I just got back from a trail maintenance overnight - work in, camp, work out, which I do 3-4 times a month.
This my 4th trip with an experimental pack made with 12 oz. marine duck. It's a basic roll-top, frameless, beltless UL design that I have made for years with lighter materials, but it weighs 32 ounces instead of the 8 to 11 or so of my UL packs. I getting hooked, nonetheless. I have long preferred canvas for bike panniers, but had never thought to make a backpack. Here are my observations:
1) Canvas has lots of body. It stands up instead of collapsing into a puddle when empty. This is both an advantage (while packing up) and a disadvantage (when stowing it).
2) Canvas doesn't get yucky, slimy, or limp when wet. It gets stiffer and stays dry inside.
3) The pack stays drier inside than silnylon in a hard rain. I haven't tried it yet with this pack but I pack my down bag loose - without a stuff sack- in my cotton bicycle panniers even in the PNW.
4) Durability is outstanding. This pack gets abused a lot in trail work. Stuff falls on it, sharp tools brush it, it gets dragged through briars, brambles and brush. It has had only 8 trail days, but they are the kind of days you would never put a UL pack through.
Next step is to try this with lighter canvas. I have some 8 oz. on order and will report on performance if the stuff looks good enough in person to make a pack. Between lighter fabric and more attention to weight in construction and design maybe I can get the total down into a more reasonable 20-24 ounces.
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