Possible variations for the butt block system
1 . The 5:1 haul system can be replaced by a 9:1 ‘ Batwing ’ system ( see image below ) for additional mechanical advantage if there is a suitable anchor .
2 . The belay device can be moved to the top anchor point for a top-belayed system .
3 . If equipment is limited , a short MA system ( set of Fours ) can be used behind the belay device for a short lift .
The image on the right shows a 9:1 Batwing system .
A 3:1 system is built as normal in the haul field between the belay device and the butt block .
The rope tail below the 3:1 is redirected again and a second 3:1 system is built on the tail below the redirect . This second ( rear ) system is used to apply force to the tail of the first ( front ) MA system .
The rear system should have approximately 3 times the length of the front system as it will collapse 3 times faster .
The total ideal mechanical advantage achieved with the combined system is approximately 9:1 .
Skills that may be required : |
Technique |
Description |
Evaluating tree anchors |
Chap . 2.3 |
Butt block |
Pg . 46 |
© Arboriculture Australia 2022 - 47 -