eli5: Why do cars seem to float in shallow water, but sink in deep water?
They can be a little buoyant depending on the shape of the floor pan, but what you’re probably seeing is cars getting swept away in floods.
This is because water is heavy, one cubic meter weighs a metric ton – which leads to people frequently underestimating the overwhelming force of rushing water during a flash flood.
They drive into seemingly innocuous knee-deep rushing water, and it blasts the car with hundreds of tons of force that drag it away or flip it over.
Some cars will briefly float in deeper water while the passenger compartment fills up, but they’re mostly metal and soon sink like a brick.
All cars unless they’re specifically waterproofed will eventually sink to rest on their wheels its simply a question of time. I think perhaps you’re thinking of where someone tries to drive their pickup truck across a swollen river that is flowing over a road or something and their vehicle gets swept down the river and they have time to climb out – in that case, its taking time for the water to seep in through all the weather proofing, so the car will be somewhat bouyant for a while. It also helps in cases like that where the water isn’t actually that deep – so maybe the rear of the car or just one wheel is touching bottom, so that will help keep the car bouyant for longer.
Now, if you drive off a pier like in the movies, well that’s deep – how long your car stays bouyant just comes down to how well the weatherproofing and door seals fit. Which if you hit the water with any speed, all your body panels are now likely warped and bent, so the water is gonna come in a lot faster. If you just drive into deeper water your body seals are going to stay pretty tight.
I put the family 626 into a drainage ditch at slow speed in highschool; had mostly stopped by the time we hit the ditch, we just kinda rolled down the embankment. We weren’t too panicked, we know the ditch wasn’t much deeper than halfway up the A pillars, but since we didn’t warp the body at all it stayed surprisingly dry for a good 20+ minutes. We were able to roll down a rear window and climb out before the front seat flooded.
A friend put their F150 into a slightly deeper ditch at speed and that sucker sank like a rock in about 60 seconds. <insert crack about Japanese vs. domestic build quality here; though he bent the truck pretty good, that probably helped.>
Cars are attempted to be made leak and draught proof but with doors windows and various mechanical parts this seal isn’t perfect so cars slowly leak water when immersed, the deeper the water the greater the pressure and the faster they leak, so in shallow water cars take longer to sink.
Cars aren’t water tight. When a car gets submerged, it will float because it’s full of air. As air escapes and water seeps in, the car will start to sink because it loses buoyancy. Its like helium leaking out of a baloon.
Cars will float until the water encroachment overcomes the buoyancy of the tires and internal compartments. Then they sink.