The line between the trees must support your food and it also tries to pull the 2 trees together.  It gets into trig to figure out how much tension will be on your lines but most calculators will do the work for you.  Estimate the angle between the 2 anchor points and the line form the anchor point to the point on the rope where the food is hanging.  It will probably be between 30° and 45°.  The 30° will put the most stress on the tree to tree line so I will use it as an example. ( (Weight of pack)/ Sin(angle)/ number of lines).  Lets say your food weighs 1LB (I know that is light but it is easy to scale to your actual food pack) and the angle is 30°.  1lb/Sin(30°)= 1/(.5)/(2) = 1LB.  So the Tree to tree rope must have a working load of your food pack weight.  So where do you find working load for a particular rope when the only list the breaking strength.  In rigging classes I have taken they say breaking strength should be 7 times working strength.  So to lift your 1 Lb food pack the tree to tree rope should have a breaking strength of at least 7 LB.**I have also seen experienced riggers use a safety factor of 5 but your ropes will wear out faster( probably only raise and lower your food pack 1000 times).
For the rope that goes over the pulleys it depends on the number of pulleys and if you use a becket.  Count the number of ropes coming down to the food pack.  Each of these ropes will share the load of your food pack. If you have a single pulley whit a becket hanging from the tree to tree line and a single pulley at the food pack you will have 2 ropes coming to the food pack. Stress on rope = Weight of pack /number of lines = 1 LB/2 lines = ½ pack weight. This line should have the same working load to breaking strength as above.  So to lift your 1 LB food pack the pulley rope needs a breaking strength of 3.5LB.

...shopping for blocks...and studying trig....on this next trip 

My thanks as well MP! I appreciate your advice on the blocks as I know absolutely nothing about sailing. Loved the calculations on rope loads (was that a 30-60-90 triangle you were describing ? ) Anyway, can't wait to get the blocks and proper rope. My wife and I have typically struggled to get that first or 2nd night food pack up in the trees and these tips will make this much easier. Thanks again -- and to ST and zski and everyone who contributed. Love these threads (like going back to school )

Magicpaddler wrote on Aug 28^th, 2014 at 8:18pm:

The limbs or once branches - now nubs are only for throwing the rock bag 'n line over. The trunk is what bears the weight. The branches (or what's left of branches) aren't important once the rope is up there. Go around the tree trunk. The trunk holds the weight. And a little slack ain't a bad thing the way trees sway in the wind.

I use 1/8" dacron kite line - non-stretch and not prone to tangle at all (as you might guess from its intended use) - google for kite supplies - forgot where i got mine - its 500# test (not a kernmantle construction like that yellow line in the link or paracord.)

I only use one cheap single wheel pulley on top, run the line down to a rescue pulley that is just a wheel over a biner (I use the cheap 99cent mini-biners) -

(You need to Login or Register to view media files and links) -

that clips to the pack with a biner, and the rope back up top thru another mini-biner - the cheap mini biners are plenty strong enough for me with the load distributed the way it is.  Once I have tension on the pack, the line stays on that orange wheel just fine.

that's my ultralight version - biners and pulley and two 50' lengths for the hanging line to two trees, one 50' hoisting line, and a thin heaving line all fit in a quart size ziplock bag - total weight is 11.4 ounces.  I can hoist food for two for 14 days with this rig, but that's at an average of 1.3# per person/day so lighter than most food packs would be.


this is the same concept as the "Z Pulley" rescue for canoes - learn one and you know the other