LH - the international maritime law on blockade became increasingly confused. You could normally stop enemy ships or cargoes of military items in neutral ships from entering a country. You then have a dispute as to what has military value (clothing? food?). In the American Civil War the Union went to great efforts to prevent cotton exports from the Confederacy in order to ruin its economy. I think that they got away with this because to Confederacy wasn't strictly-speaking a 'country'.

To enforce the blockade you had to board the suspected vessel (if it was flying a neutral flag or possibly a 'false flag') and check it over. Then if it was an enemy vessel you could sink it, first taking off the crew and passengers. If it was a neutral, or there was some dispute you could take it to an 'examination port'. I can only think that HMAS Sydney was so close to Kormoran because the captain intended to board what was thought to be a friendly freighter.

When U-boats tried this in the First World War merchant vessels were armed, so began the period of unrestricted submarine warfare (shoot first, ask questions later). Q-ships tactics used smaller vessels that were not considered worth a torpedo, or larger vessels which were filled up with additional bouyancy items so they would take a long time sinking, encouraging the submarine to surface and use its gun rather than another torpedo. They would also be seen by the submarine to abandon ship by a 'panic party' lowering a boat including the 'captain's wife' (a young seaman in a dress) and possibly even a parrot in a cage. But the concealed gun crews stayed on board.

There was an unsuccessful attempt to do this again at the start of World War 2 with what were called 'Special Service Freighters' although only 3 or 4 were commissioned. One of these was the Ellerman Liner 'City of Durban', built in Hull in 1921, probably with the assistance of my grandad and great grandad!