Published on Mar 22, 2010
The right propeller can help your boat reach its performance potential, while the wrong prop can inspire you to take up another hobby.
We use numbers to describe propellers: the first number is the diameter, and the second number is the pitch.
If you could draw a circle around a propeller’s blades, diameter would be the distance, (in inches) across this circle.
Pitch is the theoretical distance, in inches, that a propeller moves the boat forward during one revolution. (Theoretical distance, because the propeller operates in a liquid medium—water—it slips a little bit).
So, a 14 X 19 propeller has a diameter of fourteen inches, and a pitch of nineteen inches. In conversation, most people would describe this propeller as a 19-inch pitch, or a 19, leaving off the diameter altogether.
Pitch & RPM
The correct propeller will allow your boat’s engine to reach its maximum recommended RPM (revolutions per minute) at full throttle, trimmed up for best speed, with a typical load in the boat.
You say you don’t run your boat at full throttle? We usually don’t either, but if the propeller will let the engine hit its peak RPM at full throttle, the propeller’s performance should be good all the way across the engine’s RPM band: planing, mid-range cruise, etc.
If, at full throttle, the engine’s RPMs are substantially less than the manufacturer’s recommended full throttle RPMs, this indicates that the propeller probably has too much pitch (may need a lower pitch ).
Conversely, if the engine wants to grossly exceed the recommended full throttle RPMs, we’d suspect that you could use a propeller with more pitch.
This is the tricky part:
- Generically, every inch of pitch is worth about 200 RPM
- Increase pitch an inch and the full throttle RPMs should decrease by 200 RPM.
- Decrease the pitch an inch and the full throttle RPMs ought to increase by 200 RPM
Let’s say the engine in our boat is rated to operate at between 4600-4800 RPMs at full throttle.
When we tested the boat at full throttle, the 19-inch pitch propeller let the engine exceed the rated full throttle RPMs. So, we removed the 19-inch pitch prop and replaced it with a 21-inch pitch propeller (most propellers come in 2” increments of pitch).
Now the full throttle RPMs should decrease about 400 RPMs (1” pitch=200 RPMs, thus 2” pitch=400 RPMs).
Let’s use a different boat, with an engine rated to operate at 4600-4800 RPMs at full throttle.
We run the boat at full throttle, but we only see 4400 RPMs, with a 19-inch pitch propeller. On this boat, we take off the 19-inch pitch prop and replace it with a 17-inch pitch propeller.
Running the boat with the 17-inch pitch propeller, we now see that the engine reaches 4800 RPM at full throttle—an increase of 400 RPM at full throttle.
Aluminum vs. Stainless Steel
Most boats come from the factory with an aluminum propeller. Aluminum propellers are inexpensive, provide good performance, and are relatively easy to repair—but aluminum’s not very durable.
Stainless steel propellers are more expensive than aluminum props. However, because stainless steel’s much stronger than aluminum, a stainless propeller has thinner blades and can be made in a wider variety of blade shapes/styles than an aluminum prop—offering the potential for better performance.
Stainless steel is also more durable than aluminum, resisting nicks and dings better than an aluminum propeller.
Be advised that it’s stain-less steel, not stain-proof—a stainless propeller can still rust.
Three Blades vs. Four Blades
In the broadest of terms, three blade propellers offer good overall performance and the fastest top-end speed—but a three blade prop may tend to lose its grip in hard turns, and may not be the best handling propeller at top speed.
Four blade propellers can get a boat on plane faster, keep a boat on plane at a lower speed, don’t lose traction in turns, and make a boat handle better—but a four blade propeller is often a bit slower (1-2 MPH) at top speed than a three blade prop.
If you can understand and retain the information we’ve presented here, you’re on your way to becoming a propeller pro.