The later "Project 46(a)" specified even more powerful U-cruisers, of a similar displacement to the Type 139 boats, but with an increased surface speed of , and with two 88mm deck guns in addition to the two 150mm guns.
An even larger U-cruiser was proposed under "Project 47", but never reached construction; it would have displaced 2,500 tons, had a top speed of , and been armed with four 150mm guns as well as 6 torpedo tubes, 2 of which would have fired to the side.