Hello Jon,
that dimensions could be possible.
At "Bootsbau" from A.Brix (1911) i found a German-Kriegsmarine-dingi with the dimensions 3,60 m x 1,30 m. It looks like the dingi used later on type IX boats.
And at the Brix from 1929 a similar one (but for civil use) with 3,15 m x 1,13 m.
Both were disigned for harbour usage and a capacitie of 1 to 4 Persons (The bigger one).
OK, this drawings are more than 20 years older than the submarines, but they look identical.
Regards,
Uboatfan