Development of slash grapples for residue removal

Issues with woody debris mobilising after severe rainfall events and the new regulations put in place to counter the downstream effects of this, have driven a Forest Growers Research (FGR) project co-funded by MPI and the Forest Growers Levy Trust. The aim is to develop an efficient way to remove woody debris from the cutover on steep terrain as part of the harvesting operation.

The grapples are designed to be fitted to the swing yarder after the merchantable wood has been removed from the skyline corridor. The woody debris is then cleared before shifting the ropes to the next skyline corridor and changing back to the standard logging grapple. Removing woody debris during the harvesting operation is more cost effective than coming back to remove the debris after completion of the setting.

Since Feb 2024 FGR has been working with 3D Industrial Ltd of Rotorua to develop two different types of slash grapples.

The first prototype is a linear configuration designed to replace the logging grapple on a swing yarder. The change out time is typically 10 to 15 minutes. The grapple has a large opening to gather debris and vertical tines to penetrate the material on the cutover. This grapple has been successfully trialled in operations in the Hawkes Bay and produced about 50m3/hr on a 150m haul. Initial teething problems have been overcome by increasing the clamping power and overlapping the jaws as much as possible to retain the maximum amount of debris. Further trials will be undertaken over the next few months to settle on the best design.

The second grapple design is also designed for swing yarders and, like the linear slash grapple, replaces the logging grapple. This grapple is circular with large ‘claws” and is designed to overcome the issues having to orientate the grapple to line up with the debris before picking it up. This grapple design is symmetrical and is able to grab randomly oriented debris without having to be rotated. It has 5 tines, each with a spade-like tip, and chains to assist with holding the debris. This grapple has been trialled recently in the Gisborne region. Results showed it had some issues with retaining the debris during inhaul, so the grapple has been modified to correct this issue and will be trialled again later this year.

Once the designs have been proven in operational trials, full production studies will be done to establish productivity and costings. Next steps in the project are to consider appropriate slash grapple designs that can be applied in tower hauler operations.