Skidder operations under the microscope

December 14, 2013

Cable and grapple skidder travel speeds, wander rations and productivity estimates in South African Softwood sawtimber extraction operations.

by Pierre Ackerman and Elizabeth (Lise) Gleasure – Department of Forest and Wood Science, Stellenbosch University

Typical cable skidder operation on a York Timbers plantation in Mpumalanga.
Big six-wheel grapple skidder working at Weza, southern KwaZulu-Natal.

The wheeled skidder is a familiar feature of South African forest operations. Despite the increase in industry mechanisation, the use of grapple skidders is becoming more common, and cable skidding remains the norm in the majority of forest operations.

The effect of terrain
While wheeled-skidder operations have been studied in regards to productivity, one aspect that has not been thoroughly investigated is the effect of terrain on a skidder operator's path selection or wander ratio. In theory, the direct straight line path back to the roadside or landing would be the most efficient route. However, when terrain is considered, this route often is not as realistic or safe. Wander ratio can be defined as the observed loaded travel path (infield extraction distance) divided by the straight line distance from the stump site to roadside landing.

The effect of speed
The effect of terrain on skidder speed has also not been investigated. Speed can be limited by terrain, useable machine power and load sizes, but it can also be limited by the operator's path selection, driving ability or proficiency. Given the lack of information on these factors, a study was conducted in softwood sawtimber operations in South Africa over 13 operations using five different skidder types (both cable and grapple skidders) to address these issues. The goal of this study was to develop predictive models for skidder wander ratios, travel speeds and productivity. The study was completed using time studies which utilised a combination of GPS tracking and manual time studies. A maximum potential payload for each skidder, including the effect of the compartment's slope, was also determined.

Wander ratio was found to average 1.12:1 for all skidders, both grapple and cable. A wander ratio of 1.12 means that skidders are travelling 12% farther than the straight line path between choking point and the landing/roadside. Because infield extraction distance is a significant predictor of both speed and productivity (Table 1), operations managers should consider this additional 12% during operational planning.

Table 1: Summary of developed predictive models


* denotes variables that were used in the model that were significant. X denotes variables used in the model but not significant.

For speed, both unloaded and loaded, grapple skidders overall were faster in the unloaded direction, but there was no difference between cable and grapple skidders in the loaded direction. Productivity was compared and grapple skidders were significantly more productive than cable skidders.

Travel speed in the unloaded direction was found to be 7.3km/h-1 across skidder configurations. Gross power rating (kW) was a significant predictor for both cable and grapple skidders and slope played a role when determining travel speeds for cable skidders (Table 1).

Travel speeds for both cable and grapple skidders in the loaded direction showed an average sustained loaded speed of 5.5 km/h-1. Travel speed loaded was more difficult to predict for cable skidders. This was attributed to the wide variation across one of the three cable skidder types. Both models found load size (m3) (Table 1) to be significant. Again, slope (%) was a predictor only for cable skid- ders; however, gross power rating (kW) was only significant for grapple skidders.

In terms of skidder payloads, on average, both skidder types were hauling 50% of their calculated potential payload. Although maximum payloads are not always achievable given factors such as DBH, piece length, choker capacity, etc., it does provide a benchmark for discussion on improving efficiency of skidder operations.

Skidders inherently have low payload capacities by design as opposed to forwarders, for instance. But hauling low payloads reduces machine utilisation, increases the costs per unit volume extracted and can seriously reduce productivity.

Field measured productivity was significantly different between grapple skidders (123.9m3 PMH-1) and cable skidders (43.9m3 PMH-1). This is not unexpected as the higher productivity of grapple skidders has been verified by previous studies. Potential productivity was determined to be 220.5m3 PMH-1 for grapple skidders and 60.9m3 PMH-1 for cable skidders over a standardised average extraction distance of 100m. This calculation was based on the calculated optimal load, and cycle times were modelled given this load using the developed regression equations.

Significant variables for field-measured productivity for cable skidders were gross power rating (kW), average tree volume (m3), payload (m3), infield extraction distance (m), choking time (min) and dechoking time (min). For grapple skidders, significant variables included gross power rating (kW), payload (m3), infield extraction distance and choking time (min).

The study's objectives were achieved and the models developed will be used in a supply chain analysis model to provide solutions to optimising the softwood lumber chain in South Africa.

For further information regarding this study, contact Pierre Ackerman at

Published in October 2013

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