U-791 was going to be a sea-going prototype boat for the Walter turbine for main propulsion but the boat was scrapped incomplete. As with all Walter boats, her range at full speed on the turbine was quite limited, so an auxiliary diesel/electric drive was also installed. With this added machinery comes unwanted weight, and this boat had a very high weight to displacement ratio and was unworkable. Initially intended as a small boat of about 360 tons, the design was modified two more times to attempt greater operational capability.

Displacement 610/655 tons
Dimensions 171' x 13' 3" x 18'
Machinery Walter turbine; diesel/electric twin shaft
Power

(Walter turbine) 4,360 hp
(6 cyl MWM diesel) 300 hp
(electric) 150 hp

Speed

(Walter turbine) 19 knots
(diesel) 9.25 knots
(electric) 9.5 knots

Bunkers

(Perhydrol) 98 tons
(diesel) 34 tons

Radius

(Walter) 205 miles @ 19 knots
(diesel) 2,390 miles @ 9 knots
(electric) 40 miles @ 9.5 knots

Tubes

(fwd) two 21 inch tubes (carried 6 torpedoes)
(aft) none

Guns none
AA Guns none
Crew 25

This boat was contracted at Krupp's Germania Werft in Kiel and was laid down on 18 February, 1942 but she was scrapped incomplete sometime later in the War.