Cable Yarding Systems

Interest in cable yarding in the Appalachians developed in the 1970s and early 1980s due to a need to reach less accessible timber. Extending ground-based harvesting systems onto steeper slopes led to increasing public concern about harvesting impacts. Cable logging was

a proven technology used in the western United States and Europe that appeared to be operable on steep slopes and caused minimal impact. A number of studies (LeDoux 1985) documented productivity and costs for a range of cable system machines. Applications in the Appalachians focused on small to medium-size yarders and locally built versions of these machines. However, the higher cost and operational difficulties of cable yarding compared to ground-based systems has kept their niche small. Recent development of more portable designs such as tong-tossing jammers, is renewing interest.

Cable yarding is the process of transporting logs to the landing using a stationary winching machine (a yarder) and a system of heavy wire rope (Conway 1976). These systems are best suited for steep terrain or soft soils that restrict ground operations. There must be sufficient volume of high-value timber available for this system to be cost-effective, since a typical yarder is capable of removing 2 million board feet per year (Sloan 2001, Baker 2001). All cable yarding systems can move wood uphill to a landing; some systems can also bring wood downhill to the landing. Extraction distance is generally longer than with other ground-based systems and may reach over a quarter of a mile (Sloan 2001). Depending on the type of rigging and yarder capability, cable yarding can be used in selection harvests as well as clearcuts.

Yarders range in power from 90 to over 700 horsepower and can incorporate different numbers of winch drums depending on how the machine will be operated. The yarder either sits next to, or includes, a tower or spar to elevate the end of the cable. Some systems, called skyline systems, use a cable stretched from the tower to the back of the cutting unit. A carriage is pulled back and forth along the skyline to move wood to the landing. If the skyline is rigged high enough above the ground, logs can be fully suspended during the inhaul, minimizing soil disturbance. Huyler (1997) studied the use of a small skyline yarder in hardwood thinning. Results showed the sensitivity of logging costs to volume per acre removed, piece size, and extraction distance. Light thinning (about 800 cubic feet/acre) broke even economically when the average piece diameter was about 13 inches compared to only 10 inches when removal volumes reached 2400 cubic feet/acre.

Jammer yarders are the most basic type of cable yarding systems.  Single-drum jammers use some type of mechanism to cast the mainline down the hill where it will be hooked to a log.  Bridwell and Cook (1997) describe the application of tong-tossing in West Virginia using a modified shovel logger.  A double-drum jammer can be rigged with a haulback line to pull the mainline out into the woods.  Jammers are typically limited to shorter distances than skyline yarders and have limited applications in selection cuts.  However, their simplicity and relatively low cost make them competitive with ground-based systems on steeper ground.

Skyline yarders use a cable stretched from the tower to the back of the cutting unit.  A carriage is pulled back and forth along the skyline to move wood to the landing.  If the skyline is raised and lowered during operation it is called a “live skyline”, if the skyline is simply tensioned during rigging and left in position during operation it is a “standing skyline.”  When a skyline system has the yarder positioned upslope from the cutting area, gravity is often used to send the empty carriage down the cable.  The winches in the yarder then pull the load uphill to the yarder (referred to as “uphill yarding”).  Skyline systems can also be rigged with additional cables to allow the yarder to operate from the bottom of the slope, controlling the load as it moves downhill to the yarder (referred to as “downhill yarding”).  There are many possible rigging patterns to address the requirements of equipment and terrain.

A key factor that affects the productivity and cost of cable yarding is the load capacity of the cables.  This is determined by the terrain and the orientation of the yarder and the corridor.  Generally, a layout that allows more sag in the skyline can achieve higher payloads.  If the skyline has to be pulled tight to clear terrain, payload is reduced.  Kochenderfer and Wendel (1980) illustrate cable and terrain profiles.  They state that many spans longer than 500 feet in the Appalachians cannot be logged without intermediate supports because of terrain restrictions.

Cable yarding systems are often paired with manual felling and processing because of the steep terrain. Steep-slope feller-bunchers, however, are finding application with cable systems where the ability of the feller-buncher to swing material towards the cable can improve productivity and reduce costs.