I recently finished reading The Invisible Man by H.G. Wells. As with most of his books, you go into it expecting a daring tale and action and suspense, but end up finding something much more thought-provoking and strange. I'd strongly recommend reading it. It's short, entertaining, and out -of-copyright. There's no excuse not to.
While reading this book I was struck by Wells' use of a term I'd never heard before. The action of the The Invisible Man takes place around a sleepy village on the east coast of England, and as an inevitable consequence of this setting, he frequently refers to the North Sea in his descriptions of places. The odd thing is that he doesn't call it the North Sea. He calls it 'The German Ocean'.
At first I thought this was a term Wells had coined himself, perhaps a sly hint that the world of this novel was not quite the world he was living in. A few days later, however, while leafing through a set of seventeenth and eighteenth century maps of northern Germany (I have a strange job) I saw it again. A quick scan through another set of old maps confirmed that Mer d'Allemagne, Oceanus Germanicus, and German Ocean appeared just as often as North Sea or its variants. I was curious. When did this term die out, and why?
For the answer to the first part of that question, I turned to the all-knowing google hive-mind.
Here are the Google Ngram results for the relative frequency of the two terms in English-language texts since 1700. The X axis is time, the right axis is frequency. Frequency is expressed as the percentage of the total number of words published in a given year that word represented. It's a largely incomprehensible figure, so I've cropped it out.
As you can see, the two were used pretty much interchangeably throughout the eighteenth century. When you factor in variants of German Ocean -- like German Sea, Oceanus Germanicus, etc -- I think German Ocean probably has the edge. The two terms appear to have existed side by side until around 1850, when North Sea experiences a sudden rise in popularity and 'German Ocean' begins to drop away. By the time H.G. Wells used 'German Ocean' in The Invisible Man (around 1897) it was already well on its way out, and may have even sounded a little archaic to his readers.
So that's the when, but what about the why?
It is a well known fact that when a poor, innocent sea or ocean finds itself caught between two major political powers, things can get very ugly. In parallel with the fight for the physical owership of the sea -- the warships, trawlers, etc. -- there's also the more abstract fight over naming rights. Both sides typically want the official name of a sea of ocean to imply that they have undisputed strategic dominance over its waters, regardless of what the situation is on the ground (well, on the water). Sometimes the two countries can seemingly agree to disagree (as with the English Channel/La Manche), but most of the time it turns into nasty international confrontation in which the poor cartographers are caught in the crossfire (as with the Persian Gulf/Arabian Gulf).
For most of the nineteenth century, the British had enjoyed undisputed control of the North Sea. They had not bothered to change the name, however, because the name did not imply strategic dominance by any other state. It was neutral. The name 'German Ocean' was just a holdover from Pliny's Oceanus Germanicus (ocean surrounded by stinky Germanic barbarians).
In the mid-nineteenth century, however, the Kingdom of Prussia smushed into Saxony and Bavaria, making a new state called Germany. By a quirk of history and language, this new state was gifted symbolic dominance over the North Sea on English maps, even though it didn't really have a navy to speak of.
From this point, however, the English press started to become a little squeamish about writing the name 'German Ocean'. You can see in the graph above that 'German Ocean' begins to get dropped in favour of the neutral 'North Sea' the moment Germany emerges as a rival European power.
The abandonment of 'German Ocean' accelerates dramatically shortly after the passing of Kaiser Wilhelm's 1898 Fleet Act -- which began to ambitious process of bringing the Imperial German Navy up to parity with the British Royal Navy. Now that the Germans were seriously contesting England's ownership of the North Sea, to call it otherwise looked unpatriotic. The outbreak of war in 1914 made it look treasonous. By the beginning of the 1920s, the term 'German Ocean' was dead in the water (so to speak).
The odd thing is that, so far as I can tell, the name for the North Sea in German is, and always has been, Nordsee, or 'North Sea'