This was planned to be a sea-going repair and replenishment facility of sorts. This boat was to be somewhere in the area of 2,500 tons, would carry fuel, torpedoes and supplies from frontboots and would have facilities for limited repair work at sea.

The TYPE XV boats were to be a triple-hull design side-by-side with conventional twin-shaft diesel-electric power of 2,800 hp on the diesels and 750 on the electric motors.

On paper, this must have looked like a great idea - until someone pointed that any such replenishment and especially, any repair work performed at sea, would require a calm sea, something not always to be depended upon. There was also the glaringly obvious flaw - that if this boat was detected on the surface, either supplying or repairing a frontboot, there would be no escape for a boat of that size, that could not dive rapidly at all.

The project was shelved and no TYPE XV boats were ever ordered from any of the dockyards.