Hi @Scott_Draper . I have an Osprey Stratos and I LOVE it. It's my go-to day hiking pack, and i can certainly attest to its comfort and durability. I think you'll be pleased with it.
I would think that carrying 40 pounds of water (about 5 gallons) in individual containers would be much easier on whatever pack you choose - less abrasion and tearing
@Scott_Draper we would also agree with @hikermor's suggestion to consider carrying water instead of rocks, knowing that the rocks could do significant damage to the fabric of the pack, shortening the life you might get out of it with wear and tear that wouldn't be covered by our return policy. Hope this additional information helps!
Gallon jugs of water are very lumpy and it would be difficult to distribute the weight, not to mention uncomfortable. And 5 of them is quite a large volume.
Pea gravel is really the way to go; the stones are smooth and small, and not very abrasive. I'll have them in their own bags, so they shouldn't come in contact with the pack material. I'll also be able to use some of the smaller pockets if necessary by wrapping them in trash bags.
I vote for the water solution. Also, you can pour some out (or drink it) if necessary to lighten the load.
OK, I had the great idea to use the lead weights that came with my weighted vest, rather than pea gravel. These weights are, I think, lead shot wrapped up in small cordura bags and there are about two dozen of them. This works well. The pack now weighs about 45 pounds.
But: my shoulders still hurt, but in a different place. With the weighted vest, they hurt on top of my shoulders because they supported all the weight. Now the pain in on the front of chest, where the pectoral muscles meet the shoulder. I suspect the cause is that the weight is now further from the center of my body and the bag wants to rotate backwards and it's the front part of the shoulder straps that prevent this.
Is there any sort of padding that can distribute this force more broadly across the front of my chest? I'm thinking even a small piece of plywood that stretches from shoulder strap to shoulder strap might be more comfortable.
@Scott_Draper Sounds like either the back pack is not adjusted properly, doesn't fit or is not up to the task of carrying that weight. Osprey rate the Osprey Stratos 24 to 25lb I think.
which probably means it will not be that comfortable with 40lb. From the looks of it the hip belt is not that beefy enough and I doubt the framing is stiff enough for that weight. It may be ok for short lengths of time say < 1hour but ymmv.
In any case it won't be comfortable for long if it is not the right size or is adjusted incorrectly.
The most important measurement on sizing is the torso length. If this is wrong then the backpack will not sit right on the hips. It will either tend to carry on the shoulders if too short or slip down if too long. This can make the shoulder straps uncomfortable because they won't adjust in their intended range. The Stratos is a one size bag which means it fits you or it doesn't. That is fairly typical of day packs that are not designed to carry much weight because up to about 20lbs poor fit is less of a problem. The average male torso length seems to be around 19 inches so my guess is this pack is designed around that but it is just a guess. Osprey does not give a size range.
To put a backpack on, you start with the shoulder straps and load lifters loose and tighten the hip belt so that the the backpack's weight is supported on your hips. The belt should wrap around the top of your hip bones. Then you snug up the shoulder straps lightly and loosely attach the sternum strap to just keep the shoulder straps from separating. Then tighten the load lifters until they are just snug. Make minor adjustment as required. You should not feel much weight directly on your shoulders. If you do, then try again starting from the beginning. If you cannot get it to be comfortable then possibly the packs fit is wrong
You might be able to improve things by packing the weight differently. You are concentrating the weight which is not really the type of load these backpacks are designed for. As far as you can you want put the weight as close to your lower and middle back as possible. To fill the pack you may need to pack the weight with blankets or similar to suspend it in the the right place then use the pack's compression straps to fix everything in place. It may take a few tries to figure out the best way to do this so the weight doesn't slip down.
Thanks for the detailed reply. Yes, my next step was going to use some padding to try to move the weight closer to my body. I was worried about the frame strength, but it seems to be holding up OK. I don't need to be comfortable for an hour, just 30 minutes, but I'm having to stop frequently to lean over and give my shoulders a break. I'm able to slip my hand pretty easily under the shoulder strap, so I don't think it's carrying a lot of weight.
Sounds like possibly the shoulder straps are too loose. Ideally they should flow from their connection point up your back and over you shoulders without any gaps. the load lifters that connect the top of the pack to the front of the shoulder pads should be snug and you should be able to tighten them and bring the pack closer to you back without causing the shoulder straps to bunch up. Poor fit may prevent that but once you have the weight better distributed you may have better luck.