Effective Head of Water. Revolutions Efficiency. per minute. Meters. Feet. Revs. Per cent. 20.7 67.9 628 68.5 20.7 67.9 847 47.4 24.1 79.0 645 68.5 27.6 90.5 612 65.7 27.6 90.5 756 68.0 31.0 101.7 935 56.9 31.0 101.7 1,130 35.1