Depends on what type of pack you have,a panel loader (zip goes all the way around and opens like a sardine can) or traditional top loader.
I would recommend the latter anyway, the former sounds good for organisation but never works all that well and usually carries worse in comparison.
If you need to carry 40 liters of gear, get a 50 liter pack, always add 10 or even 20 liters but be super strict as voids tend to fill themselves.
What packs in easy at home will only just fit after a few days in the field, so if it only just fits at home you are f**ked after a long wet day out.
Top loaders with compression straps, floating lid and side pockets (not too big) and bottom zip opening are a must. Sleeping bag divider not necessary.
The way I always did it when working in the jungles of Malaysia (1-2 week trips in the jungle) was to have two large drybags, one for sleeping kit (hammock, night clothes and insulation) and one for fresh clothes (usually socks and underwear) and food in strong zip loc bags (with spare bags).
Tarp is kept in bottom of bag (being wet does not matter, but good to have bottom zip for quick access to it) along with any rubbish or soiled clothes. Side pockets for cooking gear (small stove and two US canteen cups) and water (easy access), low profile water bladders are OK but can be a real bitch to fill up (look at Source bladders, they can be filled with a 'UTA' attachment that means you don't have to pull it out of the pack.) Remember to suck all the air out of the bag after filling (invert and suck through tube). Always take one 'hard' 2L water container just in case.
Used to be able to get USMC Seal Line drybags before they started going for stupid money on fleabay. Any bag with a valve in the bottom is handy to keep volume down (put the bags in the pack, then use compression straps with valve open, then shut valve (again, bottom zip is handy here) but beware of cheap chinese crap.
If you are doing a full blown river crossing and using your pack as a flotation aid then you will need an oversized contractor bag to use as a pack liner. Dry bags are designed for immersion not submersion, there will always be some water getting in no matter how many times you roll the top flap. Use the contractor bag as an 'air trap' so you have a pack that floats now (don't put the pack into the bag, as then you have no straps to hold onto, and might lose it down the river.... don't ask!)
Hope that helps.
Bookmarks