Propellers - The Basics: Part 1: Get to Know Your Propeller

Propeller Basics

The propeller. It's that metal thing that spins on the back of your boat to make it move. It is often overlooked by some boaters - if they have one, and the boat works, that is all that matters. For other boaters, it is a long, sometimes frustrating process to find that "perfect" prop for their boat. For boat racers, it is one of the essential techniques of "tuning" their boat to the race course and water conditions.

What are the differences between different propellers? What type of changes can you expect when changing to a different propeller? What does some of the propeller terminology really mean? In this feature, I will discuss some of the terminology related to propellers. My next feature will discuss some of the variables that have to be considered when selecting a propeller. I will also discuss basic propeller maintenance and inspection.

Pitch and Diameter

The pitch and diameter are the two most common dimensions of a propeller. The diameter is self explanatory, and is fairly easy to measure. The pitch is the theoretical distance that the propeller moves forward for each revolution, usually measured in inches. The actual distance the boat moves forward for each propeller revolution is somewhat less, depending on the amount of propeller slip.

Aluminum vs. Stainless Steel

Most pleasure boats are factory equipped with aluminum propellers. A good quality aluminum propeller will provide satisfactory performance for most family and fishing boats. Aluminum propellers are relatively inexpensive, easy to repair, and under normal conditions can last for many years. If you are happy with the performance of your prop, and it is in good condition there is no need to change it.

If you are looking for better performance than can be provided by your aluminum propeller, such as ultimate top speed or better acceleration, a stainless steel propeller will probably be required. Stainless steel is much stronger than aluminum, and can consequently withstand far more abuse.

3 Blade vs. 4 Blade

The 3 blade prop has been the design of choice for performance boaters for many years. It works well, provides good acceleration and control and excellent top speed performance. In the past few years, however, the 4 blade propeller has been gaining in popularity due to it's different handling and performance characteristics.
The 4 blade configuration is popular with boaters who are looking for a great hole shot, good acceleration, and strong, predictable low to mid range handling, at a possible expense of top speed.

This group of boaters includes certain types of fishing boats, ski boats, and boats that are used in rough water conditions.

In general, when replacing a 3 blade propeller with a 4 blade propeller of the same diameter and pitch, expect a loss of 50 to 100 rpm at full throttle (with a corresponding decrease in top speed). This is due to the fact that 4 blade propeller has slightly more drag in the water.

Through Hub Exhaust

Through hub exhaust and over hub exhaust propellers are used on boats where the exhaust passes out through the rear of the "torpedo" on the lower unit, around the propeller shaft. Most outboards utilize this type of exhaust.

Non through hub exhaust propellers are used for inboards using shaft driven propellers, sterndrives using through hull exhaust, and on some outboards that don't route the exhaust through the lower unit torpedo.

Through hub exhaust propellers consist of a round barrel to which the blades are attached. The exhaust passes through the barrel and out the back, without making contact with the propeller blades. This provides a good clean water flow to the blades, usually resulting in good acceleration and hole shot.

Over hub exhaust propellers have the blades attached directly to the smaller tube that fits over the propeller shaft, eliminating the larger exhaust tube. These types of propellers are often used for attaining maximum top speeds. The hole shot can often suffer (at least on some boats) due to the extreme exhaust flooding that occurs around the propeller blades during acceleration. On some engines, especially those with poorer low end torque, this can be an advantage as it allows the engine to gain RPM before the propeller really starts to "grab".

There is a third design of propeller hub - a combination over/through hub design. This actually consists of a propeller with a through hub design having holes drilled through the hub ahead of the blades. This allows some exhaust to escape at lower RPMS, providing a controlled amount of exhaust flooding. This will allow the propeller to be slightly easier to turn during initial acceleration, allowing for a better hole shot on some engine/boat combinations.