Its fairly simple. If you have, for example, three snatch blocks attached to the tree, and three to the vehicle, and one rope that looped through all, you get a doubling of the pulling force every time the rope changes direction. Using round numbers, say those ten guys together pull with a combined force of 1 thousand pounds (not out of line. I just took a test where I had a measured standing horizontal pull of about 150 lbs.). That means that anywhere on the rope, there is 1000 lbs of force. If the other end of the rope is attached to the vehicle, there are the equivalent of 7 ropes pulling at 1000 lbs each pulling on the vehicle or 7000 lbs. That should be more than enough to roll a vehicle back onto its wheels.
Put another way, the first pulley has 1000 lbs on each part of the rope that passed through it (the part that goes to the guys and the part that goes to the first pulley on the tree). That means that the first pulley is tugging on the vehicle with 2000 lbs. Each additional pulley attached to the vehicle pulls with 2000 lbs. and the free end attached to the vehicle is 1000, or a total of 7000 lbs. Make sure the tree can handle a 7000 lb tug (attach the pulleys low on the trunk) or use more than one tree.
As mentioned before, the pulling guys would move a long way for very little movement of the vehicle. That is the nature of mechanical advantage (in nature
).
Typed with my thumbs
Edit: I assumed a rollover, but if the vehicle was on its wheels, even less force would be needed, unless they were tugging it up a hill. An 11000 lb. vehicle doesn't need that much to get it rolling.
