How to Choose a Prop
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| The optimum propeller selection for your particular application is one that will allow your boat's engine - with your normal load aboard - to develop
full-throttle RPM within the engine manufacturer's recommended maximum RPM range.
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Recommended RPM
Every engine manufacturer states that range as part of the engine specifications. Damage to the engine can occur if a propeller that allows the
engine to over-rev (attain a full-throttle RPM higher than the specification) is installed. Damage can also occur if the installed propeller causes
the engine to lug - not reach the minimum specification. Overall best acceleration and top-end speed will be achieved when your full-throttle
RPM is near the midpoint of the manufacturer's recommended maximum RPM range.
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Matching Prop to Engine
Outboard motors 30 HP and higher, Sterndrives and Inboards require the most attention to correct propeller selection. The smaller outboards
come with all-purpose props that meet most requirements. An onboard tachometer to monitor engine RPM is also commonly installed with the
larger engine installations. Most boats are delivered from the dealership or factory with propellers that have been properly matched for the most
common usage. Make note of the part number that is usually cast into the propeller itself, and you'll be forever able to order an exact replacement
using our Boat-Prop Finder.
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Pitch & Diameter
The two important propeller dimensions that you'll need to consider are pitch and diameter. In most cases the propeller and engine manufacturers
have pre-selected the diameter range needed for your engine. Your propeller selection can be focused almost exclusively on pitch. Your boat will
respond to different pitch propellers the way your car responds to different gear positions in your car's transmission. Moving to a higher pitch propeller
will result in a decrease in your full-throttle RPM, just like the effect when your car's transmission shifts to a higher gear. The reverse is also true: move
to a lower pitch propeller and your full-throttle RPM will increase. Most propellers are sized by two-inch intervals, example 17", 19", 21", etc, and a 2"
pitch change will result in an average 300 to 400 full-throttle RPM change.
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Spare Props
It's always a good idea to keep a spare prop aboard, but here's a way to get double-duty from that investment. If your family is into tow watersports, it's
likely that your boat's performance can benefit from a prop change to one of 2" less pitch whenever you head out for a day of skiing. The improved acceleration
will really be appreciated by the water skier you're pulling. Just don't forget to change back to your standard prop on those days that you're just out cruising.
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Prop Material
Most propellers are constructed of either aluminum or stainless steel, and utilize a softer hub material to protect your engine's drive train. Aluminum propellers are
less costly, but more prone to damage. Stainless steel propellers are usually about 3 times more expensive than their aluminum counterparts, but far more durable.
Some boaters also experience better overall performance with stainless steel because the material's tensile strength allows the manufacturer to cast the blades thinner.
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Propeller Basics
Prop Characteristics
Determining RPM's