Which is Better: 4 Blades or 3 Blades? - 04/08/2009
For many years now, boaters have debated the question of 4-blade propellers versus 3-blades. The traditional argument is that 4-blades are slow and 3-blades are fast, end of discussion. Today, new trends in the marine marketplace, higher fuel prices and challenging economic times are causing boaters to take a second look at this old debate. Speed is now a much smaller part of the boating equation, and now practical, real world performance is the name of the game for most people. Which props should you be using, 4-blade or 3?
In the beginning, the idea that 4-blades were slower than 3-blades was true to a certain extent, but that is because many of those 4-blades were not designed with speed in mind. Instead, the few designs that were available were intended to cure handling issues such as ventilation, cornering blowout, motor elevation requirements, and hole shot issues. Without many options in blade design, and very few of them truly intended to be particularly fast, the 4-blade got branded as slow, while their other performance benefits were largely dismissed.
By contrast, today’s 4-blade propeller designs have evolved into both all-purpose and highly specialized geometries. These propellers can be tailored to not only address those traditional handling issues, but can also be tailored to improve a boat’s performance envelope, which can, in some cases, even include speed.
Reasons for 4 Blades
The increased blade area afforded by the addition of the fourth blade can provide increased water displacement capability, lift, and grip, as compared to the comparable 3-blade propeller. In terms of actual boat performance, these characteristics can combine to enhance handling, hole shot, low-speed planning ability, cruise efficiency, fuel efficiency, load-carrying performance, big seas performance, following seas/down current performance, ventilation/cavitation resistance, motor elevation capability, etc. In short, a 4-blade propeller can improve all those characteristics that make for practical, all-around boat performance.
But are 4-Blades Slow?
So, why might a 4-blade generally be slower
than its 3-blade counterpart? To be honest, many 3-blade/4-blade speed comparisons
are simply not fair. That’s because the respective propellers in question are simply
different styles, designed with different purposes in mind—different diameters,
rakes, cupping, and blade shapes.
As to any actual speed loss between the two,
in many cases, it is actually quite small (generally 1-3 mph). The reason is, although
the 4-blade is one-inch lower in pitch, it runs more efficiently than its 3-blade
competitor, allowing it to run closer to its theoretical speed than the 3-blade,
thereby, effectively closing the gap presented by the pitch differential.