Firstly, what a great Post Darren,
I am also a Driver Trainer and Rescue Systems Instructor with the S.E.S. which brings a heap of personal satisfaction which I am sure you get as well doing similar with your Club.
If I may be permitted to make an observation in the interests of accuracy tho. The figures for two snatch blocks IE triple line pull are incorrect. A standard triple line pull provides a 3:1 mechanical advantage not 4:1 reduces the load on the cable to 1/3rd not 1/4.
25 Degrees = Load (2,800kgs) + 45% = 4,060kgs X 300% for mud = 12,180kgs
PLUS the wieght of the Organic mass out the front (About 3m3) and we were up around the 15,000kgs!!
Put in a Snatch block and that is halved to 7,500kgs, and what is the Hi Mount rated at? 3,632kgs!
Should have done a triple line pull...Even a Triple Line pull would be 3,250kgs!,
In that example the "triple line pull" cable load will be 1/3rd or 5,000 Kg's hence the "triple" in the M.A. rig name.
Whilst on the subject... There is one other factor in M.A. rigs for everyone to keep in mind as well, the load on the snatch block is the sum of the loads on the cable so again to use the same example whilst the load on the cable is 5,000 Kg's the load on the strap or shackle securing the snatch blocks to the vehicle and tree (or whatever) is 10,000 Kg's.
Here is a cracking good article on M.A. Rigs and loads to illustrate.
Figure 7 Rig C shows a standard or "triple" line pull 3:1 M.A. rig most people are familiar with and utilise (2 Snatch blocks, 3 anchors)
Figure 8 Rig A shows how to get a 4:1 M.A from only 2 Snatch blocks and 3 anchors with the lowest load on the Shackles and Rigging using a compound rig which would indeed reduce the line load to 3,750 KG but is no longer a "triple pull". This method needs another rope or cable tho for the 2nd 2:1. In my case I carry a length of Dyneema for this and other uses.
http://www.ncstaff.net/oed/Pulley%20MA%20Systems.pdf
Again, my compliments on a top shelf amount of info. The more we get the info out there and the simpler it is then the safer it is for everyone and hopefuly the tragedies will diminish
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