[freebeard]

Regarding the second half of the quote:

Power Waterwheel
waterwheelfactory.com/power.html
Undershot wheels are normally narrow and have to have the channel walls very close to the sides of the wheel to maximize the flow of water to pass through the wheels to generate power. This type of wheel is generally the least efficient type of wheel - usually in the 30-50% range.

Quoth Wikipedia:

Stream (also known as free surface). Ship wheels are a type of stream wheel.

• Vertical wheel with horizontal axle
• The bottom of the wheel is placed into flowing water
• Driving surfaces – blades – flat prior to 18th century, curved thereafter
• Water – very large volume, no head
• Efficiency – about 20% prior to 18th century and later 50 to 60%

Undershot

• Vertical wheel with horizontal axle
• The water hits the wheel low down, typically in the bottom quarter
• Driving surfaces – blades – flat prior to 18th century, curved thereafter
• Water – large volume, low head
• Efficiency – about 20% prior to 18th century and later 50 to 60%

Breastshot

• Vertical wheel with horizontal axle
• The water hits the wheel roughly central, typically between one quarter and three quarters of the height.
• Driving surfaces – buckets – carefully shaped to ensure that the water enters smoothly
• Water – large volume, moderate head
• Efficiency – 50 to 60%