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On an Earth-like planet with clustered continents and large oceans, like Pangaea, the easiest method of travel would be boats. However, I am looking to avoid humans using boats. I define a boat as some kind of fast transportation over water. This includes taming animals or any other water-based method of transportation. The society in question is similar to Ancient Greece in terms of the technological age and expansion. I am also looking for a hard-science answer.

My limitations are:

  • No cultural norms or governmental decrees (these tend to be misinterpreted or changed over time)

  • The oceans must remain similar to Earth, being mostly water with some minerals.

  • The continents must be clustered together

  • The humans must be human (no morphological changes).

  • The planet must stay in the tropical to temperate temperature range.

  • Whatever strategy is devised must be long-term, like how boats have been around for about 10,000 years.

What can be changed:

  • The weather
  • Any geographic features on the continent
  • Technological limitations
  • Really anything as long as the planet stays Earth-like and habitable.

Clarifications:

  • Riverboats and small inland transportation are ok, I just don't want seaworthy ships.
  • Extreme weather, like strong winds on the ocean, is preferable but there needs to be a way for humans to live normally without any extra gear, like goggles for wind.
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    $\begingroup$ Five Hundred Thousand Years? 99.99% of all human technology was invented in the last 150 years. And you want something that will remain for 500,000 years? What angry god is restricting your population's natural curiosity and inclination to solve problems? I know this isn't really intrinsic to your question ... but are you sure? That's so unrealistic it might cause cancer. 😁 $\endgroup$ Nov 30 '20 at 19:21
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    $\begingroup$ The hard-science tag may be too limiting for this question, as I expect it will be difficult to cite papers/etc that would apply to your scenario. Perhaps science-based would be better. $\endgroup$
    – rek
    Nov 30 '20 at 19:47
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    $\begingroup$ The entire history of human civilization spans less than 7,000 years. There weren't even any anatomically modern humans 500,000 years ago... Moreover, asking about a society "similar to Ancient Greece" except without ships is like asking about a society similar to the USA, except landlocked, surrounded by powerful enemies and ruled by a hereditary theocracy: the essence of Ancient Greece was seaborne trade. Romanian ditty: "Furthermode, the story seeks to expand upon the Greeks; and it's obvious to see that the Greeks trade on the sea". $\endgroup$
    – AlexP
    Nov 30 '20 at 19:49
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    $\begingroup$ @AlexP the Japanese developed ample sea tradition as fishermen, and till the 1600s had some rather large navies! It was the invasion of Korea that ended with them lacking in a navy for 250 years. $\endgroup$
    – Trish
    Nov 30 '20 at 20:34
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    $\begingroup$ Why on earth would anyone wish to avoid boats? Does not compute.... and Ratty agrees. "There is nothing in life, half so much worth doing, as simply ... messing about in boats" $\endgroup$ Dec 1 '20 at 17:10

16 Answers 16

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Just make it so wood does not last on seawater

Your planet has a variant of the Shipworm, similar to the Earth variety except that it contains its own cellulose-dissolving enzyme rather than relying on bacterial assistance to do this job.

This would result in any unprotected wood in the seawater being leaky within a day, and riddled like a sieve within a week, like this one (which took about 2 months, here on Earth)
enter image description here

While it should be possible to build metal cladding or all-metal boats to protect against this, your people would have no reason to make this discovery, as they never encounter the need for it. All wooden constructions that touch seawater fail so rapidly, that they never develop an infrastructure that uses the sea. At all.

Is this hard-sciency enough for you? One very plausible change in one organism is all that is needed.

