Tankers travel at 15-17 knots on average with some being slower, none faster. Loaded tankers have a low freeboard making boarding relatively easy whether welcome or unwelcome. Vessels used for piracy, or smuggling for that matter, are generally small and very fast. Both of which makes them not only difficult to find but difficult to engage successfully. It's a big ocean. Solution? Convoys or armed naval detachments on board the merchant ships. Where is the UN?
“Scientists are people who build the Brooklyn Bridge and then buy it.”
Wm. F. Buckley, Jr.