I mean, this one has been pretty thoroughly answered, and there’s some weird water fetishism going on or something to find some sort of use for it.
What advantage is water supposed to have here? Does it stop rounds more effectively per unit of volume? Weight? No, neither. Yes, shooting bullets into a swimming pool will stop the bullets after a while. Shooting them into a giant block of steel will shoot them much quicker. The OP seems to think that water has some sort of property that stands up to large projectiles better than steel, but that simply isn’t true.
So what is water supposed to do? Save money on tank construction? I’m sure the actual steel armor is fairly cheap - plus a lot cheaper to maintain than a bunch of separated water tanks. Is it supposed to be more effective in some way? Better protection in the same space? No. For the same weight? No. Is it simpler or otherwise offer some sort of advantage? No.
If someone were shooting at you with a rifle, would you rather hide behind a waterbed or a refrigerator sized chunk of solid steel?
The OP’s question seems to come from the Mythbusters experiment about diving into water to escape a shooter. As mentioned, the larger, more powerful, high caliber rounds disintegrated almost on impact with the water whereas the smaller, slower pistol rounds made it further. That suggests there might be some properties of water that give greater protection from more dangerous weapons. After all, if simple water can stop a .50 caliber round in less than three feet, then maybe all this plate steel and kevlar and other fancy armor isn’t really necessary.
Unfortunately the physics just don’t seem to work, as the cornstarch/oobleck experiment showed. We’re still talking feet of water as opposed to inches of steel or whatever else. Yes, water is much much better than air at stopping bullets, but that doesn’t mean it’s better than solid materials. The Mythbusters got similar results when firing into ballistics gel, where the 9mm round went through three or four blocks of the stuff that were like 18" long, whereas a .30-06 (I think) didn’t even make it through the first block. It’s not much different than water really.
As noted above, water does in fact dissipate the energy of shaped charged anti-tank warheads around 3 times more effectively than (RHA) steel per unit weight. Various moderate density materials are better than steel per unit weight against such warheads, a major threat to armored vehicles ever since WWII. Though as we know (I hope) the armor of modern tanks doesn’t mainly consist of steel, so steel RHA is just a benchmark and being better than it doesn’t mean best solution.
I would think this would suggest that higher velocity can have counterintuitive effects because some non-linear relationships are involved. If anyone can explain the physics of why high-velocity rifle bullets go shorter than low-velocity pistol bullets in water, please enlighten us.
About its use as a benchmark: I’ve seen some armor beings described as say, “900 mm RHA equivalent against HEAT” and “600mm RHA equivalent against APFSDS” which would suggest that even as a benchmark, steel RHA can be lacking; benchmarks do their job best when their data output is homogeneous. I’ll admit to not having figured out what could replace it.
One thing I’ve wondered about is protection against warheads which specialize in creating spalling. Is it possible to make it so that a tank is nearly completely protected against spalling or is that just inevitable when getting hit with a 120mm HESH round?
The bullets are made of a soft material (usually lead with a thin jacket of harder metal). When they hit the water, they deform. Pistol bullets tend to hit at velocities that allow the bullet to mostly keep it’s shape, rifle bullets hit at a high enough velocity that the bullet breaks apart. The solid bullet is better at going through a distance of water, while the broken bullet spreads the energy around and deflects away from the target. Some of the lower velocity rifles will also tumble when they hit a target (I know .223/5.56mm rounds are known for this), which has a similar effect.
A bit of a numeric check when comparing what water does to small arms projectiles vs those fired from MBT guns. Your typical small arms projectile will have a kinetic energy at the muzzle from anywhere between 430 or so foot-pounds (9mm cartridge from Beretta 92, 124 grains at 1250 FPS) to 13,250 foot-pounds or so (.50 BMG M33 cartridge, 704 grains at 2910 FPS.)
I found the exact weights for penetrators from a 120mm sabot round (M829A3) to be difficult to find. Never the less,the figure I saw for the A3 is about a 6.2 kg. Let’s use 6 kg.
Muzzle velocity once the sabot peels away is in the 5700 FPS range. KE with 6 kg at 5700 FPS is about 6 and 2/3 million foot pounds. Or about 500 times as energetic.
The energies with a main battle tank cannon are just laughably beyond anything we can infer from watching the behavior of small arms projectiles impacting water or other materials.
And what Pantastic said above me. It’s the reason 5.56mm projectiles often penetrate less through typical housing materials than do slower pistol projectiles. Not that much less, mind, but still less.
True but lacking a simple material with consistently similar resistance to kinetic and chemical energy warheads, and steel armor plate having been once the dominant form of protection, it will probably remain the standard. Although even besides the differences in theoretical thicknesses depending of the warhead, the thicknesses themselves have become detached from practical reality, eg nearly a meter of steel armor is obviously not a practical method of protection. And also the degree to which a warhead has to over-perform to do real damage behind the armor also varies (eg. how much damage a projectile/warhead capable of 800mm RHA will do against a protection array capable of resisting 700mm RHA also depends on type of projectile/warhead).
HESH warheads became largely obsolete against tanks’ main armor as soon as armor types besides homogeneous steel became common. An HE round can still damage a tank’s running gear, ports for eletro-optical devices, blow away explosive reactive armor layers for subsequent HEAT/KE hits etc. but on most modern tanks the outer armor plate you see is going to ‘spall’ onto some intermediate part of the armor array inside but the boxy shape of the turret or hull, not into the interior.
Water is better than many things. I personally have shot a 1 foot square water box at point blank range with a 30-06 and it did not penetrate the water. The projectile was destroyed. Yet, a 30-06 will penetrate 20-24 inches of live tree, and the armor piercing round will pass through 36" of oak and keep going.
Gravel and sand are very effective bullet stoppers because of the friction of so many particles against each other. One cheap bulletproof wall is a standard 3 1/2" thick wall cavity filled with 1/2" gravel. Nothing a bad guy will carry into your house will get through.
We do in fact use water to bomb-proof certain things. Our high speed flywheel facility at NASA used a containment of two rows of 5 gallon Cubitainers. That is what I was testing with the rifle, but we also put it to the test using actuall flywheel fragments in our high velocity nitrogen cannon.
There are commercial “water boxes” available for bomb squads. They consist of a 36" square cardboard box lined with Cubitainers, leaving a foot square chamber in the center to place suspicious packages. You fill them with water before use, until then they are folded and fit into the trunk of a car. I forget the target protection level but it was usable, something like a lb of TNT would be quenched.
There are temporary ammunition magazine that use water filled, half round cylinders about 8 feet tall. Actually they are crescent moon shaped and they stack into walls having a thickness of a half meter as I recall. They were designed for a 1000 lb bomb in each magazine.
And finally, the same company provided videos of their water bag blast protection. The water bags looked like saddle bags and were hung to just overlap on the sides of a small car. A similar unprotected car was parked nearby. The explosives were detonated between the two cars.
The unprotected car was demolished. The water protected car had only slightly dented panels.
Now, this is blast protection, many small particles at high speed arriving nearly simultaneously, plus the quenching of the blast wave from cooling. A high velocity shell fired at a tank is a different story.
Suppose for a minute that Bob the aquatic terrorist decides that he really hates swimming pools and and shoots one up with a soviet style rpg. What would happen (other than some pissed off lifeguards soundly beating bobs ass if he chose a public one :D) Would the liquid interfere with the molten spike from forming correctly?