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In-Reply-To: <n1419254749.16564@ma*.nd*.gt*.co*> On the Lusitania and other dives of similar depth, we used five cylinders: typically twin back mounted 15l (approx 120 cuft) cylinders of bottom mix (trimix 13/50), two side-slung 10l (approx 80 cuft), one nitrox designed for the depth of the first deco stop (40-50m depending on the bottom time), the second with Nitrox50, and staged 10l oxygen cylinders at 6m (20ft) for the last two stops. Support divers set-up the staged oxygen cylinders, two each under five buoys. Lots of spare oxygen cylinders rigged to lower in case anyone had missed the shot line - they would have been deployed by the support divers after a deco bag (delayed smb) had been sent up by diver(s) who missed the shot. If you want to carry all your gas, this limits you to two deco gases only, which can (depending on exact depth and bottom time) add a fair extra time to your deco. One deco mix would be designed to suit the first deco stop, and the other could either be oxygen or a strong nitrox to use on the last 3 or 4 stops. You would need to play with deco tables to see what was optimum, but you may find that the amount of the first deco gas you carry together with the safety factor on gas consumption you feel happy with, will affect at what depth you need to switch to the second deco gas. Generally, I found that bottom mix consumption varied markedly from day to day, but with twin 15l cylinders could plan for a return to the shot line with half my bottom mix left (ie turn at a quarter gas used) and so have a good safety margin. As it was, visibility on the Lusitania was good and people had little difficulty getting back to the shot. The condition of the wreck, the location of the shot weight, the use of stobes attached to the bottom of the shot line, and that the divers were experienced at finding their way around wrecks all helped as well. You do need to think very hard about the logistics, however, in case anything does go wrong. We opted for ten divers doing the wreck, two support divers and two fast cover boats. Two boats means you can give boat cover to any divers that come up off the shot as well as those who are on the shot/breakaway deco station, as the two groups could get separated by quite a distance in the tide. David Wilkins [UK]
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