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The Context

My mermaids are amphibians that have legs that allow them to walk on land. They live in a lagoon similar to the barrier reef. The reef is roughly 1,000 square miles, and its average depth is 100 feet with a few small islands dotting the lagoon. The Mermaids are at a technological level similar to that of medieval Europe. They have both artificial and natural portals that resemble blue holes, that ships can not cross, which act as transport between major cities and other kingdoms far outside their lagoon.

The Question

Would an aquatic race of amphibians find a reason to develop a sea faring ship and if so why?

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  • $\begingroup$ question: is the land populated in any way? and if yes: by mermaids, or by landdwellers? $\endgroup$ Jan 21 at 21:39
  • $\begingroup$ @FranzGleichmann The land is not inhabited and the only real reason any of the mermaids would venture onto land is to gather certain resources, such as wood, herbs, and fruits. $\endgroup$ Jan 21 at 21:48
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    $\begingroup$ If they are at the level of medieval Europe, they must have at least some trade. If they have trade, they have cargo carriers. Those carriers we humans call "ships". $\endgroup$
    – Alexander
    Jan 21 at 21:49
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    $\begingroup$ @Kurtalmakthekoboldkiaser You should add the existence of portals to your question body. You are asking about a method of transportation and neglected to mention that they already have access to the perfect method of transportation :) $\endgroup$
    – KeizerHarm
    Jan 21 at 22:27
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    $\begingroup$ Same reason legged humans invented the wheel: to carry heavy stuff with less effort. $\endgroup$ Jan 22 at 6:12

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Why do Humans have Airplanes?

The mermaids live in a shallow reef The reef blocks most ocean currents. This prevents any aquatic equivalent to wind-powered craft. The only option is to swim or use an animal -- like a whale, manatee, or giant seahorse -- for transport.

Above the reef there are air currents which can be used to power sailing ships. This is preferable to swimming since it is less tiring and you don't have to feed an animal.

Even without sails there are benefits. It is easier for a pack of manatees to pull a load if some of the load is above the water, than if the load is fully submerged. So easier to drag a boat than a submarine of the same dimensions.

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    $\begingroup$ There isn't really any aquatic equivalent to wind-powered craft. Wind-powered ships work precisely because they are on the boundary between water and air with different properties and moving at different speeds. Below surface, you can only drift with the current, just like a balloon can only drift with the wind. So, yeah, sailing ships are the only way to utilize the wind power, and it is thus a good reason to have them. $\endgroup$
    – Jan Hudec
    Jan 23 at 17:21
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Friction

Boats are faster than submarines. That is always going to be the case no matter your society's technology or origins.

To get from A to B through a medium, you have to press out of the way a tube of matter along your entire path, its width being the width of your vehicle. When staying submerged, that imaginary tube will be filled with water, weighing in at roughly one kilogram a litre. All that stuff is going to need to be displaced by the power of your engines. On the other hand, for a partially submerged vehicle (a boat), most of that tube will be air clocking in at 1 gram a litre. No matter your method of propulsion, be it slaves pulling oars or fusion scramjets: the boat is going to be faster and cost less fuel.

You don't need to justify the mermaids using boats; you need to find things that the mermaids would not transport above the water surface over non-trivial distances. Other mermaids is all I can think of.

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    $\begingroup$ Actually unless your boat is planing, the restriction on its speed is wave drag, which is a surface (boundary layer) problem. Traditional submarines were slower underwater because they used lead acid batteries; nuclear submarines are usually slower on the surface. Wikipedia en.wikipedia.org/wiki/Alfa-class_submarine shows the Alfa class capable of 47mph submerged but only 14mph surfaced. $\endgroup$ Jan 23 at 19:34
  • $\begingroup$ @user_1818839 Good point, I did not know that there was a resistance specifically associated with crossing a boundary. Still, that submarine can only actually surface less than a third of its mass above the waterline; it is not designed to float with minimal water resistance, which every regular ship is. So I do think that the same engine in a vessel designed for floating will always net greater efficiency than one in one designed for diving. Otherwise, why would we still build ships and not make every boat a submarine by default? $\endgroup$
    – KeizerHarm
    Jan 23 at 20:19
  • $\begingroup$ mainly because submarines are more complex, and error prone, rather than slow. Imagine making a pressure hull that would close over 10,000 containers. Not worth it, I think, for a few extra knots. There's a lot more to say about wave drag, it's a complex subject but above hull speed, the waves build up and carry off your extra power instead of letting the ship move any faster. Those elegant old yachts were designed to a maximum waterline length rule ... and when they heeled under sail, the waterline length and hull speed increased, but the rule didn't say anything about that! $\endgroup$ Jan 23 at 21:09
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Why do humans use carts and wagons?

We can walk just fine. An ox or horse drawn wagon won’t go particularly fast or far and won’t go anywhere we can’t reach by foot. Their benefit is that you can transport heavy (or bulky) goods without exerting yourself.