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    $\begingroup$ The fact that wood ships won't work would precisely be the reason for discovering metal cladding instead of trying to use not working wooden ship. Or more low tech animal skins spanned over a wood frame that is thus protected from the water. $\endgroup$
    – D.J. Klomp
    Dec 1 '20 at 12:46
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    $\begingroup$ "they never encounter the need for it" - we've explored the sky, space and the deep ocean, despite not having any need for that. If there's a way, someone will figure it out eventually, because that's what people do. Although it may be a believable enough explanation for fiction. $\endgroup$
    – NotThatGuy
    Dec 1 '20 at 13:03
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    $\begingroup$ @NotThatGuy Bad analogy. Rather mention not developing a 5-lever doorlock, for a civilization that does not use doors $\endgroup$
    – PcMan
    Dec 1 '20 at 14:21
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    $\begingroup$ Likely this would lead to earlier adoption of tarring hulls to preserve the wood. Once the utility of early rafts and dugouts is observed, there would be pressure to keep using disposable craft (you don't need it to last years if you can quickly make a replacement after your week's journey) and protect the wood or larger and more expensive craft. $\endgroup$
    – rek
    Dec 1 '20 at 14:43
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    $\begingroup$ Wood is not necessary for boats. At the low-tech end, there are things like the kayak and coracle that can be built from skin and bone. At a higher level, the US used concrete ships in both world wars, and there are ongoing concrete canoe competitions among the civil engineering departments of North American universities (and perhaps elsewhere?): asce.org/event/2021/concrete-canoe And of course these days you have fiberglass and other composites. $\endgroup$
    – jamesqf
    Dec 1 '20 at 17:00
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A second answer: enormous tides.

Your planet has a large moon like Earth does, but it's a lot closer, or in an elliptical orbit (perhaps as a result of a planetary collision a few millions of years ago), or perhaps your planet is a moon of a much larger gas giant and has not yet become tidally locked to it.

Either way, the tidal forces affecting your planet are a lot greater than those on Earth, and the result is that every day the tide comes rushing in in a giant wave, across many miles of land, all around the coast. This results in extreme coastal erosion that has created huge beaches all around the continent. Anything you try to build there will just get washed away the next time the tide comes in - there just isn't any way that you could build a functioning dock from which to launch a ship. You could launch a ship at high tide, but even if you make it to the open ocean before the tide goes out, there's no practical way for you to land again because there's no way you can navigate the white waters of the incoming tide. You could deliberately beach the ship, but then you won't get to use it more than once.

The extreme tidal forces would also make the planet a lot more tectonically active than the Earth, with a lot more volcanoes etc. Perhaps you could use this to explain why there is still a continent despite the extreme tidal erosion.

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    $\begingroup$ @Michael 20m is nothing even compared to what the Earth has experienced in the past, when the moon was closer. Hundreds of metres ought to be plausible. I'm talking about something resembling a major tsunami every day, which would just wash away any wooden jetty. The main limit on tide height is that if the tidal forces are too strong the crust can melt and you get a magma ocean instead of a land surface. (That has also happened on Earth in the past.) If this was hard-science I'd see if there was a way to put numbers on that, but for science-based I think it's ok to handwave it. $\endgroup$
    – Nathaniel
    Dec 1 '20 at 10:52
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    $\begingroup$ Note for all - I was very shocked to find that our once close moon, with enormous tides, quickly moved away to much lower tides in one hundred years. That is a blink on a geological timescale. Very surprising (and dashed many of my theories on early evolution!). I had imagined millions of years of tsunamis but that is not the case. I do not know if the slowdown is mostly due to the change in spin or the effect of massive oceans slowing it through tides $\endgroup$ Dec 1 '20 at 10:59
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    $\begingroup$ @MichaelDurrant thanks, that's useful information, and it makes sense - the energy for those huge tides has to come from somewhere, and that somewhere is the gravitational potential energy between the Moon and the Earth - which gets dissipated by the moon moving further away. So to make this realistic the planet probably would have to be a moon of a gas giant. (Io experiences much higher tidal forces than Earth - though it also has too much volcanism to be habitable, so the forces would need to be less than that.) $\endgroup$
    – Nathaniel
    Dec 1 '20 at 11:08
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    $\begingroup$ Overall I really like this answer, but there's something that undermines its validity: rivers. Rivers necessarily end up in the oceans, and the situation described in the answer doesn't prevent building littoral infrastructure on lakes and rivers. From there on out, there's a technological evolutionary path towards building ships that can survive in tidal basis, and eventually ships that can sail into the ocean. You could perhaps improve the answer with ways that prevent navigable rivers. $\endgroup$
    – Thierry
    Dec 1 '20 at 14:56
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    $\begingroup$ The height of tides is extremely influenced by geography. Ex Earth has a 0.6m tidal difference on the open ocean but 12-15m in the Bay of Fundy and almost 0 in the Mediterranean $\endgroup$ Dec 2 '20 at 8:32
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Boats started off as simple tree trunks floating on the water, which where then carved to better accommodate the passengers.

