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?
The world’s new emphasis on economy and efficiency
is causing everyone to take a second look at 4-bladed props.
You should, too.
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.
If, however, for comparison purposes, we take two propellers, identical in design (blade shape, diameter, rake, cup, etc.) that is appropriate for a given application, and simply add a propeller blade, we get a truer representation of just where the difference lies. The addition of the extra blade causes increased drag, which, in turn, requires more horsepower, in order to achieve the same rpm. Since the horsepower is limited, the rpms drop, and the speed will tend to drop with it. This is why, when going from a 3-blade to a 4-blade, the pitch is dropped an inch, or more, in order to keep rpm parity. It is this difference in pitch that causes any potential speed differentials between the 3-blade and the 4.
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.
Designing the Solution
As we have learned more about what makes a propeller work and not work, and about how boats and motors perform and do not perform, we have designed propellers that address the inherent strengths AND weaknesses of each of these respective pieces of the puzzle. These designs have become even more important as more and more specialized boat designs and motor trends have entered the marketplace and as new trends and economic challenges arise. All that said, the most important part of the equation remains the customer’s performance expectation and satisfaction; so, the goal is to match the propeller to the boat, motor, AND the customer’s needs…as always, the right tool for the right job.