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.

Bigger Holes

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.

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    $\begingroup$ Just out of curiosity, where are your merfolk getting their metal tools? Working metal underwater is very far from being as easy as it is on land. $\endgroup$
    – Monty Wild
    Commented Feb 11 at 10:44
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    $\begingroup$ @MontyWild Hello! They trade for metal and also have forges on their islands. Mermaids are also in possession of a lot of mithril. Do you think that makes sense? Also, if I may ask, did the idea I presented as a question seem alright? I worked on it for a while, so I hope it doesn't seem ridiculous. Thank you for your interest, Monty! $\endgroup$ Commented Feb 11 at 10:53
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    $\begingroup$ does the mermaid know archery and firelighting? aim for the sails then the parrot🦜 $\endgroup$
    – user6760
    Commented Feb 11 at 14:35
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    $\begingroup$ Deadheads can be very dangerous to boats so the Merfolk could intentionally drag tree trunks down (using stones as ballast), then release them shooting up at the boat. $\endgroup$
    – Sam Dean
    Commented Feb 12 at 16:56
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    $\begingroup$ The Windjammers are large sailing ships with steel hulls and steel cable for the sail lines. Nobody is cutting through that with hand tools. Once people understand what's going on, this idea will appear earlier than it otherwise would have. $\endgroup$
    – Joshua
    Commented Feb 13 at 15:02

8 Answers 8


My take on this problem - discounting the problems of availability of metal tools to merfolk - is that there are going to be lots of problems with this approach and probably better ways to achieve this.

The problems are going to be tearing a large enough hole to sink the ship, and not being detected. Hanging on to the ship would be easy - just hammer a piton with a rope into the hull.

Hammering in pitons is probably not going to be detected. People move around ships all the time, thumping on its timbers, so the occasional bang won't necessarily attract much attention. Wooden ships also leak, so a small water inflow won't be noticed, and would be pumped out as a matter of course.

However, drilling into the hull of a ship would cause a steady grinding sound that is quite distinctive. If done slowly, it could be blamed on rats gnawing.

As for tearing a breach, all you need is one reasonably large hole perhaps 10-20cm in diameter, all the way into the hull.

Into this hole, the merfolk could insert an object like a thick knife blade, with a flat plate on top just small enough to fit easily into the hole. On the bottom would be a long bar projecting in the same direction as the knife blade, to which a chain and anchor would be attached.

enter image description here Author's original work.

Once this tool was inserted into the hull, the chain and anchor could be dropped, and as soon as the anchor caught on something, the motion of the ship and resistance of the anchor and chain would lead to the blade being pulled through the hull along the grain of the timbers... as long as it was strong enough. Timbers would be split and sprung away from the ribs, and if the flat plate on top was shaped properly, it might wedge itself beneath the ship's ribs and spring timbers away from them as it continued aft. Eventually, the device will either pull out of the hull, pull up the anchor, the device or its chain will break, or the ship will come to a stop, but it will likely have a sizeable gash in its hull, with many sprung timbers.

However, this is not enough to guarantee the sinking of a wooden ship of the line, even if entire timbers are torn away. Naval sailors were used to dealing with battle damage, including shot-holes below the waterline, and they could easily fother a sail over the damaged area to slow water ingress while they fixed timbers over the breach from the inside. However, the merfolk could cut the fothering away if their presence was not suspected.

The main problem would be if the merfolk were discovered while attempting this sabotage. The sailors could probably use grappling hooks with sharp prongs, attached to cables or chain, passed under the ship and pulled from side to side to attempt to snag and surface the merfolk, where they could be killed. If this happens after the invention of safety fuse (probably sooner than your equivalent of 1831 if merfolk attacks were known to be a hazard), the sailors could drop black powder grenades alongside the ship, which would have a much greater effect than on land due to the density and incompressability of water.

