No. 3 – I didn’t get the right prop
One of the most important steps in setting up your boat is selecting the right prop. In all likelihood, your dealer put a prop on your motor that was a reasonable guess for the average customer. But it may not be the best one for you.
Boats are not Cars
Boats have only one gear ratio for forward and reverse. Unlike a car, they don’t have a transmission with multiple gears to help with acceleration and cruising under different loads. As a result, you have to pick a propeller that gives you optimal performance under a given load, at a given throttle setting.
Pitch and Diameter
The starting point is picking the right pitch and diameter. Determining the right pitch and diameter is relatively simple:
- All outboard motors have a recommended engine RPM range at Wide Open Throttle (WOT). For example, our E-TEC 90 has a WOT RPM range of 5,000- 5,500 RPM.
- The right pitch/diameter combination is one that allows your engine to spin within the WOT RPM range while your boat is carrying your typical load.
- If your engine can’t reach the WOT RPM range, decrease the pitch or reduce the diameter. If your engine spins too fast, you can decrease WOT RPMs by increasing the pitch, or using a larger diameter propeller.
- A good rule of thumb is that a 1-inch change in pitch changes WOT RPM by 150 – 200. On the other hand, the impact of a one inch change in diameter is less well-defined. Some sources claim it can change RPM by as much as 500 RPM, but in our experience it is less.
- Options for changing prop diameter are limited because the prop has to clear the engine’s anti-ventilation plate. That means for a given motor, most props will be fairly close in diameter. You’ll also find that diameter is fixed for a particular prop model. That is, prop Model A for your motor will only be available in 13.75″ diameter, while Model B is only available in 14″ diameter. You can’t order a 14″ diameter Model A.
- On the other hand, every prop is available in several different pitches, usually in 2″ increments. Therefore, it’s more practical to start by adjusting the pitch first.
The Selection Process
Here is a typical process for narrowing in on the right prop:
- Load the boat to your normal weight level (full tank of gas, full bait tank, your normal complement of tackle and ice, and something to simulate the weight of your passengers)
- Find a straight, open stretch of water
- Get the boat on plane, and push the throttle all the way forward. Trim the boat for optimal travel: no porpoising or chine walking
- Check the engine RPMs. If you are not within the WOT RPM range, change out the prop, using the guidelines above to pick the next pitch/diameter. Most dealers will let you swap out props, as long as the prop you turn in is not damaged. Or, if you have one of the composite props with interchangeable blades, you can swap the blades out.
- Repeat until your WOT RPM value is within the specified range. You may find that your boat performs better at the higher or lower end of the WOT RPM range.
There are a number of other parameters that can also affect performance – prop material, blade design and thickness, propeller cupping, rake angle, hub venting – but they have a smaller impact on performance than pitch and diameter. You can fine-tune these later.
Piranha propeller with removable blades (click for larger image)
What if I want more speed?
Let’s say that you select a prop using the above process and your boat’s not running fast enough for you at WOT. You may be tempted to increase propeller pitch. If you do that, you’ll find that you lose hole shot (acceleration), you won’t be able to reach the WOT RPM range, and the engine may seem like it’s lugging.
You can get minor increases in speed by looking at some combination of different prop materials and prop design as mentioned above. For more significant increases in speed, you’ll need to look at a bigger engine and/or adding something like a hydraulic jackplate to change engine height on the fly. Both of these options would allow you to run higher-pitch props for more speed.
Keep in mind that engines also have an optimal RPM range for cruising, usually 65-80% of WOT. In a perfect world, this would also coincide with your preferred cruising speed. In Toy Boat 2, 20-25 MPH is about as fast as we want to run in most swell/chop/wind conditions, and that corresponds to right around 80% of WOT RPM with our current prop.
What if I want more hole shot?
If you want more hole shot, you may be tempted to decrease propeller pitch. If you do that, you’ll find that you lose top speed, and you could over-rev the engine (although many engines now have RPM limiters to avoid damage from over-revving).
You can get minor increases in hole shot by looking at some combination of different prop materials and prop design as mentioned above, or perhaps hacking the prop by drilling vent holes in the hub. For more significant increases in hole shot, the simplest thing to do is add trim tabs or a hydrofoil to help lift the hull onto plane.