Part 2 of How could medieval ships protect themselves from giant mermaids?

While hominids may rule the land, giant mermaids dominate the sea! Many nations employ these giant mermaids to wage warfare and protect sea lanes.

Some basic characteristics of giant mermaids include:

  • 26 feet (8 meters) long
  • human-level intelligence
  • an excellent sense of vision and hearing
  • a thick layer of blubber
  • can hold their breaths up to 13 minutes
  • have domesticated dolphins

Given these characteristics how would these giant mermaids raid/wage war against enemy ships?

Note: Magic does not exist in my story

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    $\begingroup$ Quietly puncture/scrape off any anti-fouling coatings and plant barnacles or shipworms (en.wikipedia.org/wiki/Shipworm) to damage the hull without the crew noticing over the length of the voyage. Also, ships in the Age of Sail were highly flammable; a Molotov cocktail or two would, with luck, burn the ship right down to the waterline. $\endgroup$ Commented Mar 30, 2021 at 23:53
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    $\begingroup$ @GrumpyYoungMan - I think those are pretty funny ideas! An 8 meter monster with one attack being attaching barnacles and sneaking away (giggling), and the other a molotov cocktail it can't possibly light. Who hired these giant mermaids, anyway? Wait what's that on the ship ....AAAAH BARNACLES! $\endgroup$
    – Willk
    Commented Mar 31, 2021 at 0:12
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    $\begingroup$ I think it was established in the other question that medieval ships are largely defenseless against giant mermaids, and have to rely on protection of the latter. So any attack tactic would mirror WWII style attacks on sea transport convoys. If convoy is present, either defeat it, or do quick "hit and run". If (or when) convoy is not present, take your time and sink all ships with any method of your choice. $\endgroup$
    – Alexander
    Commented Mar 31, 2021 at 0:17
  • $\begingroup$ Other ideas: if the ship deploys a storm anchor during bad weather, simply take the opportunity to cut it loose and leave the ship to the mercy of the storm. One might also have some compatriots carry a regular anchor (using a floatation or other device to reduce the effort) tie a rope to the rudder of the ship, release the anchor and watch as the the rudder gets ripped clean off when the anchor grabs the bottom and the rope becomes taut. $\endgroup$ Commented Mar 31, 2021 at 0:18
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    $\begingroup$ Particularly, wooden ship rudders are very vulnerable to an underwater attack. Tear it off, or jam it, and the ship is done for. $\endgroup$
    – Alexander
    Commented Mar 31, 2021 at 0:20

5 Answers 5


You exposed their attack methods in your previous question:

This presents a problem as a group of giant mermaids could capsize a cargo ship and steal its loot.

Once the ship is capsized the attack is over. This can indeed be easily achieved by a coordinated attack: some of them holding onto a side of the ship to add an unbalanced load while others push the keel to further unbalance the ship. This is even more effective if done during a storm, when the ship it's already dancing on its own because of the waves.

If the ship is part of a convoy, it can do little to limit this type of attack: firing cannons or arrows against the attackers once they are on the target ship is almost suicidal, because it will also involve the defendants.

If the convoy is protected by mercenary giant mermaids, the attackers can still use fake attacks or domesticated killer whales to distract the escort and create an opening in the defense.

Plus, sabotages can be sneakily conducted from underwater, as pointed out in the comments: jamming the rudders, planting barnacles, adding ballast.


Weakspots under the sea

A famous Dutch story tells how one sailor heroically jumped over board, swam to the enemy ship and drilled some holes while clinging to it to make it sink. If memory serves correctly, the captain had some beef with the sailor and didn't save the sailir from the water afterwards.

The story shows something important. The vulnerability of the ships hull. Although the story always struck me as odd and nigh impossible, the mermaids should habe no problem.

The tools an 8m fish with arms and human intelligence can wield will be big and strong enough to penetrate hulls. They can also stay out of harms way under the hull, making them hard to stop.

Not only the hull is vulnerable. The rudder is also highly susceptible to damage. They can make the ship unsteerable or even steer the ship themselves, navigating to open sea and leaving them there to rot. Or they steer onto rocks or the like.