But that can only happen if the wood is lighter than water. There are various types of woods which are denser than water, meaning that any trunk falling in the water would sink rather than floating.

Moreover some of those woods is also extremely difficult to work, such as the ironwood.

So, if your world has only that type of wood or similar, you are basically left with a much more difficult discover of the principles of floating on water. Not much more different than what has happened in our timeline with flying, which is something we have mastered in the very recent past of our millennial history.

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    $\begingroup$ Boats and rafts are very different things. A raft is not a boat, and a boat is definitely not a raft. Boats do not float because wood is ligher than water, they float because of the air they enclose. We have found monoxyle boats 10,000 years old... $\endgroup$
    – AlexP
    Nov 30 '20 at 20:44
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    $\begingroup$ @AlexP Yes, but would you think to try to build a dugout if you had never seen wood float before? $\endgroup$ Dec 1 '20 at 0:22
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    $\begingroup$ Flying requires sufficient power vs mass and is a difficult problem. Floating is trivial - we float already. Any ordinary person that washes wooden cups in the river/sea/... would soon notice these cups float even though the wood itself sinks. Preventing discovery for an ancient Greece tech level seems impossible. $\endgroup$ Dec 1 '20 at 6:58
  • $\begingroup$ @LoganR.Kearsley: Would you think to build a telegraph if you had never seen wires carry words before? Some inventors are just crazy creative! Besides, all you have to do to "see" metal boat technology is to accidentally drop an empty metal pot into a sink full of dishwater. Any civilization that invents cooking and washing-up is basically one rich aristocrat away from inventing a giant metal pot-boat. $\endgroup$ Dec 3 '20 at 16:30
  • $\begingroup$ @Quuxplusone When you are working in the context of a civilization that already has a long history of transmitting messages by a variety of signaling methods (writing, semaphore, smoke signals, drums, battlefield pipes / trumpets / etc., bell pulls--which are literally actuated by signal-carrying wires!), and you have just discovered a new signaling method... then yes. Cf. never having seen any large inanimate object float. $\endgroup$ Dec 4 '20 at 17:49
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The easiest way to discourage the development and use of seagoing vessels may be to discourage associating with the sea itself: prolific littoral and neritic predator species too large, dangerous, and/or difficult to hunt even in groups could, from an early stage, dissuade your people from going anywhere near ocean coasts and instead focus development on rivers and other freshwater bodies.

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    $\begingroup$ Relative to your account bio: "... and a chasing after the wind." $\endgroup$
    – The Daleks
    Nov 30 '20 at 22:45
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    $\begingroup$ @Mandelbrot, just so you know, a good rule of thumb here is to wait 24 hours before accepting an answers. Not everyone is in your time-zone, so you can have great answer if you wait enough time (for example, both PcMan and Nathaniel answered after you accepted rek's answer). $\endgroup$
    – Kepotx
    Dec 1 '20 at 12:51
  • $\begingroup$ Wanted to answer exactly this. Until the Humans have conquered modern firearms and metalworking to make armored ships they simply will not dare to go out to the sea. $\endgroup$
    – Hobbamok
    Dec 2 '20 at 10:26
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Here is another way, which could be combined with other answers: make the planet cold, either permanently or because it's in the middle of an ice age.

