TIL: How to create a Block and Tackle System with Only Rope

(posted in blog, til)

Today I Learned how to construct a block and tackle system with only a length of rope.

Some Background

I have a large White Oak in my yard that has died, sadly like so many others around these parts from what seems to be a fungal infection. This tree is (still even in death) the largest tree in my yard). It is well away from any buildings so it does not pose a threat. So far it has shed two rather large limbs, the first was shed last Fall and I was able to turn it into quite a supply of firewood. Over the winter it shed an even larger limb that unfortunately got hung up in 1 medium Elm and above 2 smaller Elms that are in the understory. Its base is on the ground (embedded 6-12" from the fall).

The Problem

I would like to get the limb down from the Elms without further mangling them and, perhaps, more importantly I would like to get the limb down on the ground without maiming or killing myself, hey I can be ambitious.

The Solution - Take 1

I own a sweet pole saw that came at the high recommendation of my father. The plan was to use this pole saw to cut off of the non-load bearing sublimbs on this monster limb. Then make a series of cuts high up on the limb near where it is hung up on the Elm so that it will fall down and away from the other two Elms. There are two critical elements:

  1. Not getting the saw blade stuck as the cut reaches the critical part where the pressure causes the cut to collpase into the saw blade (if you have ever cut wood under tension you will know what I am talking about)
  2. Keeping the limb from crashing down and destroying the trunks of the 2 smaller Elms
The 2nd item was mitigated with a rope pulling away from the Elms so that when the limb comes loose it will fall slightly to the side of the Elms. The 1st, well the plan was to make multiple cuts so that the dreaded saw pinch would not trap the blade of the saw 20+ feet in the air. To do this I planned to make two cuts about 1/2 way through the limb spaced about 1/2" apart with the second angling towards the first so as to cut out a wedge and thus avoid trapping the saw blade.

The Best Laid Plans are Only as Strong as Your Execution

I got greedy on the first cut and went 2 strokes of the saw too far and it pinched and subsequently trapped my pole saw blade. Now I was in a pickle. The blade was quite stuck. No amount of pulling, pushing, or other machinations could get it out. I knew I could likely break the blade to at least free the pole and the saw head and the expense of the blade. However I did not want to then also bend one of the segments of the pole assembly.

Physics is Awesome

I knew that there was no way I could shift the limb without some muscle or mechanical help. Turns out that muscle help is not available rght now (2020 Pandemic for all future readers) so mechanical help it is. The problem is that I do not have any obvious way to create mechanical help. This is when I thought that there has got to be a way to create a simple block and tackle system with plain old rope (which I had plenty of), but I had no idea how to tie such a seemingly complex series of knots. One DuckDuckGo search later and I had this awesome video that showed me what I needed.

A Sailor I am Not, But Knots I Made

It turns out that the rope work and knots for this are very straight-forward. I crafted what I needed and started pulling. I was able to shift the limb a solid 8" or so which was enough to increase the pressure on the blade and snap it without needing to torque and potenitally bend the pole segments. Here are a few pictures of the setup:

Conclusion

While I still have not solved the intial problem of getting the limb down and I had to order a new saw blade (actually 2, just in case) I learned an excellent skill and am looking forward to using it in the future to help solve other problems where the mechanical advantage of block and tackle can save the day.

If you are curious about the physics of a block and tackle the Wikipedia article on the topic goes through the basics.