Most fishing and pleasure boat motors come from the dealer with an aluminum prop, or propeller. Most boat owners consider investing in a stainless steel propeller. There are advantages and disadvantages to each type, and the details sometimes seem confusing. However, with basic information, you can make the decision much easier.
Stainless is expensive
The primary reason that boat motor manufacturers install aluminum props at the factory is price. Aluminum propellers for most boat motors typically cost between $100 and $150, while a stainless steel propeller for the same motor usually costs twice that much.
Because stainless steel is stronger than aluminum, the blades can be made thinner. This creates less drag on the water when the prop is turning, allowing the boat's engine to run at a higher RPM at full throttle and making the boat go faster. On a 25 horsepower motor, the difference is small. On a 225 horsepower engine, however, installing a stainless steel propeller often means a 3 to 5 mph increase in boat speed.
Because of its strength, steel allows a feature called "cup" to be designed into a propeller. According to the boatbuilding.community, with a cup "a curved lip is added to either the leading edge or the tip." Also, "Cupping can increase the prop's grip on the water (less slip) and decrease blow-out (a condition caused by excessive aeration)." The result is that a boat fitted with a stainless steel propeller handles better.
Stainless steel is far less likely to be damaged upon impact than aluminum. A common misconception is that stainless steel propellers cause damage to the propeller shaft or lower unit of a boat motor in the case of an impact. Modern stainless steel propellers contain a shock-absorbing rubber hub. According to the British Navy Manual of Seamanship, this "allows the propeller to slip on the shaft to reduce damage if it encounters an underwater obstacle."
If an aluminum propeller is significantly damaged, it usually cannot be repaired. Stainless steel propellers can be welded, ground and polished by a repair shop, oftentimes for less than the cost of an aluminum propeller, and will work like new.