An undershot turbine for capturing energy from a flowing stream having a horizontal flow direction and velocity comprising: a frame; a wheel assembly comprising: a horizontal wheel axle; a support wheel having a centre and a circumference wherein the wheel axle is located at the centre of the support wheel and wherein the support wheel is rotatably mounted with respect to the frame; and a plurality of swing arm assemblies each comprising: a horizontal swing arm shaft located at the circumference of the support wheel; a swing arm having an axle end and a blade end wherein the swing arm shaft is located at the axle end of the swing arm and the swing arm is rotatably mounted with respect to the support wheel; and a blade attached to the blade end of the swing arm; and a timing assembly configured to control the rotation of each swing arm with attached blade such that, as the support wheel rotates, each swing arm with attached blade rotates counter to the direction of rotation of the support wheel and completes one full revolution for every full revolution of the support wheel and such that each blade is: i) rotated inboard of its respective swing arm shaft before it enters the stream; ii) then rotated outboard of its respective swing arm shaft as it passes through the stream; and iii) then rotated inboard of its respective swing arm shaft before it exits the stream. a frame; a wheel assembly comprising: a horizontal wheel axle; a support wheel having a centre and a circumference wherein the wheel axle is located at the centre of the support wheel and wherein the support wheel is rotatably mounted with respect to the frame; and a horizontal wheel axle; a support wheel having a centre and a circumference wherein the wheel axle is located at the centre of the support wheel and wherein the support wheel is rotatably mounted with respect to the frame; and a plurality of swing arm assemblies each comprising: a horizontal swing arm shaft located at the circumference of the support wheel; a swing arm having an axle end and a blade end wherein the swing arm shaft is located at the axle end of the swing arm and the swing arm is rotatably mounted with respect to the support wheel; and a blade attached to the blade end of the swing arm; and a horizontal swing arm shaft located at the circumference of the support wheel; a swing arm having an axle end and a blade end wherein the swing arm shaft is located at the axle end of the swing arm and the swing arm is rotatably mounted with respect to the support wheel; and a blade attached to the blade end of the swing arm; and a timing assembly configured to control the rotation of each swing arm with attached blade such that, as the support wheel rotates, each swing arm with attached blade rotates counter to the direction of rotation of the support wheel and completes one full revolution for every full revolution of the support wheel and such that each blade is: i) rotated inboard of its respective swing arm shaft before it enters the stream; ii) then rotated outboard of its respective swing arm shaft as it passes through the stream; and iii) then rotated inboard of its respective swing arm shaft before it exits the stream. i) rotated inboard of its respective swing arm shaft before it enters the stream; ii) then rotated outboard of its respective swing arm shaft as it passes through the stream; and iii) then rotated inboard of its respective swing arm shaft before it exits the stream.