Originally Posted by ouachita
Can you explain the 5:1 setup for me? I am familiar with 2:1 through 12:1 but I did not think you could do a 5:1, 7:1, or 11:1.
Here is one way of doing the 5:1
This method uses the main line in the haul system, (from the boat-these guys are arborists pulling over trees away from homes), and that has a disadvantage.
If the first cycle doesn't get the boat released, then setting up to do a new purchase is difficult.
Here is a link that shows two other 5 to 1 mechanical advantage systems.
on pages 14 and 18 that are more complicated but powerful.
Perhaps you should just go with two double pulleys working off a prusik on the main line. That 4:1 system will still have a good deal of force on the lead prusik so one needs to pay attention to just how efficient that size/type of prusik rope is in relation to the main line size/type.
You want ropes that do their job well. Generally, the closer in size the prusik is to the main line the less grip the prusik has and the more likely it is to slip. Having a main line that is not smooth is a big help.
A mechanical ascender could be a school solution for even higher ratios but they do abrade rope over time.
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The main line could/should be secured separately to a figure 8 with ears or similar. That requires one person to 'man that top belay sort of set up'. You may not have all the people you want so this 8 with ears may need to be tied off periodically.
If you do have the people you need a commander. The person taking up tension on the 8 with ears would be that person.
Personally, I would not trust just a prusik system to hold against the power of moving water. Having that 8 with ears in service makes a lot of sense to me.