Vintage pics of M809s at Aberdeen Proving Ground

[US6x4]

I'm wondering if the stock fan is simply stiff enough to resist flexing into the radiator? I don't recall any TM steps calling for fan belts to be loosened but it's been awhile since I read through those.

 

[HDN]

I just looked through the M809 operators' manual and didn't see anything about disabling the fan for fording. However, the manual says to limit speed to 3-4 MPH in 1st gear, low range. Maybe the speed limit is what keeps the fan and water pump from spinning off the truck? If you're that worried about it, follow the TM and perhaps check the fan and water pump bolts to make sure they're tight

Also, this is what the deep-water fording kit does:

 

[Ajax MD]

I agree that the slow speed is probably what prevents damage. The other thing to consider is that these trucks are aging and may need extra coddling.
I have read some incidents of 809 series trucks sending the fan into the radiator when NOT fording, just ordinary driving.

I cringe at having to source a replacement radiator.