During the Second Boer War, the engine Havelock was prepared to see action while serving on armoured trains. Unlike usual practice in such cases, the engine was not equipped with armour plate protection, but was draped in strands of thick hemp rope which covered it from front to back. This earned the loco­motive the apt nickname Hairy Mary amongst the troops.

Doesn't say why they used this instead of regular armor on wikipedia. But who could pass up having their ship being so handsome?