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The fording depth for a military (modified) HMMWV is about 5 ft.
With the military fording kit you can ford up over the roof, and yes, you can park them underwater, which they do sometimes to hide them.
The civilian HUMMER cannot ford deeper than the maximum recommended (24" or 30" depending on model) because it does not have a sealed electrical system and instruments. The first thing to go over 30" is the tranny computer, which is mounted on the engine cover.
Even at 30", without an intake extention, you are in serious danger of hydrolocking your engine if you try to go faster than a crawl. Please note that the hood shape is very effective at directing a bow wave directly into the air intake.
Just ask some of our U-Boat commanders the cost of deep-fording a commercial HUMMER.....
My HMMWV has the 60" fording kit. It didn't when I got it, so I had to research it to figure out what parts I needed. At the risk of boring those who've heard all this before, here's how it works:
All major driveline components -- tranny, transfer case, hubs, brake booster, mechanical fuel pump (military doesn't use electrical pump, and from the complaints I read here I can see why), and some others I'm forgetting -- are plumbed together to the vent system. Under dry conditions, and for fording not over two feet, these are open to the atmosphere (via the air filter housing, as I remember). When preparing for deeper fording, the operator turns a valve that plumbs all these vent lines to the CDR valve mounted on top of the engine. At that point, the vents are no longer open to the atmosphere.
The CDR valve has a hose that runs down to the bottom of the frame and has a rectangular plastic box abt. 2" x 5" x 5" that's open at the bottom. As the vehicle goes into water, the water trying to go up into that box sends a pressure signal to the CDR valve that tells the valve how deep the water is. The valve has a diaphragm that responds to this signal and sends an appropriate amount of the crankcase pressure (blow-by, I suppose) to all the above listed components. This pressure probably is just a bit less than the corresponding pressure of the water at that depth. Bottom line: the pressure on the inside of the seals is about the same as the water pressure on the outside, so nothing goes in or out (no water goes in, no oil goes out). This condition remains true for any water depth up to the rated depth (60").
I put a pressure guage in my system to see if it worked. The gauge was calibrated in inches of water. When I blew into the line leading to the plastic box, sure enough, the pressure in the system rose correspondingly. With modest breath pressure, the guage would read around 30" pressure (1 psi.).
In addition to all this, remember that the guages are waterproof, as is the wiring, and the military trucks don't use a computer controlled tranny or engine (I'm not sure what the newest HMMWVs do about this, but I assume it's the same as mine). The FI pump and the transmission control are mechanical.
And don't forget that the military trucks are "water-hose-friendly" -- no fancy upholstry, no stereos.
And finally, the military cautions users to stop at about 30" and let the vehicle fill with water to that depth before continuing, so that the truck doesn't act like a boat and drift. All in all, no fun at 35 degrees.
I would suggest that civilian Hummer owners think twice (or more) before making efforts to ford 60". Still, there are things one could do to protect for an occasional splash, e.g. intake snorkle.
I apologise for the prolixity.
LeeF. Dallas, TX
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