Water Crossings

4x4 Africa - water-crossingsThis article finds Bud back at the off-road club training area where he is going to attempt some water crossings, the water being the river that feeds the artificial wetland where Bud had undergone some eye opening mud driving training two weeks earlier.
The off-road club had gone to some trouble and expense to widen the river to 25 metres, forming a wide pool at a point where the banks are shallow and getting in and out of the water with a 4WD is relatively easy on either side. Normally around 600 mm deep at the deepest point; the bottom consists of small rocks on one side of the crossing and imported silicate sand at the other, both surfaces extending the full width of the crossing. However, while the rocky bottom does not usually present major issues, the several larger immoveable (and invisible) rocks on the sand side do; these rocks being large and hard enough to knock holes in differentials, unprotected engines, and transmissions.
Nevertheless, Bud knew nothing of this when he and the Instructor stopped a short distance from the river’s edge. Bud regarded the flowing water with some trepidation and expressed his doubts about being able to complete the water crossing successfully.
“Nothing to it Bud, all it takes is some basic precautions, finding and marking all hidden obstacles, removing the drive belt from the fan if the engine compartment is likely to be flooded, and allowing the differentials and transmission to cool down before entering the water. There is nothing to worry about with this crossing since it is not deep enough to flood the engine compartment, and the bottom is relatively solid. No one has ever gotten stuck here, except for the chap who got swept away because he tried crossing here when the river was in flood, and was more than a metre deep, but that was his own stupid fault.”
For a moment, Bud saw himself floating down the river but he had developed a large measure of trust and faith in the Instructor and banished the thought from his mind, but asked about the matter of letting the differentials and transmission cool down first.
“Good question Bud, so let us tackle this issue one thing at a time:”

Air Intake

river-crossing“I see you had a snorkel fitted as I suggested; you will of course never be able to cross water as deep as the inlet point but the problem with many 4×4 vehicles is the fact that their air intakes generally point forward at level just below where the bonnet closes. The problem with that is that during deep-water crossings, it is almost certain that water will enter the engine at this point, which invariably leads to catastrophic engine failure. Southern Africa, or the rest of Africa for that matter, does not have many water crossings but flash floods happen all the time and it may be necessary to cross one or more of those during a long off-road excursion in the wet season.”

“A snorkel is the best way to prevent water entering the engine but getting swept away in strongly flowing deep water is to my mind a bigger problem; than a ruined engine. All 4WD vehicles are designed to be able to cross water and the depth of the water they are able to cross safely is referred to as the wading depth, which differs from 4×4 to 4×4. Some new 4WD’s are able to cross water up to 800 mm deep but that has its own problems.”

Wading Depth

“An 800 mm wading depth may look good in a sales brochure but the brochures never mention the fact that the 800 mm refers to water that does not flow. The golden rule of water crossings is that if the water is flowing too strongly to walk through, you must consider it too dangerous to drive through. Think about it; if the force exerted by the flowing water on just your legs makes it difficult to walk through, think about the effect the water will have on the much larger surface area of the wheels. Add to this the fact that the tyres are filled with air, which makes them float, and you have a problem.”

“If the water is deep enough to reach above the floor plates, the 4×4 can start to float, literally like a boat, because of the buoyancy of the wheels. One way to avoid this is to open the doors on both sides to allow the water to flow through the cabin- this destroys much of the buoyancy but is also likely to cause potentially fatal short circuits in the wiring that runs under the carpets. The best thing to do is to avoid deep water altogether, or to set up camp for a day or two until the flood has subsided sufficiently to cross safely.”

“Another issue is the fact that deep water can flood the engine compartment: this is not much of a problem on diesels but it is often fatal for petrol engines because of the short circuits in the wiring that could cause it to cut out in mid-crossing. This is a major problem in most of Africa because there are usually no replacements available for the electrical parts that blew up because of the short circuits. One way to minimise the risk of this happening is to have a shroud made of tent material that fits around the nose of the 4×4, covering the grille. This creates a bow wave in front of the 4×4 that helps to keep water out of the engine compartment, but how well it does this is dependent on speed; to slow and it does not work, too fast and you run the risk of damaging your 4WD against unseen obstacles. Strongly flowing water also has a bigger effect on a fast moving 4WD than a slower moving one, so the faster you move through fast flowing water to keep the engine compartment from flooding, the more likely you are to be swept away.”