The mermaids are pretty powerful. Swimming up to a ship, drilling holes and making steering impossible. Next is attrition. Even if the ship can repair, they can't do so indefinitely. Not too much attrition is needed to make the ship stop to float amd become an involuntary submarine.

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    $\begingroup$ I remember something similar happening in the Pendragon series, by D.J. MacHale, in the second book... The author was probably inspired by the Dutch story... $\endgroup$
    – fartgeek
    Commented Mar 31, 2021 at 12:17
  • $\begingroup$ Drilling is slow and a bit high-tech. 8 m tall mermaids may prefer to use giant crowbars to tear the whole planks from the hull. $\endgroup$
    – Alexander
    Commented Mar 31, 2021 at 17:02

Ram them with a mer-boat.

A mer-boat is actually more like a mer-raft or even a mer-submarine. It needs enough floaty stuff to achieve approximately neutral buoyancy. At colder latitudes, ice would be very useful. Otherwise it could be made out of wood (from previously-taken ships) or maybe even something like pumice (we don't need any airspace in the middle, so the problems from the previous question don't apply to us).

The propulsion is a bunch of mer-people behind the boat, kicking like a child learning to swim with a kickboard.

The critical features of the mer-boat are:

  1. It is low to the water, ideally slightly submerged, avoiding detection.
  2. It provides some shelter from missiles being fired from the human ship--mer-people actually do have defense against this.
  3. At least one side has some kind of rudimentary ramming edge. It doesn't need to be a nicely polished ram, it just needs to be something that will cause an unpleasant crunching noise (followed by an unpleasant sinking feeling) when a human ship runs into it. Possibly some coral or similar, if the actual structure of the mer-boat isn't already enough.

The attack strategy is to locate a human ship and maneuver your mer-boat to be in front of the ship. You probably can't go faster than a human ship with the mer-boat so it will take some planning to do this, although maybe you send a group of your most thuggish looking mer-people over to chase the ship in the direction you want it to go. Possibly some mer-people hold the mer-boat down from underneath, keeping it low enough that it really can't be seen. Then wait, and release so that the mer-boat is at the surface at the same time as the human ship. The human ship will be quite surprised by the existence of a previously uncharted reef in the middle of the ocean.


If I'm the mermaids I'm recruit or coerce an insider to wittingly or unwittingly sabotage the ship from within and receive a percentage of the loot... WAY MORE than they'll receive from the voyage. This greatly increases the success rate of the raid and provides the mermaids important intelligence om when, where, and how to conduct the raid. Or whether or not to conduct the raid at all... Maybe the loot their carrying isn't even something the mermaids are interested in or of low value relative to the risk involved.

  • $\begingroup$ Great answer, very intelligent and practical! I hope you enjoy the site! $\endgroup$
    – Alendyias
    Commented Mar 31, 2021 at 17:40

Muck up the rudder with some rope so the ship can only travel in a circle and kill the men as they try to repair it and wait until they starve to death.

  • $\begingroup$ Interesting idea, although in the short term it's still possible to steer a ship based on sails and weight-displacement should the ship lose control of its rudder. Fine-grain control wouldn't be possible, but course could still be roughly maintained. $\endgroup$
    – Enthu5ed
    Commented Mar 31, 2021 at 20:26
  • $\begingroup$ @Enthus3d How big of a boat is that practical with though? The one in the link is a 2 man boat. $\endgroup$
    – DKNguyen
    Commented Mar 31, 2021 at 20:29
  • $\begingroup$ I don't proclaim to be any sort of nautical expert, but the techniques demonstrated in the link could presumably scale; weight shifting through the use of ballast and the use of sails should still be possible. It'll be enough at least to allow the ship to avoid sailing in circles. $\endgroup$
    – Enthu5ed
    Commented Mar 31, 2021 at 20:37
  • $\begingroup$ @Enthus3d A jammed rudder would not be merely out of control, it would be jammed in a hard-over position to left or right. No amount of sail or weight shifting will overcome that. $\endgroup$
    – PcMan
    Commented Apr 2, 2021 at 18:08
  • $\begingroup$ @PcMan I guess they would have to settle for breaking it off instead then $\endgroup$
    – Enthu5ed
    Commented Apr 2, 2021 at 18:11

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