Mechanical Skidding

Skidding logs to the landing is usually the most time-consuming process in primary extraction. Skidders, in either grapple or cable configurations pull wood from stump to landing. The form of the wood is sometimes used to further describe the type of skidding. If the trees have been processed into logs in the woods, skidding is called  “log-length”. If the tree has simply been delimbed and topped in the woods, then skidding is called “tree-length”. If the tree has not been processed other than felling, then skidding is called “whole-tree.”

Mechanical skidders are either tracked or wheeled machines. Tracked skidders are versatile and are often equipped with a bulldozer blade for minor earthwork. They may be used for skidding, road building, landing construction, and BMP work such as waterbar placement or slash piling. The tracks provide low ground pressure to reduce soil compaction, but turning tracked machines can disturb the soil. Special, flexible-tracked skidders have been developed to provide even better flotation and higher travel speeds. Flexible tracks also allow the tractor to operate on steeper slopes (Sloan 2001). With a lower center of gravity, tracked skidders tend to be more stable than wheeled machines and can be safely used on steeper slopes. However, their travel speed is slower and productivity is lower than for wheeled skidders on favorable terrain.

Wheeled, or rubber-tired, skidders are usually equipped with a small decking blade on the front and a cable winch or grapple on the rear. The decking blade is intended to be used to push logs into piles at the landing, but it may also be used for minor trailwork or for pushing slash and debris. Wheeled machines are highly maneuverable. Steering is articulated; that is, the machine is hinged in the middle. Ranges of tire sizes and types help to match the machine to logging conditions. Wide tires can be used to provide lower ground pressure and more stability on slopes. Narrower tires may be selected to operate in selection cuts or on narrow skid trails. Tire chains can also be used to improve traction and tire life, particularly on rocky soils or steep slopes.

The most significant factor affecting skidder cycle time is extraction distance, with about half of productive time spent traveling back and forth. Acquiring the load, with a cable skidder is also time-consuming. By speeding the loading and unloading times, grapple skidders can out-perform cable skidders in terms of production under the same harvesting conditions. However, it is the interaction of load size and cycle time that determines overall productivity. In a recent study in West Virginia (Reisinger and Gallagher 2001), a cable skidder actually had higher productivity than a similarly sized grapple skidder because it could acquire larger loads.

In the tactical logging plan, the skidding pattern should locate landings and main skid trails in order to minimize skid distance and to avoid problems such as tight corners on trails. Skid trails should lead as directly as possible to the landings to minimize the lengths of trails. However, slope limitations may require a more indirect path to avoid crossing sidehill. Steep skid trails should include water diversions to keep water from channeling downhill. If the prescription is a partial cut, the skidding pattern should be designed to avoid damage to the residual stand. "Rub trees," for example, may be left on the inside of corners on trails to protect the remaining stand. These trees are removed in the last pass. The skidding pattern must be defined prior to felling so that fallers will know which direction to drop the trees. A good logging plan with proper skid trail layout will significantly reduce the costs and impacts of the whole operation.