Aluminum vs Stainless Steel Propellers

The propeller choices are unending.

The choice of propellers for outboard motors is astoundingly vast and confusing. Besides diameter, pitch, and number of blades, there is also the question of aluminum vs stainless steel construction. Many boat owners might just accept the propeller recommendations from the outboard maker. Many times, this works out just fine. Sometimes it does not, and merely serves as a starting point toward zeroing in on the right propeller.

In general, any outboard motor less than 150 hp should be running an aluminum propeller. They are dramatically lighter than their stainless counterparts, and won’t load down the motor by stealing horsepower from the shaft. A heavy stainless prop is just a bit much for the smaller motors, and horsepower is too precious for those motors to lose on a heavy propeller.

In general, any outboard motor 200 hp or more should be fitted with a stainless steel propeller. This is especially so if you will be running wide open throttle at significant distances, like offshore and such. The prevailing reality is that the forces generated at the prop shaft by these bigger motors at higher rpms will actually cause the aluminum propeller blades to flex and briefly lose their shape. That will lead to performance losses, and brief losses of thrust. Other issues could include ventilation and possibly cavitation (we will cover these phenomena in another post). These issues affect fuel efficiency as well, of course. Only the stronger stainless props can perform well under the higher stresses of the bigger outboard motors.

Let’s get into some specifics:

-          Offshoring. Stainless steel are the preferred propeller, even for outboard motors as low as 150 hp. Turning high RPMs is a strong suit for the stainless propellers.

-          Flats fishing. Many folks will insist on stainless steel propellers, since they are far more durable than aluminum propellers. This is indeed true, and ground contact will dink up the aluminum quite easily.

-          Toys and Skiing. Stainless steel will certainly be a great match, especially in getting you the best hole shots.

-          Racing. Of course, stainless all the way. Not even a question.

As far as ground contact is concerned, we will challenge the wisdom of sticking with a stainless steel propeller for these scenarios. The type of bottom that you are coming into contact with matters, and it matters a lot. Here on the Gulf coast of Florida, our sandy bottom is soft, but we sometimes come into contact with those partially buried conch shells, and they are hard as rock. A stainless propeller will stand up fairly well when coming into contact with one of those shells, but think about the shock that gets transmitted to your lower unit, and your trim and tilt system as well. Outboard motor lower units and trim/tilts are profoundly expensive to service and repair. An aluminum prop will absorb the shock by bending/breaking, and it will be forgiving on your outboard motor. The "give" of the aluminum propeller protects the lower unit and trim/tilt as well. Remember that the cost of an aluminum propeller is a fraction of what stainless costs. You can literally buy 3 aluminum propellers for the cost of the same size propeller in stainless steel. Better to replace a $100 propeller, than a $3000 lower unit.

For our charter pontoons that spend the majority of their time is less than 2 feet of water, we use aluminum exclusively. Our boats rarely turn wide open throttle, and we do not go offshore, so the performance disadvantages of the aluminum are not something that impacts our boats.

We have not touched on propeller "tuning", or the art/science of finding the right combination of diameter, pitch and blades optimized for your motor/hull/activity. We will cover that in a separate post.

WHERE YOU GO FROM HERE:

Yamaha or Mercury Outboard? - Is one better than the other?

How Much Horsepower? - how many horses is enough for your boat?