The idea is that only the equatorial region is really suitable for human habitation. If you go too far north or south everything is covered in glaciers and the sea is frozen. Seen from space, the planet is more than half covered by white ice caps, with a blue-green band around the equator, something very roughly like this:

enter image description here

The Earth has probably been in this kind of state several times in the past.

Now since the continents are all clustered together, there isn't much point in using boats. The human-habitable region is a letter-box shaped piece of land that runs across a single large continent in the east-west direction. The climate there is temperate, but to the north and south it's bounded by icy wastes rather than ocean. It has coastlines to the east and west, but they're relatively short, and to get from one to the other you'd have to sail around the entire planet.

This would severely limit the usefulness of ocean-going vessels. If you had them you could use them to travel north-south along the coastlines, but this would provide a lot less economic benefit than it does on our world, so there would be a lot less economic pressure to develop them in the first place. If the coastlines are not rocky then your people might just build railways or canals along the coasts instead, and then never really feel the need to develop shipping beyond sport and fishing. (The latter of course only applies if there are edible fish.)

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Your citizens suffer from terrible sea sickness, they don't just throw up, they get unconscious and maybe suffer long term damage.

So peaceful rivers would be fine, but even estuaries would be too choppy for your would-be sailors.

Actually this would be so anchored in collective knowledge that no-one would ever even try to go to sea. So you could have a sub-plot of some outlier who finds that it is possible after all, and acts accordingly...

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    $\begingroup$ As someone who gets motion sickness easily, this would put me right off. The only ferry boat I can travel on comfortably is a catamaran, which powers through the waves with thumps and vibration, not wallowing back-and-forth. $\endgroup$
    – Criggie
    Dec 1 '20 at 21:03
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Historical precedence

The ancient Greeks didn't do any trans-oceanic travel anyway, so the question is arguably moot.

Before the armada days... there essentially was no ocean travel. Yes, brave Norse settlers made it to the Americas, but the travel was lengthy, arduous and risky enough that only a few made it to Greenland and a little beyond, and even they arguably got lucky: if North America had been a little more southerly, all they'd have found would've been sea ice.

Greek trade routes, via Ancient History Encyclopaedia [Greek trade routes, via Ancient History Encyclopaedia]

So really, you don't even need to make any changes, apart from wiping out those pesky Norse adventurers, and making it so that the continents didn't have any convenient paths between them, like "hey if you just follow the edge of the sea ice west from Norway, you get to a McDonald's!".

However... that's not very "hard science" of me. In fact, science (Feasible Ocean Routes to and from the Americas in Pre-Columbian Times, quoting Chronica-Botanica-Vol-xiv) says:

It would be foolish to assert that there were no communications across the Pacific in pre-Magellan times

I feel that there is a fairly solid case for this being at least a thing that was feasible to happen at that time, given that hundreds of plant species have been found which appear to have spread between the continents with no other explanation.

We want ten thousand years of no seafaring, so we need defense in depth! Many layers! Travel between continents only when sea levels change and expose land bridges!

So while it might be a helpful factor, merely saying "OK, there are no easy paths, and no brave adventurers" won't cut it :)

Predators

Making seawater scary with predators, as rek suggests... again, it might be a useful additional tactic, but I don't think it would work, not for thousands of years for humans, anyway.

Waterfront property has been prime property as long as humans have existed. We migrated along waterways.

We take "dangerous" as a challenge. We've obliterated almost all large animals that we don't deliberately breed to eat, and all existing species are getting smaller as we kill and eat the biggest ones. For millennia we have harvested food from waters filled with shark and piranha and crocs and gators and vipers and poisoned sea urchins and deadly jellyfish and... and we eat them.

This isn't a new habit: we probably killed off the NA mammoth and smilodon, and likely were a contributing force for much of the rest of the wave of Late Pleistocene Megafaunal Extinctions.

So, sure, have predators, but just as one of many reasons the sentient land-dwellers ("They") have, to avoid screwing with the ocean.

