The Aeroplane Speaks. Fifth Edition

For a given pitch, the gap of a four-bladed propeller is only half that of a two-bladed one. Therefore the four-bladed propeller is only suitable for large pitch, as such pitch produces spirals with a large gap, thus offsetting the decrease in gap caused by the numerous blades. The greater the speed of rotation, the less the pitch for a given aeroplane speed. Then, in order to secure a large pitch and consequently a good gap, the four-bladed propeller is usually geared to rotate at a lower speed than would be the case if directly attached to the engine crank-shaft. [Footnote 18: I have heard of temporary ones being made quickly by bending strips of lead over the convex side of the blade, but I should think it very difficult to secure a sufficient degree of accuracy in that way.]