[El Gato]

I don’t understand. Can’t you throw crap at them as they climb on board? It must be a big job to climb abord, and takover.

Boiling water, or better yet boiling oil, even crude oil, would seem likely to work well.

Of course you have to remember that the tanker crew is most likely unarmed, while the "pirates", (Jihadies for Profit), have small arms, and RPGs. Still a couple of .50s or 12.5 or 14 mm's (Soviet heavy machine gun calibers) on the bridge wings, plus a 20mm on a bow platform, would seem to be more than sufficient.

But the world can't have armed civilians running about, don't you know.

[Lonesome in Massachussets]

The IMO used to publish a handbook on piracy. Modern merchant ships are largely automated and thinly crewed, with very light security provisions. On the high seas (say 450 NM Southeast of Mombasa) there is likely only a helmsman and possibly a single officer on the bridge. The pirates like to attack at night in small boats from the stern. They throw grappling hooks over the gunnels and shimmy aboard. A handful of heavily armed pirates with automatic weapons will quickly overwhelm the bridge and, if necessary the engine room. After they stop the ship more swarm aboard. In a lot of countries (Malaysia, Nigeria) the navy and port authorities are in cahoots with the pirates, like cops during prohibition.

One countermeasure is to stand a radar watch (almost all modern navigational radar have automatic collision alarms) and play a fire hose off the stern if an approaching boat is spotted.

Personally, I think high value ships in dangerous waters should carry covert security details from interested countries navies'. The first couple of times a grappling hook is answered by a storm of lead, piracy rates will swiftly decline. Cuts down on recidivism, too.