1) I can't imagine a properly sized single tank wing thats longer than the tank, to be honest. Certainly all of my single wings are much shorter than my AL63, if that makes any difference.
2) The whole donut vs horseshoe debate is kind of silly. The lower channel of the donut that passes under the tank in use is almost certainly not going to have any air going through it unless you're in a very heads-down position.
1) There are two kinds of European steel tanks holding the same amount of gas (12 liters at 200 or 232 bars) with one of them quite akin a steel Worthington HP80 for the dimensions and weight: the short, wide ones (diameter around 203 mm, length around 520 mm) and the long, narrow ones (diameter around 170 mm, length around 710 mm). In Europe, the short ones are called "
short steel 12 liters" and the long ones "
long steel 12 liters".
(I assume that HP80s are quite similar to 12 liters tanks because here, in France, we consider AL80s as 12 liters at 200 bars, which is approximate; actually an AL80 = 11.3 liters at 200 bars. But 12 liters at 232 bars (more usual here now than the 176 or 200 bars of yore) = 2784 liters = 98 cubic feet. So Kern is right (see next post) and I have edited this post accordingly, but I think that doesn't change my point which may interest divers in Europe or South Africa).
The
short steel 12 liters tank (= quite close to a steel Worthington HP80 for outer size and weight) is too short for most single-tank doughnut wings, as Rainer already said, and for the reasons he said.
On the other hand, most (if not all) single-tank doughnut wings will be shorter than
long 12 liters, than 15 liters, and than 18 liters, steel tanks; and shorter than the AL80s ubiquitous in tropical resorts. So no issue from the length of the wing with these tanks.
Anyway, the
short steel 12 liters tank is an awkward clunker with any kind of BCD: it fits the traditional BCDs lengthwise, but it's so short and heavy it acts like a keel in your back and you feel like a turtle with it.
Short steel 12 liters tanks are often used by dive shops in Europe or South Africa because they are more stable for storage and heavier in the water (less lead for the clients) but that's their only advantages. They are very common in the dive centers (much more so than the long 12 liters) but if they don't fit your wing you can normally get a 15 liters that will be longer and OK.
Long steel 12 liters tanks are fine, but may be a bit too long for short people. They look quite like AL80s but a bit narrower. They are the mainstream for twinsets in Europe, and new European models can be quite light (nowadays 12.4 kilograms for a long steel 12 liters rated at 232 bars of service pressure from French manufacturer Roth). They feel close to AL80s underwater yet they are lighter on land. Nice tanks, really.
2) What you say about the "lower channel of the donut" doesn't match my experience. Air does go through the channel under the tank with the doughnut wings I use, without need of significantly rising my butt (I mostly need to roll from one side to the other). I find it useful. But that doesn't mean that horseshoe wings are not good.
