Glad to see a forum just for SM. It's so inventive. ... Brought it up in my local forum and got "flamed out" in a hurry. (same with the long hose)
Out of curiosity, why did you get flamed? Or, more specifically, toward what aspect of SM diving was the 'flaming' directed?
Not real good on a boat dive. (choppy waters aside)
I think there are some straightforward ways to address this issue. People have been successfully using two deco/stage bottles on (tech) boat dives for quite a while. And, these are people who also usually have some big BM doubles on. If you can giant stride into the water with a couple of AL80 deco bottles clipped on - one on each side, or two on the left - you can certainly giant stride in with just the two bottles clipped SM. A lot depends on the boat configuration, and the crew familiarity with SM, and how many other divers are on board. Yes, on a crowded cattle boat, with entry only from the stern, and a crew unfamiliar with SM, I doubt that the first experience will necessarily be the best one. But, rolling backward off the side of the boat in SM, or rolling sideways off the side of the boat for that matter, doesn't have to be that big a deal (issues of current put aside for this discussion). For exiting, stand on the ladder at the bottom, unclip each bottle and hand them up, before fully ascending the ladder. That's what we do with deco bottles. That way, when you ascend the ladder you only have your harness/wing on. Yes, you have to disconnect your inflator hoses. Yes, you are potentially in OW without a gas supply, if you fall backwards off the ladder (although you are less likely to do that if you have nothing on your back besides you harness and wing). But, I don't know that you are at particular risk. No, I don't know that I really want to do that in 7 ft seas. But, I won't be diving BM in 7 ft seas either. (Getting in is easy, it is the getting out part that isn't.)
So the necks need to be well supported. Bungees don't cut it..or will wear out doing so ..
Not necessarily. When you are standing, much of the tank weight can really be supported by the rails and buttplate, and the bungees are keeping the top of the tank in close to the body, rather than supporting a lot of weight.
What about a large strong bolt snap attached to the first 2 " below the neck of bottle. Attached the same way the back snap is banded to tank. This rock solid clip can then be attached to a reinforced D ring on each shoulder on harness. Would only need the clip> When you enter and exit...they can be clipped tight to each side. When not needed...tuck em away under a wide elastic band.
When I took the SM instructor course, one of the things that my instructor emphasized was his individual disdain for the 'chokers' that DR uses. Instead, he loops some 1/4 nylon static line, with a bolt snap, around the neck of each bottle, and clips those to chest D-rings, AND then runs the bungee around the outside of the bottle neck, to pull it in close to the chest. I do it that way as well. It works, and in that configuration, much of the the weight of the tanks can rest on the chest attachments rather than the buttplate, just as it does with deco/stage bottle clipped to the chest and a hip D-ring. But, you can also clip that top bolt snap to the bungee itself, per the DR approach. Either way works, and as your OP implies, you can have some fun experimenting to see what works for you.
There are a number of very capable and experienced divers on SB who suggest that BM may be a bit easier, or efficient, for boat diving, and I respect their opinion. Personally, I find strapping on a set of BM double AL80s for a coastal boat dive is pretty easy, and getting out, even when the seas are up a bit, isn't that much of a challenge. It is probably easier than SM. Can't say the same for BM double 130s. I am still learning how to best manage SM on a boat. But, for shore diving, where you want to double up some tanks, and either don't have a set of banded, independent or manifolded, BM doubles available, SM is a great alternative. There are also some very experienced and respected posters on SB who see SM right now as a bit of a fad, and I think they are right. It is somewhat trendy / in vogue. But, it is also an interesting gear 'frontier' and that is exciting. As you said, it is inventive. SM has been around for a long time. But, the appearance of some commercial rigs on the market has stimulated a lot of new interest, or at least expanded the interest well beyond caves and technical diving.