Pitch is becoming an increasingly familiar term to boating enthusiasts interested in purchasing aftermarket outboard propellers for their watercraft.  Usually defined as the distance a propeller will move through a soft medium such a balsa wood after one full turn of the blades, pitch is an idealized figure that is rarely seen in actual use of outboard boat props.

Boaters, therefore, might question the purpose of a pitch specification for a given propeller.  If the prop is advertised as offering 13 inches of pitch, but this is a theoretical number, what can a boater expect in terms of real world performance?

Fortunately, there is another term that helps to clear up this conundrum: slip.  When it comes to outboard boat propellers, the word “slip” gives the difference between the theoretical and the actual.  That is, slip describes the degree to which a given propeller usually achieves its stated pitch.  A propeller that is properly installed and appropriate for both motor and boat can usually achieve as much as 90 percent of “perfect pitch,” though in some cases a figure closer to 80 percent is more realistic.

Why Not Just Scale Down Pitch Numbers?

The concept of slip usually raises a simple question: why do propeller manufacturers use idealized pitch numbers in the first place? Why not just multiply ideal pitch by 80 or 90% and use that figure as the pitch? The answer to this lies in the complexity of watercraft usage.  Boats will achieve closer to their ideal pitch when conditions are optimal.  Slip, therefore, depends on context whereas pitch values remain constant.

Tags: Outboard Propellers, propeller pitch

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