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Contents
outboard propellers comparison, in stock
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Reading the numbers: diameter and pitch
Every propeller is described by two figures, such as 9 1/4 x 12. The first is the diameter: the circle traced by the blade tips, in inches. The second is the pitch: how far the prop would move in one full turn if it did not slip in the water, also in inches.
Pitch is the key. A higher pitch gives more top speed but less acceleration; a lower pitch accelerates better and pushes harder under load, but limits top speed. Each inch of pitch is worth roughly 150-200 rpm at full throttle.
How to tell whether the pitch is right
At wide-open throttle and with the boat loaded as you normally use it, the engine should reach the maximum rpm range given by the manufacturer. If it does not, the pitch is too high. If it goes over, the pitch is too low and the engine is overworked.
| Symptom | Likely cause | What to try |
|---|---|---|
| Engine will not reach max rpm | Pitch too high or boat heavily loaded | Go down one inch of pitch |
| Over-revving, little forward drive | Pitch too low or prop ventilating | Go up one inch of pitch |
| Struggles to plane with a load | High pitch or low thrust | Lower pitch or more blades |
| Vibration | Bent blade or off-center prop | Inspect or replace |
Aluminum, stainless steel or composite
- Aluminum: inexpensive and common on small and mid-size engines. It absorbs impacts by bending and is easy to repair, but it deforms easily and wears faster in heavy use.
- Stainless steel: stiffer, with thinner blades and better performance, especially at high speed. It costs more and, in a hard hit, can damage the lower unit instead of the prop.
- Composite or engineered plastic: light and corrosion resistant; common on low-power motors and on some spare safety props.
For a small recreational outboard, aluminum is usually the sensible choice. Stainless pays off if you boat often, want performance or regularly carry heavy loads.
Blade count, rotation and shaft
Three or four blades
Three-blade props are the most common and perform well at speed. Four-blade props hold better when planing, run smoother and work well with heavy loads or slow-turning engines, at the cost of a little top speed.
Right-hand or left-hand rotation
Most single-engine outboards turn right-hand. Twin installations or special setups may need counter-rotating props, so always confirm the direction before ordering.
Shaft and hub
The prop must fit your shaft exactly: number of splines, shaft diameter, hub type and retention (nut with cotter pin, locknut or cross pin). Note the make, model and horsepower of your engine and look up the manufacturer's part number.
Warning: a prop for the wrong shaft may seem to fit and then fail under load. If you are unsure, compare the spline count against the listing or your engine manual.
How to change it step by step
- Disconnect the spark plug lead or battery and shut off the fuel.
- Remove the cotter pin or nut and slide the prop off the shaft.
- Check for fishing line wrapped around the shaft and for oil leaking past the seal.
- Grease the shaft with marine grease before fitting the new prop.
- Install the thrust washer, spacer and nut (new or inspected). Use a new cotter pin.
- Test with the boat in the water and note the maximum rpm reached.
Tip: always carry a spare prop with its hardware kit. If you hit a shoal or a line, swapping it on the water is far quicker than paddling home.
Care and common mistakes
- Inspect the blades after every strike. Even small damage causes vibration.
- Remove line, weed and netting from the shaft before they damage the seal.
- Do not use the prop as a step or drag it over the bottom.
- Check the engine's sacrificial anodes while the prop is off.
- Buying by diameter alone, without checking pitch and shaft, is the most common mistake.
- Do not raise the pitch without checking the engine's maximum rpm.
If you are changing engines, see our guides to gas outboards and electric motors to choose the right power.
Frequently asked questions
What do propeller sizes like 9 1/4 x 12 mean?
The first number is the diameter in inches and the second is the pitch, meaning how far the prop would advance in one turn if it did not slip. A 9 1/4 x 12 is 9.25 inches across with a theoretical 12-inch pitch.
How do I know what pitch I need?
With the boat loaded as you use it and the throttle wide open, the engine should reach the manufacturer's maximum rpm range. If it falls short, go down one inch of pitch. If it over-revs, go up.
Is an aluminum or stainless steel prop better?
Aluminum is cheaper and bends instead of damaging the engine on impact. Stainless performs and lasts longer but costs more and is stiffer. For recreational use, aluminum is usually enough.
Is a four-blade prop better than a three-blade?
It depends on use. Four blades hold better when planing, vibrate less and work well with a load, though they give up a little top speed. Three blades are faster and more common.
