Deep wading or deep fording is a technique used by some heavy semi-amphibious vehicles to traverse water that is several metres deep - the vehicle drives on the riverbed/lakebed/seabed and uses screens or a pipe (a snorkel) that reaches above the water surface for an air supply. The technique has been used by armoured military vehicles such as tanks and armored personnel carriers. Deep fording generally implies driving through water of such a depth that the vehicle is mostly or entirely submerged by the water, sometimes to several times the height of the vehicle itself, as well as a sealed crew compartment, complete with air supply for the crew. This makes it different from the less extreme "wading" done by many off-road vehicles which are simply fitted with a snorkel that is higher than the normal engine air intake, roughly level with the top of the cab. In these cases, the crew compartment is not watertight, and the snorkel provides air only for the engine. Thus maximum height is limited by the crew's need to breathe, and very rarely completely submerges the vehicle.
In contrast, lighter, true-amphibious vehicles that float on the water surface are not limited by the depth of the water.
World War II
Although Duplex Drive allowed landing craft to release tanks farther from shore, the alternative deep wading gear permitted a tank to drive partially or completely underwater on the sea floor rather than swim. Deep wading Churchills took part in the 1942 Dieppe raid, and also operated during the D-Day assault. These tanks were given waterproofed hulls and air intake and exhaust trunking to allow them to come ashore from shallow water. Tall ducts extended from the engine deck to above the turret top and they needed to stay above water. The front duct was the air intake for the engine and crew compartment, the rear duct vented the exhaust. This device saw use in many amphibious operations, it was also used on light tanks and tank destroyers. The US had similar devices for trucks and jeeps.
The Germans gave their Tiger tank a long snorkel, essentially a long tube on the commander's hatch that allowed it to wade through up to four metres of water, as well as rubber gaskets on all openings. This was necessary due to the fact that the large tank was too heavy for most bridges in Europe and Russia at the time, meaning that they had to be able to deep ford across rivers when a bridge wasn't available. The Tiger was the first tank to come with deep fording ability as designed, although the earlier Tauchpanzer, a modification of the Panzer III and Panzer IV, was designed to drive on the sea-bed, part of the German preparations for Operation Sea Lion, the planned invasion of Britain in 1941. A long, flexible rubber hose with a floating buoy on the end supplied the engine and crew with air and gave the waterproofed tank a maximum diving depth of 15 metres (49 ft) making it an extreme example of a wading tank. They were also extremely unpopular with their crew, due to the danger of breakdown while in deep water and the possibility of drowning. Since the crew was unable to see where they were going, direction was given was by means of radio and a spotter on the surface. The Germans eventually converted 168 Panzer IIIs and 42 Panzer IVs into Tauchpanzers, although they were never used for their intended purpose in the end.
Most modern tanks since the 1960s are able to perform deep fording operations, not only to allow heavy main battle tanks to cross rivers even when there are no bridges strong enough to support them, but also to avoid bottlenecks and tough defensive positions at bridges, or to continue the attack even if the bridge is destroyed. In NATO and Soviet tanks especially, this is due to the Cold War emphasis on European warfare, which requires the crossing of numerous rivers. For example, the Leopard 2 tank carries a snorkel that is in fact a series of rings which can be stacked to create a long tube. This tube is then fitted to the crew commander's hatch and provides air and an escape route for the crew. The height of the tube is limited to around three meters. It is common NATO practice to provide a tube large enough to double as an escape route, although a more common approach is fit a single long, wide tube over the crew hatch on the turret. This allows an escape route for the crew, but must be carried to the front separately, as they are too big and bulky to easily carry on top of the tank.
All modern Soviet/Russian tanks since the 1960s (such as the T-55, T-72, T-90) are also able to perform deep fording operations. However, unlike NATO tanks, Russian tank snorkels are generally only a few inches in diameter, and do not provide a crew escape path. This is considered a worthwhile tradeoff, since the snorkel tubing can easily be carried on the tank itself and erected in a few minutes, rather than requiring logistics vehicles to bring the snorkels up to the crossing area. However, a crew in a stuck or broken down tank is forced to wait underwater until a cable can be attached and the tank pulled from the water; this makes water crossing quite unpopular with crews, and larger NATO-type snorkels are often used while on exercises, for safety reasons.
Deep fording capability is important for military vehicles because bridges may be destroyed during conflict to prevent crossing, and smaller bridges may not be able to support heavier tanks to begin with. Making a vehicle capable of navigating relatively shallow water counters this problem without the expense and technical difficulties of floating the entire vehicle.