Yarding with a grapple carriage has gained much attention in recent years in the Pacific Northwest and other parts of the world. Introduced in the 1960s, grapple yarding with either mechanical or motorized carriages has changed little until recently. Both types required the use of a swing yarder with interlock to be effective. The concept worked well where terrain, timber type, and access to equipment and skilled crew allowed.
The recent resurgence in grapple yarding is easily explained. One is labor. Not only is it more expensive to employ people on the ground as choker setters, it is also harder to find people who will do this type of work.
Second, the global market spurs competition to be more efficient, to produce more with less inputs. In the past major gains in efficiency in logging and other industries were achieved by mechanization. Grapple yarding continues to be an efficient mechanized system when employed in the right conditions.
Finally, mechanizing or removing workers from direct hazards creates a safer working environment.
Despite the benefits of improved efficiency and safety, the lack of equipment and/or the right conditions prevented many loggers from mechanizing the yarding process until recently. For many the barrier to entry has been the mechanical type grapple carriage and the prerequisite of having an interlocked swing yarder to effectively operate.
In the past decade, however, several manufacturers have successfully commercialized motorized grapple carriages. These carriages have opened the door for grapple yarding to conventional tower yarders and are also employed on swing yarders or small excavator yarders and yoaders. Many other simultaneous developments, such as camera systems for both types of carriages, improved yarder control systems, and tension monitors have made it easier to adopt and manage grapple yarding.
The final hurdle many still faced was related to adequate payload. With timber size gradually declining over the years, the most productive loggers are yarding pre-bunched timber. This has become possible by using tethered or winch-assisted machines on steep slopes for felling and-or shoveling.
Contractors now have options to purchase new or retrofit existing equipment for grapple yarding operations. They can have fully mechanized cable logging operations where every task, from felling to loading trucks, is performed by a machine. But how do they apply it successfully?
Recent research by myself and three others suggests that grapple yarding can be very productive, yarding more than 100 tons per Productive Machine Hour (PMH), which is approximately four truckloads, but it can be a struggle to sustain these high levels. Earlier research I conducted, as illustrated in the accompanying chart, showed the average is 60-80 tons, although the range and maximum were greater.
What can recent productivity studies of grapple yarding tell us?
Daily production (number of truckloads or total volume) is the standard most logging crews are evaluated by. It can be broken down to an hourly rate or productivity (often preferred by researchers for comparison). For the sake of simplicity, consider tons per hour.
If the yarder has an average turn (cycle) – a trip by the carriage back and forth – time of six minutes, that’s the same as 0.10 hours. If each turn has an average payload of five tons, average hourly productivity would be five tons divided by 0.10 hrs or 50 tons/PMH with no delays. (Simply put, productivity is payload divided by cycle time.)
In order to improve yarding production, loggers must either increase payload or reduce cycle time, or both.
There are two key factors loggers can control in order to trim cycle time. The first is average yarding distance. Grapple yarding – relative to choker setting options – tends to have smaller payloads and is therefore more sensitive to yarding distance. Research suggests that grapple yarding distances be kept less than 200 meters, or about 650 feet, to reduce cycle time. This is a shorter distance compared to other yarding systems and must be considered with planning and infrastructure development.
The second thing loggers can control is grappling time. Having a skilled operator is essential. In fact, research suggests that an experienced operator is five times more productive than a novice. An inexperienced operator may take a few minutes to grapple a stem or log, a task that an experienced operator may perform in just 10 seconds.
Tools like grapple cameras and distance outhaul measurements displayed to operators in the cab certainly help, but proper bunching and stem presentation cannot be overlooked.
When I worked in New Zealand, the first grapple yarding crews working with tethered feller bunchers went through an abrupt learning curve. The feller bunchers often formed huge ‘surge piles’ in the clearcut because they thought it would help the yarder; they shoveled all the stems into one pile under the cables. In reality these large piles of timber were difficult to grapple; it took much longer than if the timber was separated into small bunches of two or three trees. (The difference in productivity was 10 t/PMH.)
