The “Best Propeller” Decision
Published: Tuesday, August 4, 2020
So now that you have figured out the best boat for your specific interest and needs, how do you know if you have the best propeller for your application? Your new boat will typically be equipped with the right propeller… for general use… by the average user. This starting point is necessary to provide the best overall performance for most users.
There are, however, a great number of propeller designs that will fit each boat / motor application and may very well provide an opportunity to dial-in the boat’s performance to better meet your specific set of needs, desires, and circumstances. There are many propeller options in diameter, pitch, number of blades, materials, rake, blade shapes and areas, to name a few.
Let’s examine the multiple variables that should be considered in the use of your boat that will impact your propeller decision. Specifically, you should take into account your individual priorities (speed, fuel economy, towing performance, loading etc.) and optimize your boat’s performance to meet these requirements. The equation ultimately has to balance when considering available energy or power and the work that you want that power to perform.
The propeller is the final interface between your boat-engine combination and the water—where the proverbial rubber meets the road. The propeller controls traction (slip) and ultimate gear ratio (pitch) to result in the transmission of energy from the motor and drive into the boat’s motion. The characteristics of the boat’s performance depend heavily on the propeller’s design elements.
For example, a boat with a performance bias toward top speed will likely have to make some compromises in hole-shot and heavy load performance. If your pitch is too steep, the engine will struggle to get your boat onto plane and into its peak horsepower-band. However, if the boat is light enough to get there, exceptional top speeds can be realized. If a boat’s performance is biased towards towing and heavy loads, the propeller will typically have larger blade areas and a lower pitch. This boat may jump onto plane but run with engine RPM’s too high and not reach top speed potentials.
Increasing the number of blades will increase blade area and therefore improve traction/reduce slip. These are positive if you want more load capabilities for towing and cruising. Conversely, the extra blade increases drag on the propeller at high speeds and can become a deterrent if your objective is high speed.
Aluminum and bronze propellers are typically sufficient for general purposes and provide very economical propeller solutions. When higher levels of performance are desired, changing to higher strength materials, like stainless steel and NiBrAl (nickel, bronze, aluminum) allow propellers to transmit power through thinner section, more efficient designs. The cost element has to be considered as the higher strength materials naturally cost more.
The bottom line: the “best propeller” for you and your boat is not necessarily the best propeller for everyone. It is the “best propeller” for YOU. It is a very personal decision with many factors to consider. A detailed discussion with your NMPA (National Marine Propeller Association) propeller professional will certainly help you in making the right decision—for YOU.
This article first appeared in the Summer Issue (Jul/Aug) 2020 of Great Lakes Scuttlebutt magazine.
tags: Boating 101, Propellers, Retrofitting, Service & Repair
