…To Four, or Not To Four…That is the Question
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, challenging economic times, and the like have all brought new concerns to the boating world. Speed is, now, a much smaller part of the equation; while practical, real world performance becomes the consumers primary concern. In this new modern age, the question arises anew, but this time, with a different answer.
MYTH vs. LEGEND
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 holeshot 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.
THE HOW AND THE WHY
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, holeshot, 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.
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 differential 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.
As the result of many of today’s trends in the marine marketplace, 4-blade propellers have become more popular than ever. As fuel prices have risen, fuel efficiency and cruise efficiency have become prime concerns for boaters, and 4-blades can offer improvements to both. Other trends in the marine industry, such as, the move toward 4-stroke motors with their heavier weights and higher numerical gear ratios, shallow-flats boats, tunnel boats and cat boats with their ventilation-prone running characteristics, ski-boats with their pulling requirements, heavier deep-V offshores with their big seas control requirements, can all benefit from the additional grip, lift, and efficiency of 4-blade propellers. In short, 4-blade propellers have become almost indispensable in maximizing boat performance and customer satisfaction.
Where customers were once resistant to the idea of a 4-blade propeller, we are actually finding that customers come to us asking if their applications will benefit from the use of a 4-blade propeller. In many of these cases, the answer is a resounding, “yes.”
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.