Planning for adequate yarding distances and appropriate orientation of the felled timber to facilitate quick grappling will have significant effects on reducing cycle times.
Small improvements in cycle times can make a big impact because grapple yarding crews have a high number of cycles per day. One crew I previously worked with gained an extra truckload per day in production when they fixed a mechanical issue that cost them just five seconds each cycle.
Payloads should be maximized by bunching the appropriate number of stems or logs where they can be picked up. This sounds simple, especially with the ability of tethered equipment. However, it requires payload analysis and thoughtful planning. Perhaps the largest determinant of payload is the amount of deflection or sag in the cables. Contractors should ensure each skyline road will have adequate mid-span deflection, which ideally is 10 percent or more.
They also must determine how much payload can be transported by the carriage at each point along the skyline road. The payload that can be picked up at each point varies. For example, when yarding across a gully, maybe the carriage can carry 10 tons on the front face but only 3 tons from the opposing (back) face. So there is no need to bunch 10-ton payloads on the back side of the gully.
To facilitate optimal bunching, payload information can be overlayed onto digital topographic or Lidar maps along with the location of the skyline road. The feller buncher operator can access the information via a tablet in the cab; they can see in real time where they are in relation to the skyline road so they can optimally orient and bunch the timber.
Another tool that loggers are adopting is the tension monitor, which shows the yarder operator the real time tension of the cables. Knowing the tension helps the yarder operator avoid overloading the cables. It also enables them to make sure the cables are not underloaded and the crew misses out on potentially larger payloads and greater productivity.
It’s important to remember that yarding is just one step or task in the overall process of felling the trees, processing them, and loading the logs on trucks. Contractors must consider how yarding integrates with their entire operations.
I am reminded of the 5Bs of mechanized logging from an old Oregon State University video: Bottlenecks, Balance, Buffers, Breakdowns and Blunders.
The bottleneck in most logging operations is the yarding process. However, the bottleneck will not always be the yarder, and it can change from day to day or hour to hour.
It is critical to balance the flow of production from one task to the next. For example, if the yarder only produces 20 tons per hour but the processor following it can produce 40 tons per hour, the processor will run out of work quickly.
If contractors cannot maintain a steady flow of productivity between processes, then they need adequate buffers of material. For example, the processor may need a large deck of wood to work on when it is waiting for trees to be yarded or to keep working if the yarder is broken down or changing roads. Contractors can improve yarder utilization by employing a mobile anchor to reduce road change time or shoveling timber to fewer skyline roads.
With full mechanization, breakdowns become more critical to utilization. Minimize breakdowns by having appropriate preventative maintenance plans for equipment..
Finally, avoid blunders or large mistakes. Grapple yarding requires a high level of analysis and planning
Hopefully some of the information shared from others in this article will help you consider some of the challenges and be successful when adapting to grapple yarding.
(Hunter Harrill is assistant professor of forest operations at California State Polytechnic University-Humboldt in Arcata, California.)
As shown at left, yarding operations with motorized grapples average the highest levels of production, although production declines for all types as yarding distance increases.
North Bend: A simple standing skyline system that employs skyline, mainline and haulback with a simple carriage, fall block and butt-rigging.
North Bend Bridled: A variation to North Bend where the haulback blocks are offset to facilitate some lateral yarding capability.
Motorized Slack Pulling (Shotgun): A standing skyline system employing only skyline and mainline; gravity is used to outhaul the carriage. The motorized carriage can pull the mainline as slack to facilitate lateral yarding.
Motorized Slack Pulling (Slackline): A variation of Shotgun where a haulback is used to aid with carriage outhaul.
Motorized Grapple (Shotgun): A live skyline system (the skyline is raised and lowered during each cycle) with only a skyline and mainline. The carriage is outhauled by gravity. The motorized carriage opens, closes, and rotates the grapple.
Motorized Grapple (Slackline): A variation of Shotgun where a haulback is used to aid with carriage outhaul.