Radiator Fan

“And then there is the radiator fan: it takes a lot to stop a viscous fan from rotating, especially if its thermal clutch is engaged. water-crossing - BotswanaWhat happens when a fast spinning fan encounters water is that the fan starts to act like a boat propeller, but being made of plastic, the blades bend forward and you end up with a fan-sized hole cut out of the radiator core. This can happen in less than a second and unless you have a spare fan, radiator and belts to replace the ones that the deep water destroyed, your expedition is pretty much over. The only way to prevent the fan destroying the radiator is to remove the drive belt, which is relatively easy on many diesels because in many cases only the alternator and fan is belt driven. If the alternator has an integral vacuum pump for the brakes, you will not have brakes during the crossing but that is not a major problem, since you do not want to stop in the middle of the water crossing anyway. You also may not have power steering, which can be difficult but I would rather do a water crossing with no power steering than end up with a shredded radiator and no fan. But don’t worry about the water pump not being driven; since you are entering the water crossing with a cold engine, the amount of time spent without a functioning water pump is not long enough to cause your engine to overheat; you will be on the other side even before the engine realises that it has no coolant circulation.”

“Petrol engines are a bit different because they often have serpentine belts that drive everything. You sometimes get them on diesels as well but removing them is not always easy, and it is almost always less of a hassle to find an alternative crossing point or waiting for the water to subside enough to cross safely than removing and re-installing a serpentine belt.”

Water in the diff oil

“The reason why you have to let the differentials cool down before entering the water is because the hot air inside the differentials expands, and if you cool them down suddenly, this hot air contracts and it then sucks in water past the oil seals, much like you draw liquid into a syringe. Water can also enter through the breathers, which is why I told you to have breathers extended with vacuum tubing. There is no ay water can now enter through the breathers where they are now, high up in the engine compartment.”

“Water can also be sucked into the engine and transmission if they are hot and you suddenly enter cold water, so it is of critical importance that you allow the engine, transmission and the differentials to cool down completely before you enter water. Water does not only diminish the lubricity of oil, it causes rust that will eventually destroy everything it meets. The same thing goes for wheel bearings, if they are hot when you enter water, you are guaranteed to get water sucked in past the seals, which will mix with the grease and before you know it, you will have destroyed the bearings because they have lost their lubricant.”


“A mechanical clutch depends on friction to work: if you depress the clutch pedal, the clutch plate physically moves away from the flywheel, which is what allows you to change gears, because the link between the engine and transmission is interrupted. Letting out the clutch pedal restores the friction between the flywheel and the clutch plate. However, if you are in deep water and you use the clutch water enters the space between the friction surfaces of the clutch, where it acts as a lubricant and you lose the friction it needs to work, so you end up with lost momentum, and you are stuck in the middle of the water crossing.”

“If the clutch is undisturbed, the pressure between the friction surfaces keeps out the water, so resist the temptation to change gears while you are in the water, because you cannot change gears quick enough to stop water getting into the clutch.”

Find and mark the obstacles

4x4 crossing-water“All water crossings are potentially dangerous, since you cannot see under the surface of the water; just because there are visible obstacles above the surface, it does not mean there are no obstacles under the surface. A flash flood can carry obstacles such as large rocks, tree branches, dead animals and who knows what else, and all of those could destroy your 4WD. So, when you get to a water obstacle the very first thing you do is find a way around it; if you can’t, or there is no other way, the next thing you do is to see how deep it is, and how strong the flow is. More often than not, you will be able to see how fast the flow is by watching the surface, but fast flowing water is not always deep, so you walk into the water to measure its depth.”

If you can comfortably walk through the water all the way to the other side, without feeling the current is going to sweep you off your feet, you have won half the battle, but while you are in the water, test the surface by pushing a tent pole into the bottom. If there is no resistance you are going to get stuck, but river bottoms in Southern Africa usually have sandy bottoms, as opposed to muddy ones, but do not take this as gospel- test the bottom before you enter the water.”

“While you are in the water testing the current and depth, mark out a route where the water is shallowest. The object is to get to the other side, not to see how your 4WD copes with the deepest water you can find, so, once you have established a possible route, take a bunch of tent poles or sticks and mark all the rocks, gullies, tree branches and other obstacles you can find. Just push a marker into the bottom where you find an obstacle, but between the line you want to take and the obstacle, so you do not have to worry about remembering just how far the marker is from the obstacle. Doing it this way means that if you get past the marker, you are past the obstacle as well.”

“Once you have marked out a line, and are satisfied that you will be able to cross the water safely, with the emphasis on “safely”, make sure your 4WD has cooled down completely, and if you have to remove the fan belt, do it now, and fit your shroud to keep the water from flooding the engine compartment. However, we do not have to this here, because the water level has gone down quite a bit, and the deepest part is no more than around 500 mm or so, plus your body lift will keep the water away from the doors.”