The most valuable kind of predator could be the ship-attacking kind. If any large craft going past the continental shelf runs a very good chance of getting capsized boned by aggressive or amorous whales, then most craft won't go past that line, at least until they realize how much ambergris, whalebone and whalemeat are worth.

Hrm, OK, we don't want to give them an economic incentive to venture out: maybe kraken, rather than whales, then.

Nutrition vs Poison

(I see @Kari beat me to this one, darn! Updoot for Kari!)

On Earth, we may well have taken to the water due to an aquatic phase in our evolution... but whether or not that is the case, we stuck by the water because it's a fantastic source of nutrition, such as DHA, an omega-3 found abundantly in human brains and in aquatic foods. This aquatic aspect of our diet may well have contributed to our large brains. Oh, and salt, we use a LOT of that, with our abnormal behaviors like sweating and crying.

If everything in the ocean was toxic to Them, They'd have no real interest. Add some heavy metals, radioactives, some lead and arsenic and some faster-acting things, to the seaweeds and shellfish, and have the toxins concentrate more and more up the entire food chain.

Perhaps make rock salt toxic to Them. That's extremely believable, given how toxic it is anyway. Or have other soluble minerals leech out of the rocks of the land and into the oceans.

But you don't even need to specify what toxins affect Them; just that sea animals have the ability to process ocean foods that They as land animals have since lost. Remove the omega-3s, or remove Their need or ability to process them.

Provisions

With no seals, polar bears, edible fish, etc to catch and eat, and nowhere to stop and find freshwater or meltable snow, travelers would be constrained by supplies they could take with them. With a large enough distance between, this would be a hindrance, and set a hard date for having to turn around, equal to less than half the provisions a craft could carry.

Non-swimming

We can swim and float, so can spend time in water, make dugout canoes that can tip us in without dying, and so on. Let's get rid of all that. Their species sinks in water. Heavy bones, dense muscles, small lungs, non-closing airways, no diving reflex, poorly formed for swimming... so not real fond of swimming in the first place, even less fond of swimming anywhere near the deep, toxic ocean.

Conceptual leaps

As L. Dutch pointed out, if the only buoyant stuff They ever see is bloated corpses, They'd take a while to even come up with the idea of floating airbags for travel, and those just aren't very ocean-worthy, but do well enough on rivers if you stick them under some kinda raft framework. Waterway dirigibles, pulled by draft animals trudging along the waterside, as we did with canals; or by pulling on a rope alongside the waterway.

It's admittedly a "small leap" from there to surrounding them in a solid sealed box instead of a mere framework, and from there to the discovery that it doesn't matter if the solid box has airbag in, or even whether it has holes in, so long as they're above the waterline, and from there to realizing that you can just float an open-topped box in a way that it won't flip over and drown everyone, and from there to ballast and sails and masts and rudders and rigging and then reinventing rigging to sail into the wind...

But even so... look how long it took us to develop wheeled vehicles (5500ya)! And another three thousand years after that (2400ya) to put teeth on it and make a cog, a thousand years after that (1300ya) to make a clock, and six hundred years more (700ya) to make a worm gear! Each of these, looking back, is a "small leap". And none of them kills us much if we screw it up. And even then, the wheel didn't really penetrate into sub-Saharan Africa until the Europeans turned up in the 1800s.

Prevailing winds

Hot air on land goes up. This means wind blows from the sea: a "sea breeze". In the night, it blows the other way, as the land cools, but the sea stays warm. So Their more sensible sailors make sure to be home before that happens, rather than get stranded away from shore.

Why would they be stranded? Because with poor rigging tech, They couldn't tack into the wind very well.

If you also have a climate without any constant trade winds for their navigators to follow, the inconsistent and mercurial climate would mean that there'd be no obvious paths for people to try exploring in. I'm not sure how realistic a world without tradewinds and currents would be, though...

Construction materials

The Armadas of Britain, Spain, France, and Holland, were all formed from forestry. The devastating impact of the formation of the British armada on the countryside of the UK can hardly be overstated. Maintaining the armada required the formation of a proper forestry commission with control over sizeable tracts of land doing nothing but the slow process of growing trees long enough to lay straight keels, masts, and so on.