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    $\begingroup$ Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on Worldbuilding Meta, or in Worldbuilding Chat. Comments continuing discussion may be removed. $\endgroup$
    – Monty Wild
    Commented Feb 11 at 12:21
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    $\begingroup$ Question: how do mermaids lift and maneuver a 3000 pound anchor? Let alone deal with the heavy chains they're attached to? $\endgroup$
    – elemtilas
    Commented Feb 12 at 2:02
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    $\begingroup$ Egad! I was beaten to it on my favorite topic! Excellent answer, @MontyWild! $\endgroup$
    – Mermaker
    Commented Feb 15 at 11:07
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    $\begingroup$ @elemtilas If you make the anchor able to fold, it becomes trivial to carry underwater. If you have it "compact" to a mostly straight object, it can be carried underwater relatively easily by a bunch of merfolk and their beasts. Once it is in place, remove the locking pins that keep it straight and leave it to unfold under its own weight, creating an irregular shape that can get stuck on things. Somewhat like foldable anchors used in climbing, just larger. $\endgroup$
    – Mermaker
    Commented Feb 15 at 11:13
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    $\begingroup$ @ArchduchessFerdinand Humans have more or less the density of water, which means the displacement in liters is more or less the same as their weight. A human that weights 70kg will displace 70l of water, which is a little under 20 US gallons. For a human to displace 250 gallons, they would have to weight close to a metric ton. $\endgroup$
    – Mermaker
    Commented Feb 15 at 11:18

Seems a lot of work just to capture a ship which is now sinking. There's not much point asking them to surrender in a sinking ship unless you want them in boats and the ship at the bottom of the sea where your people can loot it at leisure.

If you want them still on the ship, why not just destroy the rudder. Affix it to your anchored point and the 100+ tonne ship will rip its own rudder off for you, probably snap its masts from the shock as well.

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    $\begingroup$ came here to say this - in addition, if you do it in any sort of rough seas, the loss of the rudder will cause the ship to swing in the wrong direction, possibly sinking it anyway. $\endgroup$
    – lupe
    Commented Feb 12 at 12:09
  • $\begingroup$ I also agree with this. Breaching the hull and sinking the ship is one thing, but you don't breach the hull just to use the breach as an entry point to capture the ship. $\endgroup$
    – Monty Wild
    Commented Feb 16 at 3:37

It's not hard to sabotage a ship. Just jam its rudder and prevent the sailors from fixing it. They will be at your complete mercy.

Humans would be as great at repelling a mermaid attack as fishes are against fishermen. Having access to both aquatic and surface environments will be tactically paramount.

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    $\begingroup$ We might hack through the rudder shaft from the inside to get rid of the rudder, then steer by jib sail though. Yes, it's possible to steer by jib sail. I have done it. $\endgroup$
    – Joshua
    Commented Feb 13 at 4:01
  • $\begingroup$ @Joshua, it's entirely possibly to sail a dinghy without a rudder by balancing the rig, but can you do it in a full-rigged ship? $\endgroup$
    – Separatrix
    Commented Feb 14 at 8:30
  • $\begingroup$ @Separatrix: If you don't have to dock, sure. The open ocean is surprisingly forgiving to poor steerage. The coastline, not so much. $\endgroup$
    – Joshua
    Commented Feb 14 at 14:58
  • $\begingroup$ @Joshua, Mermaids would be operating in shallower waters where there are resources worth fighting for, open water is effectively a desert. On that count you're probably stuffed without your rudder. $\endgroup$
    – Separatrix
    Commented Feb 14 at 15:13
  • $\begingroup$ @Separatrix: I was thinking tropical islands for some reason. Most trajectories will take you away from islands. And even with a continental coastline if you weren't foolish enough to be right up against the coast you're fine. The main problem is it's going to take quite some time to turn. $\endgroup$
    – Joshua
    Commented Feb 14 at 15:17

Frame challenge - Why bother with a risky anchor and trying to rip a piece of planking out?

