after the disastrous failure of operation Sealion, the Kriegsmarine and the German military looked over what apsects of the invasion had aided in this failure.
in addition to Germany's shortcomings in Naval power and air range, a lack of sufficient landing craft was discovered to have attributed to the failure of the invasion. the vessels available to the Germans, often Ad-hoc constructs made from Barges, or attempts at submersible or otherwise amphibious vehicles, had left the German forces without sufficient resources to carry out their advance into the English heartland: the Barge-craft, designed for the Rhine, not the choppy waters of the Channel, often sank with their vital cargoes of supplies and vehicles, and vehicles such as the SchwimmPanzer III ended up stuck on the bottom of the channel. few of the latter were able to be recovered under the pounding of the RAF, dooming their crews to horrifying deaths at the bottom of the English Channel.
Despite the unlikelihood of further amphibious attacks, the kriegsmarine and the Heer, the German Army, put aside their differences and collaborate on a vessel suitable for their task.
a vessel able to carry vehicles up to the 60+ ton weight of the new German heavy tanks, or their equivalent weight in other vehicles, infantry or cargo.
in addition, analysis of American operations in South America lead to the development of armed craft, carrying anti-aircraft guns and rocket tubes.