Batteries are the real design constraint
Why energy density decides the shape of the whole vehicle.
Batteries are the real design constraint on an underwater vehicle because, unlike a car that can simply refuel from a tank holding far more usable energy per kilogram than any battery pack, an underwater vehicle has to carry every unit of energy it will ever draw on for an entire mission sealed inside its own hull from the moment it launches, and the sheer bulk needed to carry a meaningful amount of that energy ends up dictating the size, weight, and shape of practically everything else on board.
The mechanism behind energy density
A car's engine burns fuel by combining it with oxygen drawn continuously from the surrounding air, which means the vehicle only has to carry the fuel itself and gets the oxidiser it needs entirely for free from the atmosphere it is driving through. An underwater vehicle has no such atmosphere to draw on, so even a fuel-burning design would have to carry its own oxidiser sealed alongside the fuel, closing much of the apparent energy advantage combustion would otherwise hold over a battery and helping explain why batteries, despite storing meaningfully less energy per kilogram than fuel does on land, remain the practical choice underwater rather than an obvious compromise. What a battery stores is fixed the moment it is sealed and charged, with no way to top it up from the surrounding environment the way a car quietly tops up its own oxidiser supply with every breath its engine takes, so the total energy available for an entire underwater mission is set once, in full, before the vehicle ever leaves the surface. There is no equivalent of coasting into a petrol station partway through the day, no way to draw a little more from the world outside the hull once the mission is under way, which turns the initial sizing decision, how much battery to carry, into one of the single most consequential choices made about the whole vehicle.
The torch-battery comparison
A torch running on disposable batteries lasts only as long as the cells packed inside it, and getting more runtime out of it means building a bigger torch body with room for more cells, the battery's own bulk directly deciding how large the torch has to be. The same bulb wired instead to the mains runs indefinitely from a wall socket, adding no extra bulk to the torch at all, since the energy source lives somewhere else entirely and simply flows down a wire on demand. An underwater vehicle is, in this one respect, exactly like the battery-powered torch and never like the one on the mains, because it can never plug into anything mid-mission, and its hull ends up shaped, in large part, around however many cells its intended mission length actually demands. A designer asked to double a torch's runtime reaches, almost without thinking, for a longer body with room for more cells rather than a cleverer bulb, and a designer asked to double an underwater vehicle's endurance is reaching for exactly the same lever, just wrapped in a pressure-rated hull rather than a plastic torch casing.
The number that matters here
A tank of diesel or petrol the same size as a battery pack stores several times more usable energy for the same weight, a gap wide enough that a vehicle designed around combustion would need to carry only a modest fraction of the mass an equivalent battery-powered vehicle carries purely for energy, if it could draw its oxidiser for free the way a car does. Underwater it largely cannot, which narrows that gap substantially, but even the narrowed version leaves batteries delivering markedly less usable energy per kilogram than the fuel a surface vehicle takes for granted, and that shortfall is exactly what a battery-powered hull has to make up for in sheer volume and weight rather than in cleverness. A flywheel offers a useful point of comparison rather than a real alternative here, since it stores energy mechanically instead of chemically and can release it far faster than a battery ever could, but it holds a comparatively small amount of energy for its own weight, which is exactly why it turns up as a short, sharp power boost elsewhere in engineering rather than as the main store for a mission lasting hours.
Why this matters in practice
Because batteries are heavy for the energy they hold, they routinely make up a large share of an underwater vehicle's total weight, which then has to be balanced against buoyant foam and displaced volume elsewhere in the hull using exactly the density arguments an earlier set in this archive covered, so a battery bay is never sized in isolation, it is sized alongside the buoyancy budget the whole vehicle depends on to float correctly in the first place. Mission duration and hull size end up coupled far more tightly underwater than on land as a result, since doubling a car's range mostly means fitting a slightly larger fuel tank into space that was already going spare, while doubling an underwater vehicle's endurance can mean finding room for twice the battery volume, which then forces a larger hull, which then displaces more water, which then demands still more ballast and buoyancy to balance correctly, a chain of consequences a fuel tank on land never sets off in the same way.
Where this stops being true
The whole constraint disappears for a vehicle tethered to a surface ship or platform by a cable, since power then flows down that cable continuously rather than being carried on board at all, freeing the hull from the energy-storage problem this article describes entirely and letting its size be set instead by whatever sensors, tools, or cameras it actually needs to carry. It is only the vehicle that has to be genuinely self-contained, free to travel wherever a mission requires without a cable trailing back to safety, where the battery this article describes becomes the single loudest voice in how the whole vehicle ends up shaped, quietly outranking almost every other design decision made about it.