Simply ram the boat! Rams on boats (usually oar powered rather than sail) have been used since antiquity to sink enemy ships. I can envision a mermaid-powered and -aimed shaped log being used as a ram. You could even line the tree-trunk with metal and sharpen the to provide more damage.

Green wood has a maximum density of about a ton (1000 kg) per meter cubed (64 lb/cu foot), and water-logged wood will be even heavier, so a 20 m x 0.7x0.7 m (that's a pretty big tree, but not Redwood-sized) length would weigh about 10 tons and have consequent inertia. In addition it could be fitted with a unidirectional skirt that opens out when something pushes in in the direction opposite to travel (think umbrella opening with wind)

A "fixed" object would do just as well, so colliding the ship with a submerged solid object could also work. This could be something as simple as a tree-trunk floating below the surface, perhaps held in place with a sea anchor or in place by a fixed anchor point in shallower waters.

Collisions have lost more ships than just about any other cause (as far as I can tell from the list of shipwrecks on Wikipedia) - admittedly most of these have been from collisions with rocks/land rather than floating objects.

As to efficacy, there are reports of even non-solid objects such as whales sinking ships, though whether these are true or not is up for debate.

  • $\begingroup$ Not the answer I was expecting, but a really great one nonetheless! Thank you, Bob! That's an interesting point about the skirt. I had been thinking about parachutes in water, though using it to lengthen impact duration isn't something I considered--that seems like a cool idea! Mermaids can also have whales or the like push the ram. Thanks so much, Bob :). I will mention that ships have deflected cannonballs, though, and historically rams tended to be bladed. So the effect will be limited, compared to that. But orcas have sunk some ships near Spain, lately, so it's still great! $\endgroup$ Commented Feb 12 at 3:05
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    $\begingroup$ @ArchduchessFerdinand Cannonball protection would mostly be above water. As far as I can tell, naval sailing ships (British) had about 8" (20 cm) planking below water and much thicker above -see first answer in this question on history SE $\endgroup$
    – bob1
    Commented Feb 12 at 3:12
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    $\begingroup$ In recent times one of the big hazards to shipping is the vast number of shipping containers that have been lost overboard, especially since these are typically lost from ships travelling along busy sea lanes. Some of the damn things float, very low in the water, for months before sinking. So, yes, this is a very plausible alternate attack mechanism. $\endgroup$ Commented Feb 12 at 3:45
  • $\begingroup$ @bob1 That is a good point, so you would have a definite advantage in ramming. Now I'm excited for rams attached to one or more whales/creatures, with sharp blades. You don't even need to worry about them getting stuck, since they can easily be retrieved from a sunken ship, the animals detached! Thanks very much Bob. I'd like to thank KerrAvon, too, but I'm not sure if that'd work with the rules. $\endgroup$ Commented Feb 12 at 12:35
  • $\begingroup$ @ArchduchessFerdinand There's no rules against politeness here that I know of. You can upvote the comment and thanking in a reply to their comment (use the format @[username] to notify them, the author of an answer or question will get notified of any comment on the question or answer they wrote). If they get enough upvotes on a comment I think it gives a badge. $\endgroup$
    – bob1
    Commented Feb 12 at 19:15

Not at all feasible

The proposed plan has two huge, somewhat interconnected problems.

  1. Big ships stay in deep water, with "age of sail" vessels travelling on courses that are not predictable in sufficient detail for the plan. Unless the mermaids (merfolk?) are attacking a ship on its way out of a known harbour - which means that they have been doing their preparations very close to a major human habitation - they can have no idea of where to prepare their anchored chains. Which does not matter that much, because...
  2. A chain strong enough to rip the planks off a ship is much too heavy for even a large team of mermaids to lift in the water. Displacement in water will only reduce the weight of an iron chain by about 1/8, which is a negligible difference. No matter how good a swimmer a mermaid is, they simply cannot swim upwards with any large fraction of their own body weight in chain, and the chain is heavy enough per length that not enough mermaids can fit around it to work as a team. There are no feasible options at that technology level for adding lots and lots of floats to the chain spaced along its length to make it less heavy in the water, especially since the floatation aids need to be added to lots and lots of chains - someone might notice an order for 100,000 pigs' bladders or five forests of cork.

