Thanks everyone for all the feedback. I didn't make it clear in my initial post, but I'm sure it's fairly obvious that I don't currently do any side mount or even BM doubles diving. I just recently had the opportunity to try side mount at a Demo day here in Atlanta hosted by Divers Supply so that is my only experience so far with using two tanks at once. I'm most surprised by how many responded with using 3 tanks as that number never occurred to me as being an option. Is there a particular rule of thumb as far as how much air should be remaining in the lightly used tank after the first dive?
Still not real sure if dealing with two tanks on a rec charter boat will be worth it though, but I intended to try it out. Although I like JamesK's thinking a bit more as far as just diving with one side mounted tank when on a rec boat. I made a point to try a single tank at the Demo day and was suprised that trim wasn't as big an issue as I thought it would be.
As you can tell from above, I currently don't have one. But your later posts brings up a good point for me to think about with regards to repeatability with a particular rigging system.
Thanks Again,
Paul
Just a few more things to think about Paul ...
Diving a single in sidemount works best if you're using an AL80 ... because it's relatively lightweight and becomes buoyant as it gets toward empty. Doesn't work so well with something like an HP100 ... it's doable (I've done it), but it requires some technique balancing the cylinder, and even with technique it's work.
As for your question about how far to empty one tank, it depends on whether you're diving with a buddy or not. If you are, you should be breathing your cylinders somewhat evenly, with the notion that since they're independent cylinders if you have to donate a reg you're not going to have that gas available to you at all ... so it wouldn't do to breathe one cylinder down much below what you'd consider a rock-bottom pressure for either you or a buddy. On the other hand, if you're solo diving, then that's not a concern, since it's unlikely in the extreme that you're going to need to hand off a reg to anyone. But you do still have to remember that as you breathe one tank down, it will become more buoyant than the one on the other side. Personally, even when solo diving, I still prefer to keep my cylinders within a few hundred psi of each other, both to keep my options open and to maintain a better balance on the rig.
Sidemount ... particularly for recreational divers ... is a fairly new thing, and there are no standardized methods of doing much of anything. Rigging, for example ... some people use cam bands, which are relatively simple to switch from tank to tank. Other people use hose clamps, which tend to stay with the tank. Which you use will determine your strategy for swapping tanks.
When I said I bring a third tank and a transfill whip, what I meant was that I'll swap out one cylinder and then transfill between the two I'm using in order to balance them out. Some folks wouldn't do that, but it works well for me.
As for extra tanks on boats ... it really depends on the boat. It's best to check ahead and make sure it's OK before making it part of your gas plan. Most of my local charters have plenty of room for a third tank, as long as everyone on board doesn't bring one ... and even then some can accommodate it.
The biggest problem I have with sidemount on boats is one that beano alluded to ... boat entries weren't made with sidemount in mind, and unless you're going off a swim platform you're often going to be forced to go through the entryway somewhat sideways, and doing an "angled" stride off the boat just to fit the tanks through. On smaller boats, I often find it easier to just drop the tanks overboard on a lanyard and clip them on once I'm in the water. But again, it's going to be situation-dependent.
There's lots to consider ... which is why this forum's handy. Seems like we all do something a little bit differently ...
... Bob (Grateful Diver)