I have heard that in the great expedition of 1588 it was expressly enjoined the Spanish commanders of that signal Armada that if, when landed, they should not subdue our nation, and make good their conquest, they should yet be sure not to leave a tree standing in the Forest of Dean. -- https://www.jstor.org/stable/550221?seq=1

If your civilization lacks wood as a construction material, and instead uses mostly stone, leather and metal, then that both sets a cool aesthetic, and helps to explain why large ships might not get built for some time: only perhaps coracles, for shallow freshwater marshes.

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No non-perishable food which contains Vitamin C.

This will only prevent long travel over open ocean.

Scurvy was a huge problem for Earth’s sailors and only really solved with lemons, limes and Sauerkraut. Just get rid of those foods and you should be fine.

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    $\begingroup$ Except that humans invented ships and used them long before we knew about it, and even long after we knew about it. $\endgroup$
    – pipe
    Dec 2 '20 at 9:15
  • $\begingroup$ Yes but it did create huge problems for ocean crossings. If average travel time had been slightly longer (more than ~3 months) I imagine it would have prevented it completely. Interestingly effective cures (e.g. lemons or pine needles) popped up throughout history several times but then were forgotten again. $\endgroup$
    – Michael
    Dec 2 '20 at 9:55
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    $\begingroup$ @Michael yeah, but OP sounds like he's not just talking about 3 month plus travel, but also blocking people from Madagaskar and similar (or island hopping in southeast asia. Because these two things are perfectly feasible, if you REALLY want it probably without food even if you get a bit lucky $\endgroup$
    – Hobbamok
    Dec 2 '20 at 10:27
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Here Be Dragons

For one reason or another (maybe not having an ice age), the sea life is... large. Very large. Whales are the size of large boats and armored. Their predators (think Mosasaurus from Jurassic World) have beaks capable of crushing that armor. They roam all coastal seas and boats get bitten in half before the Mosasaurus discovers there are only tiny snacks inside.

So humans use boats on rivers all the time, but get anywhere near the sea and it's just too dangerous. Every generation some foolhardy adventurers take a boat out to open sea and get themselves killed in front of the townsfolk just to remind the rest that it's still a really bad idea.

The aquatic predators can be of any type you like of course, but since they're still in the middle of an evolutionary arms race with the other huge sea life, they should be huge and mostly immune to anything humans can come up with.

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If you have no sea fishery, because all oceans are infested with poisonous jellyfish or something, that would keep people away from the sea.

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As you can change geography, you could make the sea full or sharp and point rocks, and add that is possible to change the weather, make it in a way that the cluster of the continents made significant changes in the atmosphere, as the Bermuda triangle, which resulted in strong storms at deep sea, making the previous detail even more dangerous.

But going a bit further, if you agree, its possible to make the geography of deep sea being composed of mainly volcanic like structures, which could heat up the water at dangerous levels to human life.

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Eliminate boat-focused transportation in your world the same way that we eliminate unicycle-focused transportation in ours. Sure, unicycles exist, but they're a novelty. Whatever you're using a unicycle to do, there's a better, easier way to do it. There's no real reason to use one as a primary transportation device.

On an Earth-like planet with clustered continents and large oceans, like Pangaea, the easiest method of travel would be boats.

Is it, though? If your land mass is clustered and your oceans massive, there doesn't seem to be any reason to build ships capable of traversing the ocean. Where exactly would you be going? Crossing the ocean in a ship is just a really long, dangerous, difficult way of traveling to the other side of your continent. Somebody might do it once for the glory, but it will never be seen as a practical mode of transport.

The only remaining use cases for your ocean would be for fishing, or for shuttling people/cargo up and down the coast. Eliminating the first is easy: you have ample sources of fish available inland, and the only fish in your ocean worth eating are significantly harder to locate and catch. Building ships and going out to sea to catch fish is such a terrible return-on-investment that it's pointless.