Point 2 is such a problem that examining the other major issues is not worth the effort. (Eg how to communicate and coordinate with the chain team, how to drill or hammer into something with any force as you are swimming alongside and how the mermaids obtained the massive amount of chain and moved it to their ambush area. Also there is the major issue that if a ship is travelling at high speed due to strong wind it will be very difficult to attach the chain, but if it is travelling slowly then it is likely to merely stop rather than rip itself to pieces.)

  • $\begingroup$ Hello, Kerr. We did some calculations and even a 100m long 30mm thick steel cable seemed to only weigh 1.3 tons. Assuming the equivalent weight of that would work for a So a 1.3 m3 balloon would be enough to make it neutral. Which isn't particularly big for a military operation, about 6 square meters of material. You'd likely have one balloon, the chain balled up, and transferred by barge till you got near the objective. I figure some shallows will have regular ship traffic with specific popular routes, so it shouldn't be too hard to be waiting for a ship, anticipating its direction? $\endgroup$ Commented Feb 11 at 23:54
  • $\begingroup$ Monty gave a pretty good answer on the point of pitons (originally thought of using a drill) to stay in place. Ideally you'd only spring the key point of the plan while the ship was at high speeds, but even low speeds on a thousand ton ship has tremendous power, and I had thought this could shear planking. Overall, I feel like this answer assumed too little of the character's capabilities, and didn't explore the main question, only covering some of the premise. $\endgroup$ Commented Feb 12 at 0:00
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    $\begingroup$ @ArchduchessFerdinand steel wire rope was introduced after steamships, so it is anachronistic. As regards the course, the method you described requires 3 attachments, one after the other, which means it has to run across 3 of these prepared chains with exactly the correct timing so that the mermaids have time to prepare for the next one and the sailors do not have time to deal with the previous one. Even if the other problems were not insurmountable, that's the kind of overly complicated plan with far too many failure points that no competent commander would use. $\endgroup$ Commented Feb 12 at 0:22
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    $\begingroup$ @ArchduchessFerdinand The problem is though, you've underestimated the depth of the ocean by 1 or 2 orders of magnitude. That's long enough for a very shallow area of water like the North Sea, which until recent geological time was above water. But anywhere else, you only need to be barely off the coast and you're not going to touch bottom with 100m of cable. $\endgroup$
    – Graham
    Commented Feb 12 at 0:23
  • $\begingroup$ @Graham Hi Graham. I mentioned that largely you would lay this out in areas of shallows with ship traffic. The average depth of the sea hardly matters, unless you mean there are no areas that are relatively shallow ships cross? A lot of ships did tend to sail along the coastlines as well, as it was a great navigational aid. Additionally, there seems to be an underestimation of what a warship is worth. If you had to carry 20 tons of chain on a barge, or use a 20 m3 balloon, this seems pretty reasonable for disabling a potentially 2,000 ton warship. I expect 500m of chain would be overkill. $\endgroup$ Commented Feb 12 at 2:10

This is a frame challenge.

Captain Nyx was losing her mind already. This couldn't be happening again! Why those damn fish-folk couldn't leave her alone?!

The first raid had Nyx replace the entirety of the crew for female sailors. She couldn't trust men on those seas - not for any fault of their own, but because the siren's song was too powerful. Their chorus of death was enough to force any male-crewed ship towards their reefs, so that's not a risk she could take anymore.

Nyx's second ship saw herself assaulted during the dead of the night. The merfolk climbed up the ship's hull and used blow darts with puffer fish venom to dispatch her crew silently under the moonlight. Wasn't for her dwarvish constitution, she would be one extra body in the cold waters below that night. She barely managed to escape.

