Each sheave in the tackle system can be expected to create a resistance equal to approximately 10 percent of the weight of the load. The actual pull that would be required on the fall line of the tackle system is equal to the summation of the weight of the load and the friction in the tackle system divided by the theoretical mechanical advantage of the tackle system. There are other types of resistance that may have to be considered in addition to tackle resistance. FM 20-22 presents a thorough discussion of resistance. In all cases where manpower is used for hoisting, the system must be arranged to consider the most satisfactory method of utilizing that source of power.
More men can pull on a single horizontal line along the ground than on a single vertical line.
On a vertical pull, men of average weight can pull approximately 100 pounds per man, and on a horizontal pull approximately 60 pounds per man. If the force required on the fall line is 300 pounds or less, the fall line can lead directly down from the upper block of a tackle vertical line. If 300 pounds times the mechanical advantage of the system is not enough to lift a given load, the tackle must be rigged again to increase the mechanical advantage, or the fall line must be led through a leading block to provide a horizontal pull. This will permit more people to pull on the line. Similarly, if a heavy load is to be lifted and the fall line is led through a leading block to a winch mounted on a vehicle, the full power available at the winch is multiplied by the mechanical advantage of the system.