Gears: A Cog in the Wheel

Gears: A Cog in the Wheel

Sometimes people have a difficult time with props and they wonder if changing gear ratios is the solution. The question usually is: “Is changing the gear ratio the same as changing the prop pitch?” No, it is not. Here’s why. 

Higher pitch props are harder to turn, and are less efficient as you go higher in pitch. That’s true with comparable props, obviously there are other factors like rake, diameter but it is true overall. Generally, you want to find where your engine makes its most usable power. Typically, you can find dyno results online with popular engines, if your motor is custom built you have may have those numbers already and if you don’t, well you’ll just have to guesstimate. First, what kind of boat is it? Is it a lightweight pad bottom or a heavy center console? Second, what is the primary use, are you running this thing fast most of the time? 5000 RPM and more for sustained periods or are you running at cruising speeds with heavy loads? The reason the type of boat and how it is used is important is because generally, lighter more efficient boats can get away with running a taller gear (lower numeric gear ratio) without bogging the engine down. It is really simple, a lightweight efficient boat is easier to push than a heavy boat with lots of resistance. 

What does this mean? Find where your engine is making its power most efficiently and match that up with how you’re using your boat so that getting the most out of your prop. If you are having a hard time getting on plane and it takes a really high pitch prop to keep you in WOT operating range, then you could probably look at your gear ratio. (keep in mind, higher numeric is a lower gear. First gear in your car is a higher ratio numerically than 5th gear). A 1.62:1 ratio means the engine cranks 1.62 times for every prop shaft rotation.

By having a gear ratio that lets you use a prop that keeps your engine in it’s best power-band, where you run it the most, then you’re going to be more efficient overall. But, the biggest mistake is assuming a lower ratio (higher numeric) is going to give you a better hole shot; it's not necessarily. The reason is the prop gets you out of the hole more than the gear ratio. When in doubt, on an efficient hull, opt for the taller gear; that way you can run lower pitch props. This often means better acceleration, mid range efficiency and top end. Plus, not only will your prop be more efficient, but you'll have better prop selection.

Powerband

Find out where your engine makes its power as a guidline

So, yes, you can change your prop and change your operating range but if you’re running out of prop and bogging down at your cruising speed or you can spin big props too easily, you should consider changing your gear ratio. The popular adage is: your prop will get you out of the hole, not your gears. A high pitch prop will struggle out of the hole, and be less efficient overall.  

You can get a better idea of how efficiently you are running by using a prop slip calculator like the one on the Mercury Racing site here, or by doing the math yourself. 

Gear Ratio Example

As an example, imagine a prop pitch range you want to be in, let's say 25"

 As an example, 10% slip, 1.75:1 ratio, 25" pitch propeller. Theoretical speed of 95 MPH. 

As an example, 10% slip, 1.75:1 ratio, 25" pitch propeller. Theoretical speed of 95 MPH. 

If we adjust the ratio to 2.15:1, we see that a drastic change in pitch is needed

 Theoretically, you would need 30" pitch to come close to the 1.75 ratio performance. In reality, the dynamics of the 30" prop would alter the overall performance drastically, acceleration would suffer and so would overall efficiency. 

Theoretically, you would need 30" pitch to come close to the 1.75 ratio performance. In reality, the dynamics of the 30" prop would alter the overall performance drastically, acceleration would suffer and so would overall efficiency. 

A fantastic source and mandatory reading for any performance boater is a book called Everything You Need to Know About Propellers produced by Quicksilver. 
 

 A quintessential guide to understanding propeller dynamics and setting up your boat. 

A quintessential guide to understanding propeller dynamics and setting up your boat. 

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