Diameter is the distance from the center of the hub to the tip of one of the blades times two. Typically you want to use the same diameter size prop that your engine came with. However, diameters between different propeller manufacturers may vary slightly. For example, one manufacturer might use a 14-1/4" diameter where another manufacturer may use a 14-1/2" diameter. This small variance does not affect your performance as much as the pitch and overall design of the blades. So, although you might need to get a different size diameter, it is recommended that you stay within ½" of your current diameter. Diameter is the first number listed in the propeller description. For example 14-1/4 x 19 (14-1/4 is the diameter, 19 is the pitch.)
Pitch is defined as the theoretical forward movement of a propeller during one revolution assuming there is no "slippage" between the propeller blade and the water. For example, a 21 pitch propeller will theoretically move 21 inches forward with every revolution. Propeller slip occurs with every propeller, but the amount of slip varies depending on propeller design. More aggressively and efficiently designed propellers will slip less.
When selecting a propeller pitch for your boat, it is important that the propeller runs at the upper end of your engines wide-open-throttle RPM range. Find the manufacturer’s recommended RPM range in the owner’s manual or ask your dealer. If you want your RPM's to increase, go down in pitch. To decrease RPM's, go up in pitch. As a general guide, for every 2" of pitch, RPM's will change approximately 400 RPM's.
For water sports or extra people on board, you should generally drop 2" of pitch to help compensate for the added weight and drag on your boat. It makes a noticeable difference in your boat's hole shot, fuel efficiency, RPM's, and overall performance. You should ALWAYS carry a spare propeller on board, and if you're into water sports or occasionally load the boat with extra people, a spare prop with a lesser pitch is a good idea. Propeller pitch can be compared to a gear on a car - lower gear, higher RPM's. Pitch is the second number listed in the propeller description. For example 14-1/4 x 19 (19 is the pitch, 14-1/4 is the diameter.)
Aluminum vs. Stainless Steel
Many people argue that stainless steel propellers perform better than aluminum propellers. This isn’t always true. Material (Stainless vs. Aluminum) only accounts for approximately 10% of the actual performance of the propeller. The other 90% of performance is in the blade design. Well-designed aluminum propellers will outperform an average stainless steel propeller. The advantage of a stainless steel propeller over aluminum is durability. Stainless props can withstand more of the damage caused by small rocks, sand, or other loose objects in the water.
However, the disadvantage to a stainless propeller is that there is “minimal give" to the blades, so if you hit an object hard enough, there is the possibility of causing major damage to your lower unit. With aluminum props, the blades will most likely sacrifice themselves before any damage is caused to your lower unit.
If you run in deep or familiar waters, or in salt/brackish water, a stainless steel propeller can be a good choice. But it is important to always carry a spare propeller on board, and a spare aluminum is an inexpensive choice.
Cavitation, (which is often confused with ventilation), is a phenomena of water vaporizing or "boiling" due to the extreme reduction of pressure on the back of the propeller blade. Many propellers partially cavitate during normal operation, but excessive cavitation can result in physical damage to the propeller's blade surface due to the collapse of microscopic bubbles on the blade.
There may be numerous causes of cavitation such as incorrect matching of propeller style to application, incorrect pitch, physical damage to the blade edges, etc...
a spare propeller in case of emergency
You carry a spare tire for your car, so why don’t you have a spare prop for your boat? Don’t let a good time on the water be ruined by a damaged prop. Order a spare prop today and stay on the water longer tomorrow. And don't forget the tools... If you have a Mercruiser drive, a combination prop wrench will do the trick. For OMC, Evinrude and Johnson, a pair of good plies will be needed to remove the cotter key. A 10 inch block of 2X4 makes a great block between the prop blade and ventilation plate will make it easy to loosen and tighten the prop nut. Be sure to tighten the prop nut really tight.