What are good limbs to set your anchors on and what are not? Most live limbs over four inches in diameter are good. As with all rules though, there are exceptions. These are the things to consider when selecting your anchor:
1. What species is the tree? Limbs as small as three inches in diameter can usually be used for anchors in hardwoods like oak and beech. Softer wooded trees require a larger limb than 4 inches for safe anchoring. Leam your tree species.
2. What type of anchor are you using? If you are anchored around the tree trunk (single rope climbing) and only using the limb to keep the rope from sliding, a very small limb can be safely used. Even a fairly sound dead limb is sometimes okay, especially if there is a collar on the limb that is still alive as there frequently is in hardwood trees.
3. How far out on the limb will you be setting the anchor? Close to the trunk can bear more weight safely than further out on the limb.
4. What is the angle of the limb? Limbs near vertical can bear more weight than limbs that are nearly horizontal if you are anchoring out on the limb and away from the base.
5. What is the angle of the anchored line? Is your line of pull (your weight on the rope) going to be vertical as when you are hanging on only one anchor?
Or is it approaching horizontal as when you are crossing laterally between two anchors (traversing). The closer you are to a line drawn between the two anchors the more force is placed upon them. Catenary physics is deceptive in that once the angle between the ropes formed by the weighted object (you) exceeds 90 degrees, the weight on the anchors increases rapidly. This means that the weight on each anchor will rapidly exceed
half your body weight (which is the weight carried by each of the anchors when the angle is less than 90 degrees). This law also applies when you use a Treeboat, so anchor to good, solid limbs.
6. What is the soundness of the limb between the trunk and your anchor? If there is deadwood between your anchor and the trunk, as there occasionally is when side limbs die or when limbs rub together, think twice about anchoring there. That is the weak point on the limb.
7. What is the angle of the crotch or limb. ? Narrow angled crotches should be avoided, especially for use as descending anchors. It is veiy easy for knots to get hung up in this type of crotch when removing the climbing rope. Narrow crotches also bind on the climbing rope as you are climbing, adding friction and making a double rope ascent much more difficult than it should be.
Branches that have a pronounced downward slope should be avoided, especially as a double rope anchor. The rope tends to work its way outward from the trunk as the rope slides over it.
Finally, all life-safety rigging is a question of calculating where the weak link is. Any good rope, webbing, or climbing hardware will have a new breaking strength of anywhere from four to ten thousand pounds. Many branches you will feel perfectly safe on would tear right off the tree before your gear failed. The trick is never to put yourself in a position where you could drop more than a foot or two before your safety catches you. More fall than that and you start building up kinetic energy that can severely test both the branch you are rigged to, your gear and your body. Fortunately, it is easy in tree climbing to keep the distance you could drop before coming up tight in your rope to a safe minimum.
All of this may seem somewhat intimidating, but as you gain experience and learn your trees the selection of anchors will become almost subconscious.
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