Wookie, this is a purely hypothetical, but what would you consider someone diving mini doubles? I'm starting to lean that direction even for recreational dives as well as I enjoy the redundancy and similarity in setup to my normal doubles rig, From an "advanced" diver standpoint, a set of twin 40's or 50's makes more sense to me than a single 80. Not that your answer would prevent me from diving with you if I ever make it down that way, just curious what the answer from a captain's perspective wold be. I totally understand if a blanket "no doubles" policy exists on your recreational charters.
I will explain the history of the rule. The Fling and Spree were patterned after the California liveaboards. When first in business, we speared, we slept in sleeping bags on the open deck, and you brought as many cylinders as you wanted and piled them on the deck. A weekend trip to the Flower Gardens was $215 or so, and the boat would take 50 divers if that many wanted to go. We'd just put on another case of eggs. Doubles, not a problem.
As time passed, we got a covered deck, installed bunks, limited trips to 34 passengers, and set up dive benches on the main deck. On the dive benches there were 40 dive stations, 34 for guests and 6 for crew. That made things a little tight, and we started attracting a more genteel clientele, although divers were still tough. The spearing ended the day the former owner stepped out of the house and on his brand new deck carpet was a big stinky amberjack someone had speared with all of the requisite blood and slime. That was the last day anyone had a speargun on either the Fling or the Spree.
At about the same time, some group from Louisiana wanted to bring doubles. No sweat the owner says, but they won't fit in the tank rack, you will have to secure them against a rail. No problem the shop says. Shop shows up with 15 hoovers in double LP 130s that they had all bought special just for this trip. They figured out the first day that the air compressor could in no way keep up with 20 single 80's and 15 double 130s in a surface interval. Then, the folks in doubles wanted bench space to kit up. The folks in singles (Burton's third law of diving: divers will expand to take up all available space) got their masks smashed, regulator hoses kinked, it was just a mess. Then, it was rough on the way back in. Since the diving was over,the beering had begun, and the doubles were not properly secured to the rail. Some sets of double 130s were sliding around on the back deck and banging into other folks stuff. The final straw was that a number of manifolds were bent in the banging tanks and the owners wanted the boat to replace the manifolds.
That was the last time doubles were allowed on a sport trip. What we found was that it doesn't make any difference. Folks who routinely dive doubles aren't really keen liveaboard customers anyway. Liveaboards operate on a schedule. Since the majority of liveaboard divers are all about the number of dives, and we don't limit bottom time, it makes no sense to disrupt the normal routine of the majority of the customers to satisfy one who may not come anyway. When someone wants to put together a doubles trip, we welcome them and their 17 friends who want to dive doubles (See what I did there, I cut 6 folks off the boat and raised the price a little). We don't have trouble with room on the boat, everyone gets a fair amount of bench space, and life is good. When TS&M goes to the red sea, she charters the whole boat, they dive doubles and scooters, and the whole boat is dedicated to what they want to do, not what the 2 strays on the boat want to do, because there are not 2 strays on that boat. Same when she goes to channel islands. They charter the whole boat, bring their doubles and scooters. I think they limited their last trip on a 34 passenger boat to 21 or so. We do that too, if someone wants the whole boat. Add Helium has the whole boat week after next to go dive 300 foot deep wrecks. We offer those charters, but they aren't your standard recreational charter.
Now, when we talk tiny doubles, if you mean something like the old AL triples, the UDS-1 system, I've had a few folks bring them and bungee them to the rail. They aren't obtrusive. They aren't very practical, but whatever. If you are talking twin 50's with an isolation manifold, they are the same size as HP100s, or 120s. You'd get more gas with a 120 and a pony for redundancy.