Prop Shaft Design

In the 1950s, boating was booming. In response to a boom where standards were non-existent, one part of the U.S. Coast Guard's Merchant Marine Council -- the Motorboat and Yacht Advisory Panel -- formed the American Yacht and Boat Council to set standards of safety in the industry, including standards for propulsion, construction, design and materials. Its Standard P-06 addresses propeller shaft design in the areas of materials, straightness, shaft diameter and couplings.

  1. Materials

    • The Standard recommends shafts be made of one of seven different materials, conforming to various ASTM and federal Standards. The materials include three alloys of copper and four grades of steel. The first group, copper alloys, consists of naval brass that conforms to ASTM Standard B21, nickel-copper conforming to ASTM standard and an alloy of nickel, copper and aluminum, known as Ni-Cu-Al, that meets Federal Standard QQ-N-286.

      Naval brass is a mixture of nickel, copper and tin, while nickel-copper is a mix of those two metals. The yield strength of the nickel-copper is nearly twice – 44.75 percent – higher than the naval brass. The Ni-Cu-Al has greater yield strength than either of the others, with a per-cubic-inch weight only slightly higher – 0.306 pounds per cubic inch to 0.304 pounds per cubic inch – than naval brass.

      The steel alloys are Type 304, Type 316 and Type 630 steel that meet ASTM Standard A276. Type 410 steel hardend with martensite – martensitic stainless steel – is acceptable if it meets the mechanical properties of Standard A276.

    Shaft Straightness

    • The straightness requirement for a propeller shaft is based on the single revolution of a shaft supported once for every 42 inches of its length. The straightness specified by the ABYC Standard is based on the diameter of the shaft in 1/2-inch increments, beginning with a variation of 0.005 inch for shafts between 1/2 inch and 1 inch in diameter, going up to a variance of 0.008 inch for shafts 4 inches in diameter.

    The "Ideal" Diameter

    • Section 6.5.1 of the Standard says that the boat’s use is the primary consideration for the diameter of a propeller shaft. This means that a recreational boat won’t need the same diameter propeller shaft as a boat the same size that’s engaged in transporting cargo, saying that “boats intended for light pleasure service” don’t require as high a design coefficient as “racing crafts, work boats and diesel boats.” The design coefficient should be chosen based on the designer’s experience.

      The basic formula for shaft diameter first multiplies the engine’s shaft horsepower by the chosen design coefficient by 321,000, then divides that result by the yield strength of the shaft material times the shaft rpm. The cube root of the answer is the diameter of the shaft in inches.

      Additionally, the ratio of propeller diameter to shaft diameter should not exceed 15-to-1, to avoid excessive stress, in normal service. In high-performance service, a lower ratio – around 12-to-1 – should be considered.

    Coupling Design

    • Couplings -- the part of the propeller shaft that join the shaft to a marine engine's gear box -- are part of propeller shaft design. The ABYC standard addresses the matter of coupling design by reference, with an exception that puts the ABYC stamp on the process. The dimensions and tolerances of propeller shaft couplings are specified in the Society of Automotive Engineers Standard J756, "Marine Propeller Shaft Couplings," except that the ABYC standard specifies that, “key-way fillet radii are mandatory for all sizes of shafting.”