What was Germany's most effective tactic against Great Britain's warships during World War l?
It has to be the hasty yet dangerous submarine warfare that Germany utilized.
The downing of the British ship Lusitania in 1915, prompted many European leaders to demand Germany to end its use of U-boats, or submarines. And Germany complied; by the end of 1915 the Germans did stop using submarines.
However, when World War 1 became more intense, Germany found that to win the war, U-boats would be critical. Thus, these machines were reintroduced, and Germany, ignoring the agreement it had come to with Britain and its allies, began a strong campaign at sea.
German U-boats are clumsy, and obviously their re-introduction didn't do alot to help the German war efforts. But they were pesky, especially at annoying the Allied naval blockade, which was at the time created chiefly by Britain's warships. Several tons of shipped materials were salvaged as German U-boats, placed strategically around the Mediterranean and Atlantic, struck fear. In fact, Britain lost nearly a quarter of all its ships (war and trade) due to U-boats.
So, these U-boats AKA German submarines were a dangerous pest for the British and their warships.