DOUBLES the pulling capacity? so i have a 12000 lb winch for another 2lb object? would it be 3X rated capacity if i used two snatch blocks? how exactly do you rig up a snatch block?
The attached picture is a page out of my rope rescue manual, and hopefully large enough to read. The principals of mechanical advantage are the same for ropes as they are for winches/cable. Simply put, just adding more than one snatch block does not necessarily mean that you will increase your mechanical advantage. Yes, adding one snatch block will double your rated capacity. However, to get the 3:1, 4:1 or 5:1 advantage, you have to set it up correctly or you will simply have a change of direction. See the picture below for clarification. In the picture, "A" is anchor, which could be your vehicle in a 2:1 pull. "L" is the load. The red circles would be your snatch blocks. In the explanation, "MA" refers to mechanical advantage.
Just remember, your line speed is a direct inversion of your mechanical advantage. In other words, a 3:1 advantage gives you 1/3 your original line speed. Also, you have to factor in your line strength, anchor strength, and snatch block capacity when increasing your mechanical advantage.
Finally, to interpret the picture, the "arrow" end of the rope would be where you are pulling from (i.e. the winch). So, a 2:1 (first drawing on the left) would be a snatch block around a tree "L" anchored back to your vehicle "A".
A 3:1 system would be your winch line out to a snatch block attached to a tree "L", back to a snatch block on your rig "A", back to the same tree "L".
With all that said, a 2:1 is typically all anyone ever would realisticly need. I would never use more than a 3:1 advantage. If that doesn't get you out, then you need to re-think your extraction plan. A 3:1 puts tremendous loads on everything in the system and beyond that the chance of a catastrophic failure (i.e. line or hardware failure) increase exponentially.
Good luck!