Blog 5b – Fourrrrrrrrrrrrrrrrrrr – a bad Golf Shot or not?
In this two-part issue of our Blog 5a & 5b: We are going to focus on the idea of 4 blade propellers that can be often incorrectly used interchangeably. If not done properly with the correct style/pitch/diameter, a 4-blade propeller can be a blessing or curse.
So, the 4th blade can provide increased water displacement capability, lift, and grip, as compared to the comparable 3-blade propeller. In terms of actual boat performance, the 4-blade can 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 better boating boat performance.
Then why might a 4-blade generally be slower than its 3-blade counterpart? Most of the time, 3 to 4-blade speed test are a lot of time, not fair or correct because the respective propellers in question are simply different styles, designed with different purposes in mind—different diameters, rakes, cupping, and blade shapes. However, if you 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, to achieve the same rpm. Since the horsepower is limited, the RPMs drop, and the speed will tend to drop with it. Therefore, when going from a 3-blade to a 4-blade, the pitch is dropped an inch or two 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 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.
Hopefully this has completely confused your brains (welcome to my world) – Have a great time on the water. The PropTologist