Now she had an all-female crewed ship built specifically to block those types of invasions, with a specially designed hull and magically enhanced night watchers. She had all her bases covered.

Well, not quite.

The merfolk had come for her again. She screamed at the top of her lungs as she saw her sails burst into flames out of nowhere, one after the other. A bright light, a shining beam, and them flames.

It took but a few minutes for her ship to be stranded, with no way to use the wind to get anywhere. She then saw the merfolk swimming closer, from all around her ship, ready for a raid. There was a thumping on the hull - no doubt some sort of ram, trying to breach it open so they could sink the ship.

How did they do it this time? How they set her sails aflame?!

Then she saw it - glowing in the distance, mounted on top of whales - a myriad of perfect mirrors with the unique shine of mithril, each one tall as a man and perfectly circular, focusing the warm light of the sun into a deadly beam of death.

They truly had it for her this time.

If you want to stop a boat mid-sea to make it hostage using merfolk, what you need is not to damage the hull.

Go for the sails instead, using some sort of Archimedes' Death Ray.

While the feasibility of this weapon is disputed in our real world, it is far more realistic in a fantasy world with special materials and magic available.

  • $\begingroup$ Ooh, nice! An interesting take on the whole situation, and a brilliant use of logic! $\endgroup$
    – Alendyias
    Commented Feb 15 at 19:47
  • $\begingroup$ @Alendyias "Brilliant", hehe... I see what you did there! Thank you! $\endgroup$
    – Mermaker
    Commented Feb 15 at 19:50
  • $\begingroup$ Very well written, thank you for the excellent answer. The only issue with mirrors is that I doubt you could keep the kind of steadiness you need. Plus changing the sails or turning the ship would mess it up. If we're going with hard physics, I think we already have mirrors that reflect 99.9% of light, IIRC? Mithril could magically reflect more than 100% of light, of course, or you could have special solar mirrors that actually store light for later. Either way, great to see a reference to dwarves and Archimedes' death ray! $\endgroup$ Commented Feb 17 at 17:48

The main problem with this idea is that, barring unusual circumstances, wooden ships do not sink all that well. You can riddle a wooden ship with holes (this is what cannonballs used to do) and will still float, and sail along, if slower and worse for wear.

Which means that the ship will keep on ripping itself free, thus the sailors would be more motivated to to fight the mermaids than just give up.

I would advise more complex attack, like for example:

  • one team to drill into the bottom, like in your plan,
  • one team to throw grappling hooks over the starboard, to make the ship canter badly,
  • one team to throw (or better, shoot) lighter grappling hooks at the rigging and cloth to tear it off. Suddenly sail-less ship that canters so badly it takes water through the board is a scary (but reversible) issue, which will panic the seamen much more than a hole in the bottom.

The sealife itself can slow down a ship to a near standstill via biofouling. https://en.wikipedia.org/wiki/Biofouling

So your aquarians appear, seed the ship with growth, the ship halts, the growth deepens and destroys the planks, water starts to fill compartments.

The aquarians force themselves in, conquering department after department.

In addition: Biofouling, if properly bio-engineered could consist of well anchored material, with parachutes, going down deep, into water flowing into another direction. With a proper engineering, a whole horseload of power could pull on one plank constantly.

  • $\begingroup$ This mostly happens when at anchor or in the harbor though. Fouling is much less of an issue as long as the ship is moving, thats why its a major problem for private boats / yachts that only get moved ocassionally but much less for commercial ships. $\endgroup$
    – LazyLizard
    Commented Feb 15 at 12:12
  • $\begingroup$ No, it happens independent of usage or not. mdpi.com/2077-1312/8/10/748 It happens less in comercial vessels, due to constant anti-fouling measures and maintenance. $\endgroup$
    – Pica
    Commented Feb 15 at 12:25

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