I hope this is the right forum for this question.
I am new to doubles and have begun the process of educating myself and piecing together a rig.
Can someone explain to me why the primary hose needs to be 7ft long? In an emergency situation diving a single tank where my buddy needs air, I would dish off my primary to my buddy and breath my backup, no? And my backup hose is longer than my primary. Why is the primary 7ft long in a doubles setup?
Obviously I'm new to doubles, but I'm trying to figure it all out. Many thanks!
Your single tank set up is not "correct" for donating the primary if your backup 2nd stage is on a longer hose. Regardless of single/double tanks, if you want to donate the primary (a good idea IMO) you should have it on the longer hose, and keep your alternate on a bungied necklace where you can get it immediately and even hands-free if necessary.
The idea of the long hose is often associated with doubles because it's part of the hogarthian technical diving set up, and most technical dives are done with double tanks. But, the long hose is a great set up for single tanks too, because the air sharing is so much more convenient.
Now, as sax player said, 7ft is the standard length for overhead environments in which divers might need to exit a restriction single file while sharing air. This length also routs conveniently if you are wearing a canister light on your waist belt. For OW diving, a 5 ft long hose routs very well under the right arm, across the chest, over the left shoulder, behind the head. For larger divers 5 ft might not be quite enough, so it's worth experimenting with slightly different lengths. You can do that by using a coupler to connect two standard length hoses and come up 5'6", for example.
With doubles in OW, the primary hose leaves from the right post and consequently has less distance to travel, so for me 5ft is perfect. Keep in mind this is only for OW environments; in restrictions (of course you would get trained for the dangers of diving in restrictions/overheads and the instructor would explain all this) you really need the longer hose.
I wish custom hoses were cheaper and easier to get. I'd get a 5'6" hose for OW singles, keep the 5ft for OW doubles, and the 7ft for cavern/cave.
