This is a fork from
this thread which has was starting to wander from it's initial purpose
Here is a packing method promoted by Cliff Jacobson,
who has been leading adult and youth groups on northern Canada canoe trips for a few decades.
The assumption is that some mishaps leave you with near zero probability of getting outside help.
The objective is to have dry pack contents when you've retrieved it after it's been floating for several hours.
(Cliff uses down insulation)
This does not pretend to be SUL, hardly even UL ... but it does not just pretend to be waterproof. I must confess, however, I don't use this for backpacking
The "Sandwich Packing Method"
1) The pack itself need not be made of waterproof material.
Uncoated fabric has an advantage that it does not trap water between the pack body and what I'll describe below.
2) Use a somewhat sturdy pack liner made of a material that'll resist a strong hydrostatic head.
3) Use an "abrasion liner" inside the pack liner. Need not be waterproof.
Folks often use an old pack liner that they've tired of patching.
4) Sleeping bag goes into a stuff sack, need not be waterproof.
5) That bag goes inside a somewhat sturdy poly bag,
purge excess air from the poly bag, twist closed,
fold the twist over double, bind with a shock cord loop or short piece of small line.
6) That assembly goes inside another stuff sack, need not be waterproof
7) Other things you want double protection for get the same treatment, the rest goes into any old stuff sack.
8) all your gear bags go inside the pack-liner-liner sandwich, the abrasion liner gets folded down,
push the excess air out of the first liner and give it the twist/fold double/bind treatment
9) close the pack
This does work. When I give lakeside pre-trip orientations to BWCAW crews I start by tossing my pack into the lake. After about an hour of presentation and Q&A, we retrieve the pack and inspect it's dry contents.
Hasn't failed yet.