i get what you are saying, and i took the same physics and geometry level classes too so dont take me as an expert physicist or anything like that.
lets say you have a 9k winch, but its mounted on a earthroamer xv-lt expo truck(if this is you, your an idiot, cause you paid at least $250k and couldnt pony up $2500 for a nice 16.5k winch or higher...?) anyhow the earthroamer is rated at 17,950 gvwr(lets call it 18k to make things easy.
now lets take that truck and get it stuck in the mud for sake of this argument. now every situation will be different, as variables such as how much weight or gear you actually have with you, how stuck are you, -is it just i cant get traction, or am i to the frame in the bog ect., and are you on level ground, down hill, or up hill ect ect ect. but for scenario lets just say flat/level ground, stuck to the axles in mud. so now you need to pull yourself out, but your vehicle weighs 18k and you only have a 9k winch. you are going to need a snatch block to double that 9k power to 18k. that means that every part there is going to be taking on the full 18k weight. that single line, its having to do all the work(there is no second line, and second winch, yes you have doubled the line over, but all you did was basically change the gear ratio. that stanch block just took the load for the motor in the winch. so you start to pull and all things being ideal you have only 6 wraps on the drum of the winch so you are getting max pulling power, and the mud is not sticky so its not creating a suction or holding power -ill cover this in a bit. so you are pulling your 18k truck with your 9k winch and its working. but is your winch line rated to pull that -sure it can, but is it rated to do so? and just cause it can once, how many times do you want to trust that it will keep doing that? and again is your snatch block rated to do that? no, ok but your are stuck and its all you have, so you give it a shot, and it works. great, but do you want to trust it again and again to be over loaded every time you have to use it?
maybe you should get a bigger snatch block. -i would not own a snatch block that is under half the load of my winch, unless you plan on only using it for a change of direction. -now i have a snatch block that is only a 16k sntach block and i have used it with my 9.5ti winch, to where the winch bogged down and almost stopped, had the truck almost completely off the ground. -i was pulling a tree over at my parents house and had anchored my 1985 toyota pickup to another tree behind me and using the winch to pull the tree in the middle of their yard out, and no my snatch block did not fail(and i did not tear the frame in two). so yes it can be done of needed, these things are under rated, but why do this if you can avoid it. i have sense bought a 18k snatch block and keep both of them with me at all times in my rig. i will use the smaller one for change of directions only. i would never put the smaller one into a compound system knowing that i have already over stressed it, that would be asking for a disaster.
so now to cover issues with snow, sand or mud... all of these elements can get you stuck, and when winching out of them your actually might be pulling more weight than the rig itself weighs. that is because these elements will cause a suction or vacuum if you will, and the can and will fill into all the small crevices and openings at or below their level. thus the initial pull will require more force or effort to get the vehicle moving or unstuck than will be required once the vehicle starts moving to keep it moving. so lets say our same rig the earthroamer xv-lt is stuck to the doors in a mud bog and the bumper is half under mud, the axles are completely covered, and the frame is buried along with the transmission transfer-case ect. so we get out our 18k snatch block, and our cable, and we start rigging uo our system to pull out the truck, get it all set up and go for it. and the winch just bogs down... no pull nothing. so we are now needing more power to pull the truck on level ground that we would need to pick it vertically up in a parking lot.... that is the power of suction. and i have experienced this in snow and sand as well. some would say well just toss in another snatch block... ok well hold on, not so fast. you might only need another 1000lbs of pulling power, so that might work, but your are creating the ability to now pull 27k lbs so anything between 18 and 27k lbs you will be able to move, but your snatch blocks are only rated for 18.... so moving 19 or 20k might be doable -remember these are undrated, but by how much, and again thats always in optimal conditions never used before ect., but 27k is not gonna work. -well you say thats no biggie, i got me two of the Warn 24k lb snatch blocks, we will just use those... ok so now your snatch blocks are good for it, but is your bumper and how its mounted? or the winch and its mounts? -the drum might not be able to take that kinda abuse, because while the motor is only pulling 9k the drum and line have (up to) 27k lbs on them... and i know the line on a 9k winch will not hold a load of 27k... sorry not gonna happen.
so in short yes snatch blocks are great, but dont get ahead of yourself and rip your system apart.
i have seen videos of 2 or 3 guys rip the guts out of a rope system moving a huge truck. they literally broke pulleys and shredded the rope they had it so built up.
im trying to find some images or videos of such disasters, but but right now i can not. now i could be wrong on my thinking, but i know i have seen systems fail that seamed like they would work, but didnt because one or more of the items it the system were not rated high enough. and if i remember correctly the only thing that the power or load rating does not matter on, is the unit doing the work ie the motor, that can be as small as it wants, you will just have to over come that with gearing(pulleys/snatch blocks) -also the more you gear down or every snatch block you use cuts your length of pull in half and the distance of the the pull. if you have just one snatch block, and you pull in 10' of line you will have only moved 5', say you have pulled in 15' of line, but you have two snatch blocks, well you only moved 5' still... cause you have 3 lines.
a note to the admins... this thread or group of post might be nice to be added to the general section board index under winching and recovery tips and techniques as well as here. just a thought.