I had an idea for a story, where mermaids sabotage a ship during the age of sail. I wanted to ask for advice on it, and if it makes sense. Perspective would be especially helpful, since someone told me it'd take 80 tons of steel to rip the planking of a warship and told me to abandon the story.
Breaching a Ship
Getting Ready to Work
The mermaids are planning to make a hole in a sailing ship, to force it to surrender and capture it. The first step, is they swim alongside it--they can easily keep up a cruising speed of 5kts, and can move at about 17kts for a while, so this should be possible. They can also breathe underwater. They then achieve the difficult task, while swimming, to use a small drill to fix a harness to the ship. This allows them to work while it's sailing, without having to swim.
The First Tear
Then, more small holes are drilled. This all a grappling hook to be hooked into the planking. Tying the grapple to the sea floor, you would then use the weight and speed of the heavy ship (possibly 2,000 tons in displacement) to have the grapple rip out a chunk of the ship. I will note they can easily set up points to attach their chain all along the sea floor, ahead of the ship, so it won't be hard to find one to hook onto.
This is done three times, until they isolate a larger section of hull, its grain exposed. Drilling into the grain, and using a (possibly larger) grapple, they tear off a larger chunk out of the ship.
This process may be applied three more times, making three larger holes, until that isolate an even larger piece of the ship. And this should be the finale layer. When they rip off this chunk, they breach through the hull with a small hole, perhaps 20cm in size, perhaps smaller.
The Final Breach and Securing the Ship
Through this hole, they slip two iron bars, making an X shape (a cross), with a chain tied to them. The iron bars are about a meter long. You then attach them via a long chain to the seafloor, as before. I figure that a First Rate warship of over a thousand tons would be able o rip some planking, and create a 1-meter diameter hole. I was told this would take 80 tons of steel.
Once the hole is made, mermaids surge through it, and protect the breach from being repaired, as the hold fills. When the sailors give up, they patch the breach themselves, and offer assistance.
Questioning Myself, and How this might be Countered
I think this is an interesting and plausible way to use a ship's strength and that of the sea floor to breach a ship's hull in reasonable time? It may take a couple of hours, I would guess. And I figure if the sailors were alert, listening for unusual sounds at the bottom of the ship, they would notice. But if they don't expect it, or aren't vigilant, perhaps this would be a plausible way to breach the very thick hulls of a First Rate Warship?
Wanting a Satisfying Solution
I often see suggestions that give too little credit to sailing warships, whose planking was something like half a meter thick. That you could simply have a mermaid drill a little hole through one with a hand drill and sink it. That's what made me curious about coming up with a solution that gives due respect to the thickness and strength of these mighty warships, while still showing that human (and mermaid) ingenuity can still defeat them, with some clever engineering and physics.
I hope that something like this can work out as a plausible way to sink one of the great warships, and hope you found it an interesting thought experiment.