Gilldiver
Contributor
Sidemount is a great system for wreck diving, but I agree that backmount would be better for swimming down the hatch on the 853, just like backmount sucks for trying to slide under a deck plate on the Oregon. I think you're a ways off from this yet, but at a certain point you might want to consider cave training. It helped me a lot with wreck diving to have that training--gave me a whole different mindset.
I have been diving the 853 for 30 years and know the in's and outs of that sub and have gone through all the hatches with the exception of the engine room and crews quarter escape hatches, with double 72's, 80's, or100's (the escape hatches or just too narrow for me). But, I have seen a good side mount diver do it by unhooking the bottoms of the tanks and swinging them in front like DevonDiver is doing in his profile photo. The drawback is that your hand are needed to control the tank and are not as useful for pulling you through the hatch. They are also not available to clear any hangs like a camera, wreck bag, etc, when used to control the tanks. But that is where practice comes in. You also would need to rely on you fins a lot more if you can't just grab the sides of a hatch combing to pull through. The tightest hatches I have gone through are the Engine room hatch and the Engine room to Officer quarters hatch on the USS Bass, they are just narrow but tall so getting doubles through is not all that much of a problem, but you have to be clean with nothing dangling off of you. They should be just as easy/difficult in side mount.