I was thinking maybe because the ocean is a lot flatter than a lot of terrain but there’s also large waves and storms and sea. I also can’t imagine that something like an aircraft carrier would be significantly less complicated or difficult to build than an equally sized land vehicle.
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In the ocean, the water supports the weight of the ship.
On land, the wheels/tracks have to support the weight of the vehicle.
#1 You need to move it. Building gigantic tracks or wheels would get expensive fast.
#2 Where would you even use it?
#3 What would be the purpose? Something like that wouldn’t be able to move very fast.
As for size, larger ships are much more stable than smaller ships because it takes a lot more energy to rock them about.
The water supports the entire weight of the vessel without limit. On land, there are limits to the weight you can have per axle, plus there are limits to the size and length of the vehicle.
It doesn’t make sense to build a city sized truck if you can’t drive it anywhere. On the water, you can have nearly unlimited sized vessels (look at ocean going tankers) you just won’t be able to dock in certain ports. That’s why larger vessels use smaller tenders or ferrys-to access ports they couldn’t normally reach.
It is as simple as the fact that you don’t need to build roads on the water.
To build a land based vehicle, it needs to be able to fit the infrastructure of the land – roads and bridges for example are designed to be a certain width, and strong enough to support a certain weight of vehicle. Countries will then place limits on the size of the vehicles they allow to use their roads so that they are appropriate for the infrastructure built – a standard truck will fit on the road and won’t over load a bridge and cause it to collapse by driving over it.
If you want to make a bigger vehicle? You now need to replace every road with a wider and stronger one able to support it, much to the annoyance of everyone that lived alongside the previous road whose homes and businesses are now being demolished to widen the road.
On the water, this doesn’t really matter – the oceans and seas don’t need any preparation or infrastructure to allow a bigger boat to travel over them. As long as your boat is able to float safely and deal with things like storms and waves safely then you are good to go.
The biggest issue with larger boats is the infrastructure in ports which needs to be able to berth and load/unload them. The trick is that you only need to build a port at the very beginning and end of the journey, so the cost and practicality of this is far more reasonable than on land where you would also have to upgrade every meter of roads between those two points as well.
You’ve probably heard an explanation for why a person can survive lying on a bed of nails that runs a bit like this: their weight is spread out over many many points, which reduces the weight pushing on any obe point. That’s almost enough info already. The thing is, they’re not infinitely small points, the tip of the nail has some non-zero area, even if it’s 1 mm² the amount of weight (or force) applied to the tip of each nail is (on average) the entire weight of the person divided by the total number of nails.
So let me put some numbers to it. A 150 lb person manages to spread their weight evenly over 1000 nails. That means 0.15 lbs is pressing on each nail. I’m sure you can already imagine that if you pressed a nail into yourself with 0.15 lbs (0.667 N or the same force as 68 g would experience due to gravity. So imagine a 68 g nail sitting point down in your hand. According to Google, the area of the tip of the nail is 1/100th of 1 in². That’s actually closer to 6 mm² than my original estimate, but here’s the key part. Pressure is force divided by area. So our force is 0.15 lb and our area is 0.01 square inches. That’s 15 pounds per square inch (psi). Which is actually really close to 1 atmosphere of pressure. The air around you pushes you with 14.8 psi. Google says 100 psi is needed to puncture skin.
So, now that I’ve become far too long-winded, let me bring it back to ships. Water is a fluid. The water pushes on you, if you go a little deeper, the weight of the water above pushes on the water below, which pushes on you. So the deeper you go, the higher the pressure. But more importantly, it’s pushing everywhere it touches, all at once. It’s like a bed of nails, but the nails are as closely packed as molecules. Every square inch is being pressed on by quadrillions of “nails”
When people talk about the weight of the vehicle being supported, this is what they mean. The ocean is just the world’s best bed of nails. It distributes the weight over the entire surface of the ship’s hull. And it’s not perfect evenly distributed (deeper bits of the hull experience higher pressure) but there are no gaps between nails, so it’s more like just lying in a bed, than a bed of nails. Land vehicles aren’t vehicles unless they move, and it’s impractical to make the entire bottom of the vehicle 1 giant track. Which means the weight is being spread out over a smaller surface area, meaning higher pressure. This means stronger materials are required. Usually bigger thickness axels and stuff, meaning it’s heavier, meaning stronger materials. Either the embiggening cycle never stops or it does, but it costs so much that it’s no longer feasible.
The biggest land vehicles are limited by the weight they can carry and the amount of friction they create with the ground. Boats are not limited by these factors because they float on water.
1. Where would you put all these large land vehicles on the roads?
2. Ever heard of trains?
The maximum size of vehicles is decided by the other things they have to interact with to do the jobs they have to do.
Max boat sizes will be decided by the docks they have to visit, or the harbor depths they need to come into. That can be pretty big.
The job a boat does is to move a bunch of stuff from one place to one other place.
Max truck sizes might be decided by the height of bridges they need to go under.
The job of trucks is to move just a few things to a whole range of different places.
Then there are trains. Cubic storage of a train might be comparable to a big ocean vehicle, but the length of a train is going to have to at LEAST be small enough to fit between one place and the next, right? And the width of the train is very much decided by the tracks it runs on.
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