I can totally see this applying to Mermaids and ships. Sails provide free energy and unlike horses or oxen you don’t even have to feed and water them.

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SIZE

Size matters. And not just on the scale of individual units, but also on the full loads.

No matter how fast and convenient a portal might be, carrying a wagonload of ... anything ... will always be more convenient if you use a wagon, rather than many hands (no matter how light they make the work).

Any time they are using a portal that is not landlocked, the boat serves the purpose of the wagon to get the load to the portals, and back again from portals.

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  • $\begingroup$ While I agree on the the idea that if the Mermaids made ships they would you them as wagons, I forgot to clarify that the portals resemble blue holes so a ship would not be able to enter it. $\endgroup$ Jan 21 at 23:23
  • $\begingroup$ @Kurtalmakthekoboldkiaser I've updated my answer accordingly. You might also consider adding that detail to the main body of the question, if you haven't already. Going back to "medieval" correlations, even if the wagon were not allowed to pass through the city gait, wagons would still be used to get from one city gate to the next. So other than actually passing through the portal (gate) itself, I feel the analogy still stands well. They'd just likely plan to have another boat (wagon) on the other side (inside the city), ready to continue once the cargo is transferred. $\endgroup$
    – Harthag
    Jan 21 at 23:36
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Habitation

They live on them. It might seem counter-intuitive, but if the merfolks' biology operates more according to physics and less according to magic, it makes sense.

Without magic, or sufficiently advanced technology, a human-sized critter with a metabolism similar to a human (which a mermaid would probably have, since they presumably have a human brain) can't really get enough oxygen from the water using gills. However, if you assume that merfolk are mammals, then realistically they'd probably breathe on the surface, and then dive for extended periods. And if they live in a relatively shallow lagoon, they have no real need to stay submerged for hours or days on end.

I asked a question on this, and the general consensus is that half-an-hour to an hour dive time isn't unreasonable for a mammal of that size. The main issues are that the human body isn't particularly hydrodynamic - so a merfolk would sacrifice speed in order to get hands - and they'd have crap body insulation, so they'd lose heat rapidly.

So for air-breathing merfolk, having a ship provides a mobile base where they can sit around in relative safety to rest between dives, warm up, store their stuff, float around looking for the next dive location, and sleep.

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Their ships were always for trade.. and nowadays, for the tourists

Merfolk need ships to deliver goods.. and human guests.

Merfolk have always been trade partners

Of course, humans discovered the merfolk 1000s of years ago. At first, they regarded merfolk as any other opponent, fighting wars over certain strategic places, like sea straits and resources. The merfolk won these wars.. and in the old days, they controlled a lot of our supplies.. Merfolk got as powerful and rich as the Arabs were on 20th century Earth. They developed ships themselves, to deliver the goods: fish, oil, cobalt, and other resources.

(to complete the scenario, I let the time line proceed)

Resources exhausted

In a few centuries, the resources so precious to the merfolk dried up.. No new oil sources were found, gas fields lacked the pressure, to get it out.. and slowly, the merfolk changed plan. The oil money was invested in luxury underwater hotels, they converted their economy to tourism.

Tourism

It worked out. Merfolk love to cook and invite guests. Humans who visited the pacific, mediterranean and carribean regions soon discovered their underwater facilities, the friendly and handsome personnel, their swimming pools, the beautiful reefs, the underwater colors..

Paris Agreement

To bring the human tourists in, merfolk decided to avoid air traffic, because they signed the Paris Agreement. You'd land somewhere near, then you proceed the journey on merfolk ships, who bring you to your hotel. These ships are CO2 neutral and really advanced: they look shiny, fully electric, traveling as fast as hovercraft.

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Warfare, primarily*

Your mermaids are amphibious and can walk on land, but you haven't clarified if they can stay on land for very long. That doesn't change that they might have a vested interest in keeping a shoreline clear of land enemies, especially if the land enemies take ships out to harpoon into the sea, or if they're just using cannonballs from cliffsides as a way to harm mermaid groups at sea.

(Seeing that landlubbers aren't apparently very much on the land according to your comments, these could still be other, temporary forays by enemy mermaids - even towards waterlocked enemies, attacking in a way such that your projectiles use the sky and gravity is a good way to get the drop on them. If other mermaid groups are on the land in question).

They can fight on land themselves, but that's probably not as good either way - especially when ranged weapons come into play.

So - if they want to go to war with a shoreline nation - one way to get around that is to use siege equipment or catapults - and attacking from the sea. But siege equipment and catapults don't naturally buoy presumably, and even if they did - would have a hard time maintaining accuracy on the waves on their own. But with a boat, they could place the siege/catapult weapons on the boat, and use it as a platform to solve the stabilization problem.

