Materials for construction

Although traditionally water wheels have been made mostly from wood, the use of steel in overshot (and pitchback) wheels allows faster rotation (possibly reducing the need for gearing) without extreme reductions in available torque. Although, a wooden wheel with a wooden axle, that can easily turn low-speed, cannot necessarily sustain high-speed, high-torque loads such as a run of millstones or high speeds that are needed for the hydropower/hydroelectric power generation.

Until around 1820 water wheels in North America were generally built with a wooden axle (usually from seasoned white oak) with 3 or more of the spokes extending through the wood axle and interlocked inside. Additional spokes attached to pocket holes in the axle and were wedged inside the wood axle. These wheels generally had metal gudgeons held in place on the ends of the shafts using wedges and steel hoops which allowed the wood axle to have a small metal tip on the end. These metal tips or “journals” would then ride on wood or stone bearings. A water wheel made this wheel was called a Compass Wheel . Sometimes a wood axle would need to be replaced after only a year or two prompting the development of “hybrid wheels”. After 1820 water wheels began to have steel hubs and later steel axles with wood spokes, rims, and paddles. These hybrid wheels eliminated the often problematic wood axle and allowed the addition of more spokes. Later they went to cast Iron water wheels and eventually they went to all steel water wheels.

Overshot (and particularly backshot) wheels are the most efficient type; a backshot steel wheel can be more efficient (about 60%) than all but the most advanced and well-constructed turbines. Nevertheless, in some situations an overshot wheel is vastly preferable to any turbine.

The development of the hydraulic type water wheels with their improved efficiency (>67%) opens up an alternative path for the installation of waterwheels in existing mills, or redevelopment of abandoned mills.