DIY Boat Propeller

Boat propellers push a boat through the water through screw action: as the propeller turns on a shaft, its curved blades cut into the water like a screw, pushing the boat through the water. The larger the blades and the faster the shaft revolutions, the greater the boat's speed through the water. Although naval architects spend millions of dollars designing ever more efficient propellers, many do it yourself hobbyists can build serviceable propellers in their own garage or workshop.

  1. Propeller Material

    • The traditional material for propellers is steel, for its durability and ability to hold shape despite being thin. Brass is also used, as well as aluminum for lighter duty applications. Avoid using materials that are subject to corrosion, such as unpainted iron or materials that may be damaged or destroyed on contact with floating debris such as wood or plastic. A bent metal propeller blade can be beaten back into shape but a shattered wooden or plastic increases the danger at sea during a survival situation.

    Points of Connection

    • The propeller blades push the boat through the water. Therefore, the propeller must be attached to the shaft with durable long lasting materials that resist shearing damage and that are resistant to corrosion from a marine environment. Consider stainless steel or heavy brass fittings and use locking nuts where possible to avoid having a nut come loose.

    Blade Shape

    • The blades of a propeller are essentially screws, so a well designed blade has a defined curve to maximize thrust efficiency while reducing cavitation. Cavitation occurs when a blade operates inefficiently; it is loud, makes vibrations and reduces overall speed. If efficiency is important, consult with a naval architect or use an existing propeller blade as a template and follow its curves and shape carefully.

    Shaft Safety

    • Some sailors allow their shafts and propellers to spin freely while the transmission is disengaged, others prefer that the shaft lock into position. Both approaches have their advantages and drawbacks. Locked shafts increase drag and the risk of damage during a grounding; freewheeling shafts can increase friction on the shaft stuffing, snag on weeds and lines and more quickly wear out bearing connecting the shaft to the transmission. Consider a build that allows for the captain to lock or unlock a disengaged propeller at will.

    Feathering Blades

    • Some commercial propellers have feathering blades; when the shaft is disengaged, the propellers fold inward to reduce overall drag. This may be of importance for sailing vessels, where any extra drag beneath the waterline negatively affects performance under sail. If feathering is a useful option, consider a cable pull system within the shaft that connects or disconnects latches holding the blades into working position.