To ensure ships aren't used to move cargo from port to port, ensure your world has much easier ways to get stuff from point A to point B. Perhaps there's a vast system of navigable lakes and rivers that meet your transportation needs via riverboats and barges while being safer and more predictable than ocean travel. Add a few canals and you can have a rather extensive system of inland waterways (example, the Intracoastal Waterway in the US stretches 3000 miles from Boston to the southern tip of Texas). Having population centers towards the center of the continent and farther from the coast helps your case as well, as the extra work required to haul cargo back and forth to the coast might be greater than what's require to ship it overland. You can add navigational hazards to the oceans to further reduce the attractiveness of ocean shipping. Reefs and rocks can make certain areas impassable. Natural ocean currents and trade winds can be oriented in the wrong direction, or could pull ships out to sea (similar to how it's harder to sail from Europe to South Africa by following the coastline than it is to cross the ocean to Brazil and then cross all the way back). If ocean transport is significantly more difficult than using inland waterways or transporting things overland, then people aren't going to bother building oceangoing ships. Your port cities and shipping centers will naturally develop on rivers and trail crossroads instead of coastal areas.

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You could design your world around a specific type of weather which would result in frequent or permanent high pressure areas over oceans or specific parts of it. Combined with volcanic activity, this could lead to gases which disperse oxygen accumulating in pockets over vast swathes of ocean. This would make travel in these areas impossible for a civilization with the technology level you mentioned.

Signs of these deadly areas are quite easy to spot (no animals, smell, etc) but the areas themselves are hard to overcome.

However, I am not sure of the hard science behind the feasibility. While the premise, in my opinion, is sound, the gases might disperse into the sea water over time. It would also mean having a sustained level of volcanism below sea level which might have other side effects such as drifting continents.

However, it could also be a story element in which changes to the weather conditions or volcanic activity could easily open up sea routes.

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Your world could have had a big asteroid crash into the ocean and cause the shore around the world to be toxic. Or maybe you just have different chemical properties that happen when fresh water mixes with the type of water in your seas which causes a big toxic area bordering all the oceans that is stinky and poisonous.

So there would be organisms, large and small and bacterial processes in that area that stink really, really bad. Humans just stay away because of the stink. They don't even travel within 10 miles of the ocean. Most of them don't even know there are oceans, they just know there are areas that stink so bad. Only people who have bad olfactory glands have ever dared venture near the sea. :) But the air is actually poisonous so those who didn't smell it die before they realize it's too late.

It isn't just the stink, but the air around it is poisonous, like sulfurous or something. You could make it so that the shoreline has organisms that grow with this toxicity or you could make the rocks interact with something in the water that causes this toxicity, but really it's just the shore that's bad, so if humans were able to fly over the shore they'd eventually get past the stinky/poisonous area and maybe reach a normal non-toxic ocean which contributes to normal non-toxic weather patterns, but the wide areas bordering the shoreline are so bad they never go to the ocean.

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Simple: The trees of the planet have dense wood to protect them from harsh conditions and insect attacks. However, their density means that while they can definitely be used for construction, making boats out of them is a bad idea. Or the wood is too waterproof for a boat to balance, and would thus make it capsize as soon as it hit the water.

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Frequent invariably fatal phenomena - eg

  1. Kraken / Sea Monsters - large, unpredictable, undefeatable, ever vigilant, always fatal.
    (These MAY exist on earth currently but, if so, they are very stealthy and rapid in attack and invariable fatal :-). )

  2. Frequent rogue waves - again unpredictable and immense and invariably fatal.
    These occur on earth very occasionally and very occasionally sink even large liners.

  3. Your choice ...

If the frequency of either of these was somewhat greater than on earth
(much greater for Kraken :-) ) and if they were invariably catastrophic for boats of any sensibly achievable size then boats above that size would not evolve. Boats well above "sensible size" may be built as a trial, but if these always met with disaster the attempts would be 'discouraged'.

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