Number of Blades
The choice of the number of blades is one of the first decisions to be made in the design of a screw propeller.
Marine screw propellers usually have 3, 4 or 5 blades, of which four blades are the most common.
Two-bladed propellers are used on sailing ships with auxiliary power, as they offer the lower resistance when in the sailing condition.
The problem with two-bladed propellers for most vessels is that such propellers require very large diameters to get the blade area required for effective thrust.
Three-bladed propellers have generally proven to be the best compromise between blade area and efficiency.
Four or five-bladed propellers and even more blades are useful for two reasons. First, their extra blades create more total blade area with the same or less diameter. 4 blades propellers, however, would seldom be as efficient as the three-bladed because the closer blades create additional turbulence, literally scrambling up each other's water flow.
Another reason to use more than three blades is to reduce vibration. If a propeller is in the habit of producing annoying, rhythmic thumping and humming, a propeller with more blades will often solve the problem. Every time the blades of the propeller pass under the hull or by the strut, they cause a change in pressure that causes a push. If the push is strong enough, it generates a bang. Lots of rapid bangs equal vibration.
Conclusion: the less number of blades the more efficiency, the higher number of blades the smoothest and uniform performance. This must be always taken in consideration when selecting the proper Diameter, Pitch, Blade Area and Shape.
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