NiBrAl propellers maximize thrust for performance watersports
- Heavy-duty brass propellers are typically only found on large vessels undertaking heavy boating duties.
On performance boats, stainless steel propellers with 5-6 blades are the most common choice. They provide excellent propulsion and great durability over the life of your boat. But if you’re buying one of the best pontoon boats for slow-speed cruises, an aluminum propeller might be a great alternative.
What is Cupping in Boat Propellers?
Modern boat propellers are designed with a curved lip (i.e. a ‘cup’) on the trailing edge of the propeller blades. This ‘cup’ increases propeller performance in several ways, including quicker acceleration, reduced ventilation and slippage, and better ‘bite’ on the water.
For outboard motors, like those found on Nepallo pontoon boats, cupping helps when the motor is trimmed (aka raised) so that the propeller is near the water’s surface. Cupped propellers are also typically capable of producing higher top-end speeds.
What is Rake in Boat Propellers?
In relation to the hub, rake is the degree of slant (forward or backward) in the propeller blades. Rake impacts how water flows through the propeller and around the blades, greatly impacting your boat’s performance.
Aft rake (i.e. blades slanted backward) raises the bow, reduces the hull’s wet surface area, and increased top-end speed. The more aggressively a propeller’s blades are raked, the more likely you’ll need to add trim tabs.
With highly-raked propellers, the blade tips can strike your engine’s older trim tabs. So keep this in mind when looking at replacement propellers if you don’t want to install new performance trim tabs as well.
What About Location and Boat Weight?
Where you’re living, and the size of your boat also impact your propeller selection. Let’s explore these two factors more closely.
Location (aka Elevation)
This factor is really about elevation over the general location. Engines produce less power at higher elevations due to reduced concentrations of oxygen. In fact, engine power decreases by about 20 percent with every 7,000-foot elevation increase.
One method to account for this loss in power is to install a propeller with reduced pitch, which will increase your engine’s RPMs. If you frequently travel from sea level to high alpine lakes, you may need to carry two props and change them out depending upon your location.
The prop installed by your boat manufacturer was most likely made to handle the weight of the gear loaded onto your boat. For performance ski and wakeboard boats, and even some pontoon boats, it’s also designed to handle the weight of towing someone behind the boat.
That said, some boat propellers have too much pitch when the boat is loaded down close to its maximum weight capacity. In this case, your RPMs will be low, and your boat may accelerate sluggishly or struggle to get on plane.
If you frequently load down your boat before you head out, you may improve performance by installing a prop with less pitch. Or, if you go from running light to running heavy, keeping two props with different pitches may be your best bet.
Other Boat Propeller Selection Criteria
In addition to the aforementioned, compiling this data will aid your prop selection process:
- Prop rotation (clockwise or counterclockwise)
- Number of blades
- OEM part number of current prop
- Propeller shaft diameter
- Number of splines or keyway type on the propeller shaft
- Engine data
- Number of engines
- Gear case size
- RPMs at wide-open throttle (WOT)
- Manufacturer, model, and year
- Engine displacement (in cubic inches or centimeters)
- Power trim or trim tabs
- Overall boat length
- Boat hull material
- Present top speed
- Boat manufacturer, make, and model
- Boat hull shape
Boat Performance Issues to Avoid
Two major issues that decrease the performance of your boat are ventilation and cavitation. Here’s an overview of these issues and how you can avoid them:
Ventilation refers to exhaust gasses or surface air being drawn into the propeller’s blades. When this occurs, engine RPMs will increase dramatically, but your boat will lose speed. To prevent this, you should avoid tight cornering and over-trimming the engine (i.e. raising the prop too close to the water’s surface).
To reduce ventilation, you can lower your motor on the transom or replace your propeller. Ventilation happens more frequently with poorly designed propellers that offer little to no cup, worn propellers with damaged blades or cup profiles, or propellers that simply do not match how you’re using your boat.
Cavitation happens when water boils or vaporizes due to a lack of pressure on the back of the propeller’s blades. This phenomenon naturally occurs with most propellers during normal operation. But when it occurs excessively, it can result in severe metal erosion or pitting of the blade surface.
The most common causes include poor blade design, excessive cupping, over-trimming the engine, dings or sharp corners on the leading edges of the prop blades, and protrusions under your boat’s hull obstructing proper water flow, such as thru-hulls or sensors.
Boat Propeller Maintenance Tips
Whether you just bought a new propeller or you want to keep your existing prop in the best condition possible, regular maintenance is important. Set a schedule to check your boat propeller for the following conditions at least twice a year:
- Nicks, dings, or missing material
- Loose, missing, or corroded hardware
- Bent blades
- Entangled objects, such as fishing line
- Decomposing blades
- Slippage in the rubber hub
- Paint chipping off near blade tips (aluminum props only)
If your propeller fails to pass any of these checks, it’s time to contact a boat service professional for a more thorough inspection. A replacement propeller may be necessary, depending on the extent of the issues.
If you determine this to be the case, search for the right propeller for your boat today.
Fortunately, you now know what to look for when selecting the right propeller for your boat. Once your new propeller is installed, you’ll be right back on the water to enjoy wakeboarding, tubing, water skiing, and your other favorite watersports.
Do you have any experience replacing the propeller on your boat? If so, please share any additional tips with us in the comment below!