A Good Brit's Guide to Defeating a German U-Boat

 (World War 1) 

-Train pigeons to poo on the periscope/ sights/ the thing where they see out of

-Hire Blacksmiths to hit them with big hammers  

-Get angry British sailors to hit them with big hammers 

-Cover the sights

-Pour paint in the water 

-Depth charges (from 1916 onwards)