Did you know? Two of these can ambush a tanker.
This is real, I got it from a sailor’s first person narrative. He didn’t witness it in action, but he was warned about it as his cargo ship was leaving the port of Singapore in 1994. Apparently it was common practice then, though I don’t know if (or where) it’s still a thing. Technically, it only works with modern ships, but below I’ll discuss adapting it for D&D scenarios.
Nighttime. (It’s got to be dark.) Two fishing boats get positioned left and right of the target ship’s expected route – typically on its way out a port. They’re small and made of wood, so the radar can’t pick them up. Their bows are connected with a 60-100m (roughly 200-330 ft) rope. And the rope is slightly submerged, to a depth calculated to get caught by the ship’s bulbous bow.
“A bulbous bow is a protruding bulb at the bow (or front) of a ship just below the waterline.” In this picture: 1 – Profile of bow with bulb. 2 – Profile of bow without bulb.
As the ship moves between the two boats, the bulb catches the rope, and as it moves past them, it drags both boats behind it. Soon enough, they end up right next to the ship’s sides, without having to match its speed on their own power. No engines, no oars, no sails, just a rope. Then the pirates throw grappling hooks and climb on the deck, and now they can go about their business of robbing the place. Surprise!
As is, this trick only works for ships with a bulbous bow. But these didn’t exist before 1910, and they only benefit “large vessels that cross large bodies of water near their best speed” – cargo ships, ocean liners, tankers etc, not sailing vessels.
That said, any protrusion at the bow of the ship would work, as long as it’s below the waterline and a rope can plausibly get caught there. So what can we come up with?
An Elf Wingship from Stormwrack / D&D 3.5
This is the obvious, quick and dirty solution. If you can buy fire-breathing flying reptiles in your world, mayhaps you can buy a ship design with sails and an inexplicable protrusion at the bow, bulbous or otherwise. Maybe it’s efficient because the laws of physics are a bit different here. Maybe it’s inefficient but carries a cultural significance, so people build their ships like that anyway. Maybe a wizard did it, maybe it was elves. Probably elves.
A trireme-type prow might or might not work. Its ram isn’t pointy, it is more… accordion-shaped, so catching a rope is actually feasible. But it isn’t below the waterline, it is exactly on it. This makes the trick either impossible, or very hard to pull off. At best, you’d have to eyeball it and hope for convenient waves at that exact moment.
Idea #1: Pirates conspiring with dock workers, who compromise the ship by slightly frontloading it, or overloading it. Now the ram is slightly below the waterline, and the trick can work. Plausible? No idea, I don’t actually know much about ships, I’m winging it here. 😛
Idea #2: A floating rope would help. Now, ropes made of natural fibre normally sink in the water, and polypropylene ropes have probably not been invented in your setting yet (these do float). But it’s not unreasonable to assume there’s an exotic material for this purpose, or a cheap magic item.
So what if the ship’s protrusion is above the waterline (like a trireme’s ram, or an imaginary design), and the rope is stretched above it? You’ll say, that’s very hard to pull off in practice. I say, Levitate is a 2nd lvl spell, and a 3rd lvl Sorcerer on a little pirate crew isn’t unthinkable. Paint the rope black for camouflage, and the surprise is on!