They could even mount cannons on these boats and aim then, since they'll have a hard time using the saltpeter black powder gunpowder, as well as other gunpowder types that aren`t very useful underwater that they might find/mine on land..

(As an aside, you mentioned that there are both artificial and natural portals that ships can't pass through but that mermaids can; for mermaid city to mermaid city combat, I would ask the following question as some food for thought - what happens if a ship shoots a cannonball through the Blue Hole Portals? What about launched catapult loads? If they don't destroy the portal themselves, do they keep the momentum going out the side, ala the game Portal?)

Mermaid Warfare Boats might look a bit different for these needs

A lot of this "Above-water Weaponry" does sort of relying on solving the ability to stabilize siege weapons and catapults on a boat, while waves are an issue. While not a medieval tech level boat, this Martini One gives us an idea as to how mermaids might build such boats.

Though we can do older technology, the idea of negating waves and try to avoid turbulence is the main focus of that - they could add wave breakers to it, or use a medieval approach to automated wave crest correction on their floats, or simply course correct by sheer scale. After all, these boats are being manned by mermen, so the ability to sail fast might not actually be the biggest point - they could even be easily sinkable, since if they sink into the water, the mermen can easily recover them and repair/rebuild them, and none of their crew will drown.

Since they're not needing to be particularly fast in these cases if mermen can speed them up, and the size of the ship doesn't matter much either, nor redundant sinking failsafes, we can expect that they might look a bit like rafts, or Pontoon bridges - just pontoon bridges to nowhere. Or if you want to use the seabed as a level area to stabilize from regardless of the waves, sort of like Eiffel Tower style structures that can detach from the sea floor but then dig in when they're within firing range of the target - you've got the ability to go to the sea floor and ensure each possible column holding up the psuedo-tower is locked in place into the seabed when necessary.

In peacetime, some other uses

With the catapult designs, sometimes you don't want to be at war. They're still potentially useful for launching large rocks or a lot of water at a cliffside of an island or continent, breaking up the rocks on the edge of the land so that it drains into your waterways more freely broken up, either as a means to build more stone weaponry, or stole materials for other structures, or even to just make mer-made rivers and deltas so you can swim further up the the landmass to get to areas to forage where No Merman Has Gone Before.

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    $\begingroup$ To answer your questions, the mermaids (assuming a tropical climate and no outside hydration or disturbances) can last 4 hours out of water and the portals do keep momentum of the objects that go through. $\endgroup$ Jan 24 at 14:17
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    $\begingroup$ Ah, then yeah - with 4 hours out of water, it's probably more likely they won't be forging cannons and trying to use gunpowder - although an initial check seemed to indicate at least for fireworks, mixing it might be relatively quick and simple, forging cannons might be a bit much. Catapults would still work, using gravity as main way to increase force of impact on other mermaid coral reefs. $\endgroup$ Jan 24 at 22:35
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    $\begingroup$ @AlexanderTheFirst The only way I could see them getting cannons is getting them through trade with other races. $\endgroup$ Jan 24 at 23:12
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Science!

There is all sorts of knowledge to be gained from the dry environment. For example, they can do research on phlogiston and the luminiferous aether much more easily in an atmosphere than underwater.

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  • $\begingroup$ You can do that research from a submarine just as well, assuming the OP meant surface ships... Or a simple platform. $\endgroup$ Jan 22 at 6:42
  • $\begingroup$ @terrywendt yes, but ships are easier to build and more mobile. $\endgroup$ Jan 22 at 15:51
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    $\begingroup$ @TerryWendt: The thing about submarines - mermaids have no need to worry about keeping the inside waterproof, as they're mermaids - so regular ships that are brought down below the water's edge by mermaid strength and then resurfaced as needed basically are submarines for them. $\endgroup$ Jan 23 at 23:39
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Alright, I reread the question. I don't understand why this question was asked. Given the conditions the OP presupposes, "why did humans build ships" is essentially the same question.

You might ask "Why wouldn't mermaids build ships?" I'm sure you could think of some reasons, and then come up with other reasons to counter those...

Or you could ask "Why wouldn't mermaids build submarines?". That might lead to some very interesting world building ideas, like submarines that use giant underwater sails to harness undersea currents, etc,...

But back to the original question: do they use gills to breath or lungs? Or both? They don't have legs, so how do they move around on deck?, How do they rig the sails? If they don't use sails, what do they use to propel the ship? Do they use fossil fuels? How did they figure out fire under water? If they use electric motors, how'd they figure out electricity under water? Are the ships made of wood? Where do they get their wood? There's no wood under water. Metal? how do they smelt metal under water?

All of that was just to presuppose the question of "why would they build ships", with how would ships even work for them.Why they might has some rather obvious answers: You can move faster on the surface then below. You can move heavier and/or larger loads on the surface then below. And you can navigate shallower waters on the surface then below. Every other reason is a sub-category of those three primary reasons.

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