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My novel taking place in an early 1800s setting (circa 1800 - 1850) has a human versus merfolk war. It started with disputes over fishing resources, but now has escalated, for the merfolk, to a fight for survival, as their empire over the ocean is collapsing, and they need to raid human shipping just to survive.

The merfolk can attack human ships by cutting into the hulls and sinking them, or using grappling hooks to pull attackers into the water. Humans can attack them with firearms or melee weapons if they get too close.

Problem is, neither side can directly affect each other - until the humans, desperate for the "impossible war" to end, spearhead research into what ultimately becomes the depth charge. Using this, without warning, they launch a coordinated barrage against the merfolk capital city, destroying the empire, and fragmenting the merfolk into tribes and city states.

Now, all that out of the way, is it realistic for depth charges to exist with 1800s technology? Could they detonate just by contacting the seafloor? Would they need to detonate via some kind of fuse? What difficulty would the humans have in acquiring enough of these to level a city? What kind of supporting technologies would be needed for this?

Answers to any of these sub-questions are much appreciated.

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    $\begingroup$ Just to clarify: a depth charge is an explosive that has a fuse that triggers at a specific depth. ie detonate at depth of 20m to hit a submarine. That is different from a contact trigger device that is dropped from surface and explodes on hitting ground or merfolk- structure. $\endgroup$
    – Gault Drakkor
    Sep 17, 2021 at 23:39
  • $\begingroup$ Thank you. I was wondering what terminology I should make up for them, I'll make sure to remember that. $\endgroup$
    – WasatchWind
    Sep 17, 2021 at 23:50
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    $\begingroup$ Frame challenge: with limited technology, another way of making an ocean floor area uninhabitable would be pollutants. For example, archive.epa.gov/emergencies/content/fss/web/pdf/fingas_2.pdf notes that "Many historic spills showed at 2 to 3% uptake of sediment is sufficient to sink [heavy fuel] oil" and that it continues to emit hydrocarbons over time. Another example is newscientist.com/article/… which might be possible to trigger using dumped fertilizers. $\endgroup$
    – GrumpyYoungMan
    Sep 18, 2021 at 1:49
  • $\begingroup$ @Current If Sperm Whales are intelligent beings who can communicate, they might dispute the idea that the merefolk ever ruled the ocean. $\endgroup$
    – M. A. Golding
    Sep 18, 2021 at 21:13
  • $\begingroup$ @M.A.Golding okay.... I wasn't asking that. I was asking about depth charges. My story also takes place in a fantasy world, so I make the rules. $\endgroup$
    – WasatchWind
    Sep 18, 2021 at 21:16

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Crude naval mines existed before the 1800s and the concept can naturally be extended to depth charges, given that your universe has a dire need for such a weapon. 1854 was the invention of nitroglycerine and the late 1860s saw both the development of both dynamite and considerable advancement in naval mines with contact fusing. So, if you move the timeline for some of these inventions ahead a bit in your universe, you can indeed have contact fused, depth (pressure) fused, or timer fused depth charges either loaded with black powder or nitroglycerine/dynamite.

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  • $\begingroup$ I'm assuming the explosive would be stored in some kind of metal container? How deep would it be capable of going and still being effective? $\endgroup$
    – WasatchWind
    Sep 17, 2021 at 22:25
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    $\begingroup$ @Current That depends on the design of the mine and its casing. For example, a mine filled with nitroglycerine (which, being a liquid, is not very compressible) would probably reach the ocean floor intact even with a relatively thin casing. I don't think having depth charges that can reach whatever depth you need would be implausible. $\endgroup$
    – GrumpyYoungMan
    Sep 17, 2021 at 22:40
  • $\begingroup$ yeah essentially the humans are bombing the merfolk in this scenario. I also wonder if just the effect of shockwaves underwater would have a super devastating effect. For the purposes of my story, the depth charges don't need to be powerful enough (or in enough number) to destroy the entire merfolk city, their simple unfamiliar nature, as merfolk aren't familiar with explosives, could be enough to trigger their defeat. $\endgroup$
    – WasatchWind
    Sep 17, 2021 at 22:46
  • $\begingroup$ @Current The Wikipedia article on depth charges says: "The killing radius of a depth charge depends on the depth of detonation, the payload of the depth charge and the size and strength of the submarine hull. A depth charge of approximately 100 kg of TNT (400 MJ) would normally have a killing radius (hull breach) of only 3–4 meters (10–13 ft) against a conventional 1000-ton submarine, while the disablement radius (where the submarine is not sunk but put out of commission) would be approximately 8–10 meters (26–33 ft). ..." $\endgroup$
    – GrumpyYoungMan
    Sep 18, 2021 at 1:27
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    $\begingroup$ @Current "... A larger payload increases the radius only relatively little because the effect of an underwater explosion decreases as the cube of the distance to the target." Extrapolating from that, a 800 kg TNT charge gives a ~20 m disablement radius, a 2700 kg TNT charge gives a ~30 m disablement radius, and so forth. The merfolk are unprotected so the injury radius for them would be somewhat higher and, if they possess hearing, would be deafened beyond that. So that's something to consider for your worldbuilding scenario. $\endgroup$
    – GrumpyYoungMan
    Sep 18, 2021 at 1:32

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