Stainless Steel vs. Aluminum
The most common misunderstanding is stainless steel propellers out perform aluminum propellers. Only about 10% of performance has to do with the material. The other 90% has to do with the propeller design. A well designed aluminum propeller will out perform an average stainless steel prop. Now don't misunderstand what I am saying, typically stainless is better. If you take two identical props, one stainless and one aluminum, the stainless will perform better due to the fact it does not flex nearly as much as aluminum. What does all this mean to the average boater?
Stainless is about 5 times harder than aluminum which helps protect it against sand, small rocks or loose objects in the water so your propeller will last longer with normal use. With aluminum, every time you go through a little sand the likelihood you will pick up some dings and scratches is much greater than with stainless. Since stainless is so hard, it does not give near as much when a collision with the bottom occurs.
With little give or flex, a stainless will out perform its aluminum counterpart. With little, if any, flex your performance is increased.
Three vs. Four Blade
In the past, most boaters shied away from four blades mainly because they thought they were slower than 3 blades. Typically they were. When gas was cheap, everyone wanted to go faster. Now with changes in the market place and new advances in propeller technology boaters are changing the way they think about four blades. Who benefits from four blades? Almost everyone does. A four blade will give you a better hole shot, get the stern up faster, get you up on plane quicker and have a lower plane speed (translates into gas savings), give better grip with the extra blade, vibrate less that a three blade and generally perform better than a three blade.
Everyone wants a better hole shot. By having an extra blade in the water, the boat will come out of the hole better. This will enable the boat to get up on plane easier and once there, it will have a lower plane RPM (again more fuel savings). Typically, this is where most boats are ran, at their mid range RPM. This increases efficiency and saves fuel. The extra blade will give you better grip and the more blades on a propeller the less vibration felt in the boat. What's the down side?
You will lose some top end speed, possibly 1 to 4 miles per hour. That’s not much considering the gain in all other areas.
How to choose the right propeller
The best propeller for your boat/motor combination will result with you being able to run your motor at wide open throttle and remain within your manufacturers recommended RPM range. That being said, you really want the propeller to run at the upper end of the wide open throttle RPM for maximum performance. How do we adjust for that, by changing the propeller. Changing the propeller to maximize your motors performance is a lot like shifting gears on a bicycle. In low gear (smaller pitch), you can take off faster (better hole shot), but you have limited RPM (how fast you can peddle). As you shift gears (go up in pitch) you can go faster, but it takes more RPM (harder to pedal). Remember the golden rule, if you go down in pitch your RPM will increase; as you go up in pitch, your RPM will decrease. The calculation for this is for every inch in pitch you change, your RPM will change approximately by 200. If you want greater RPM, go down in pitch. If you want to reduce your RPM, go up in pitch. Most propeller manufacturers increase and decrease their propellers in 2” increments. So for the typical prop size you will increase or decrease by 400 RPM.
Once you have a propeller that runs within the recommended RPM range, you have a prop that is suited to your engine/boat combination with regards to RPM. After that, if more performance is required, you need to address the design or geometry of the blade. Large blade, small blade, cleaver style, etc. the list goes on. That’s when you need to speak to a person that understands propeller design.
The main point out of all this is to understand that the propeller is the number one thing that anyone can change easily that has the greatest effect on your engine. It all goes back to understanding the trade off of pitch. If you drop in pitch your RPM will increase. As you go up in pitch, your RPM will decrease.
Terminology:
Propeller Size: Prop size is determined by diameter and pitch. When speaking of prop size, it is given as two numbers. The first being the diameter and the second being the pitch. For example: 14 x 21 translates into 14" diameter by 21" pitch.
Diameter: The distance from the center of the hub to the tip of a blade multiplied by 2. So the above example would have a radius of 7" from the center of the propeller to one tip. Multiplied by 2 = 14".
Pitch: Technically speaking, pitch is the theoretical distance a propeller will travel through the water in one revolution with no slippage. Looking back to our example again, the 14 x 21 propeller will theoretically travel 21" in one complete revolution.
Cupping: Many propellers today have cupping. Cupping is the curved lip on the trailing edge of a propeller. What does it do? It helps reduce ventilation, gives better grip, more top end speed, and a better hole shot in many cases.
Rake: Rake is the degree that the blades slant forward or backward in relation to the hub. What does it do? It affects the water flow through the blades. Aft rake helps keep the bow up resulting in less wetted surface. Aft rake props also bite better and are used in many applications where the engine is mounted high on the transom.
Ventilation: Often confused with cavitation, ventilation is surface air or exhaust gases drawn into the prop. This occurs when the engine is riding near the surface or sometimes on tight turns. When this happens the RPM goes up and you lose speed.
Cavitation: is when water vaporizes or boils do to the extreme reduction of pressure on the back side of the blade. Some cavitation issues can cause damage to the propeller over time such as cavitation burns.