“So what you must do now is pick a spot on the opposite side that will give you the shortest distance to cover; then take some of those long branches over there under that tree, and walk the line you have chosen. A good idea is to take a zigzag route and push a stick into the water all around you- that way you cover the most ground and is more likely to find hidden obstacles. If you were a party of several members you could all from a line and test the bottom in one pass, but since you are alone, take all the time you need to cover the entire route you want to take, even if you have to do it in three or four passes.”

“I know it sounds like a major hassle and more trouble that it is worth, but believe me, in Africa you are on your own; there is no one around to pull you out of a river if you get stuck, and if you are unable to use your winch to get yourself out, you are in big trouble. Also never walk a water obstacle bare foot; you never know what you might step into and the rain forests in central Africa is no place to suffer as serious cut- you could lose your foot to gangrene in less than a week if you can’t get it treated in time. Getting hurt in South Africa, or even much of Southern Africa is not too bad because there are usually hospitals or clinics not too far apart, but don’t bank on it; you could get gangrene or any number of deadly infections before you can get medical attention. So be safe, and never walk a water obstacle barefoot, so now you know why I told you bring a wader, it will protect your feet while keeping the leeches off you, especially in equatorial Africa.”

Bud reminded himself never to believe a TV advert in which a 4WD barges into a water obstacle at full tilt: he would never have thought that there were so many things involved merely in driving across a small river, so he donned his new wader, grabbed a bunch of sticks and set off towards the opposite bank. It took him all of two hours, but in the end, he had a clearly marked channel; all the submerged rocks were marked by sticks, and he even managed to mark two gullies, both of which were deeper than his waistline. He felt proud of his achievement but resisted the temptation to think about driving across the Niger or Congo rivers. He would be content for the moment just by making it across this pool and back.

“OK, Bud, I know where all the obstacles are so I am not going to check, but you have done well, you got them all, so if the 4×4 has cooled down enough, we can attack the obstacle as soon as you are ready.”

Bud was ready and the 4×4 had cooled to the Instructor’s satisfaction, but he made Bud touch the differentials and the engine himself to get an idea of the temperatures involved; while they were talking and Bud marked out a channel, the 4×4 had cooled down to around ambient temperature, which was what the Instructor wanted.

Gear selection

water-crossings-in Africa“OK Bud, the sand on the bottom of this pool is reasonably solid, but there are some muddy patches, so we want to avoid wheel spin at all costs. Even though the bottom is not mud, it is soft and traction is all important, wheel spin will destroy your momentum immediately, so we are going to use the traction control and low range second gear in 4WD mode.”

“Traction control will control the wheel spin, while low range second gear will give you enough momentum; remember, we are pushing a box through the water and rolling resistance will be higher than you might have thought. We are using 4WD mode because we want to use all the available traction, but we are not using the diff locks because you need to able to steer around those obstacles you marked out over there in the middle of the pool. We could use the diff locks, but you could still get wheel spin, even in low range, so in this case, we will use the traction control to eliminate the possibility of wheel spin altogether, or at least, reduce the chances of it happening to almost zero.”

Bud made the proper selections and entered the water at a steady 5 km/h, and to his surprise there was no slipping and sliding. He had fully expected to fight all the way to the other side but the traction control did all the work. Steering around the obstacles he had marked presented no problem- the 4×4 followed his steering inputs flawlessly. Keeping his rpm’s above 2500, he concentrated on missing the obstacle markers, and only once did he nearly make a mistake by putting his foot on the clutch pedal, but the Instructor warned him in time. Then, just as he was beginning to enjoy himself, he was through the obstacle and on solid ground, but before he could say anything, the Instructor told him to turn around and repeat the exercise.

Bud’s first water crossing

Africa - river-crossingThe same thing happened the second time round: the stability control system prevented wheel spin and the 4WD mode making use of all the available traction, got him through the water crossing without a hitch. However, he had an idea that once he started his epic solo off-road trip through Southern Africa, things might not go as smoothly.

As the Instructor had told him last week, all he had accomplished thus far was operating a 4×4 with lots of electronic driving aids on the most basic level. He had not carried any real weight, and he has not been called upon to make major decisions such as what gear to use, the correct speed at which to enter an obstacle, or even when and by how much to deflate his tyres. He was beginning to think that as limited as his knowledge of electronic driving aids and how to use them in the off-road driving context was, he would be completely at sea if he had to repeat the exercises of the last few weeks with his other 4×4, the one with no electronic driving aids. Nonetheless, he was fully resolved to complete his course and accompany the club on their annual tour through six countries in Southern Africa.