The English author used a lot of sources but the most important and credible source for this data up above is this guy:
…who was the 19th Century chief archivist and chronicler for Hamburg and a Hamburg senator. He was the author of the Hamburgische Chroniken in niedersächsischer Sprache “Hamburg Chronicle in Low German”. He also published the Bremen chronicle. These are available online but in Low German. I painstakingly translated some stuff from the naval battles in 1401 which included the fact that Hamburg captured a 12 cubit long ‘feldschlange’ cannon from the Mad Dog. We estimate this to be about 19 feet which is huge.