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How to make a wooden propeller
by Philip Thiel

Laminated plywood propeller, 16"diameter x 24" pitch

    The following notes describe a method for constructing a propeller of epoxied laminations of marine-grade plywood, suitable for the low power and rpm of human-powered watercraft, and within the capabilities of one competent in elementary geometry and basic woodworking. We will illustrate the procedure as applied to the making of a three-bladed "right-hand" propeller, one which rotates clockwise in driving the boat ahead when viewed from behind the boat, with a 16-inch (400-mm) diameter and a 24-inch (610-mm) pitch, based on the Troost B3.35 model of 0.35 developed-area ratio to absorb 1/5-hp (150 watts) at 240 rpm and produce about 13 lb. (58 N) of thrust in open water at 4.2 knots (2.2m/s) with 80% efficiency (1). The same procedure, of course, may be used for the construction of propellers of similar characteristics and other dimensions.
    As is the case with most propellers we will use a helicoidal surface for the "face", or after side, of the propeller blade. This helicoidal surface is generated when a straight line (the "element") revolves with uniform speed about an axis through one of its ends and at the same time moves with uniform speed parallel to itself along the axis. Any point of the straight line then generates a curve in space called a helix, which lies on the surface of a co-axial right circular cylinder. This distance along the element between the axis and the given point is the radius, r, and the distance this point moves parallel to the axis during one revolution (360 degrees) is the pitch, H. The successive postitions of the element constitute the helicoidal surface.

Philip Thiel  *  Sea/Land Design  *  4720 7th Avenue NE   *  Seattle WA 98105 USA
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