Sidemount diving with aluminium tanks is very different to sidemount using steel tanks, sadly there still seems to be some denial in this my many manufacturers, the Hollis SMS50 being a very good example of this.
Personally, I’d always use a valve wrap for attaching the aluminium tank to the bungee in sidemount. I’ve tried many configuration on this and quite simply nothing works anywhere near as well. The wrap around the valve pulls the tank up securely to your armpits and keeps the valves in easy reach; allowing the tanks to fall behind the armpits is a problem, the tanks tend to move more, aren’t as easy to reach – it also leaves the tanks more prone to falling our of trim as you are reducing the critical dimension between the bungee support and the anchor point on the waist. Basically they should end up similar to the image below.
The valve wrap technique should always be made from the outside of the tank to the inside, otherwise the reg will rotate outward exposing the first stage, plus the tank will not be secure if moving to a tank forward no-mount position. For lightweight aluminium tanks this is the most secure mounting technique, it’s harder to apply than loop type techniques but becomes easy after some practice.
Sadly most rental tanks are either non-left/right tank configurations or don't have the extented stam. Without an extended stem you’ll often need to double (well, one and a half) wrap on the left tank (shown below also, the right tank is still basically the same wrap as before). This is the main reason I like the single continuous bungee over two independent bungees – it allows the tanks to be easily balanced when different valves or valve connections are used on the right/left side. If you attach the primary left tank first it also means the bungee has much more play so the double wrap is easy to achieve.
As Andy said, the bungee needs to be smaller than the ridiculous system that comes with the SMS50, 6-8mm is the best fit in most cases. Run the bungee from the chest d-rings with clips if possible. I try to keep the bungee attachment as high as possible, using attachment points under the d-rings doesn’t feel as comfortable for me. I also like being able to unclip the bungee from the d-rings if required. The bungee can be run from d-ring to d-ring through most harnesses as shown below.
The biggest problem with the SMS50 is the door handle style d-rings at the rear. My advice for aluminium tanks is to remove the whole butt plate and either keep it for later use with steel tanks or simply bin it, that’s how useful it is
Using aluminium tanks is best done with two d-rings mounted on the waist strap, one towards the rear for the full tanks and one towards the front for the buoyant tanks
The spacing of these d-rings is dependent upon the cam band set-ups so it’s best to have your own bands you’ll use on each dive. If you can find the low profile d-rings (
D-Rings, Belt Slides, and Clamps - Dive Gear Express), these are ideal for the front d-rings. Don’t use the offset d-rings at the rear for mounting tanks, they have the same problems as the door handle rails on the butt plate. the d-ring positions should be something similar to what is shown below, using the rear offset d-rings results in too long a leash and the front d-rings are either way too far forward or the tanks flap around too much from the movement in the leash.
The cam bands should use the shortest possible leash length and a short clip, as small as you can comfortable work with. A common mistake for aluminium tanks to set the leash length using the distance from the rear offset d-rings to the tank, it’s best to keep the leash as short as possible and use the d-ring positions to maintain tank position. The tank should sit along the side our your body, the position of the d-rings should be the length of the leash connection off this ideal position (towards the rear for the full tank d-ring, towards the front for the buoyant tank d-ring). The cam band position on the tank should suit the distance from roughly your armpit to the waist band. The lower the better is the general rule I’d follow for cam bands (although not too low to keep the tank out of trim). If you keep the tanks up by your armpits it’s possible to have the bands quite low on the tank.
The type of regs used also have an impact on aluminium tanks, heavy first stages like the Apeks DST or Hog D1 tend to pull the tank down, so it’s becomes buoyant much earlier, even around 170-180 bar/2700 psi in some cases, lighter regs such as Mk25s tend to become buoyant much later, normally around 120-140 bar/2000 psi mark. I know a few people using the heavier regs who just use the forward (buoyant) d-rings from the start, it’s not recommended as they hang low at first but it’s probably better to have this at the start rather than the tanks sticking upright for 70% of the dive (as would happen if just butt plate the door handles are used).
I think if you try ditching the butt plate, change the bungee and add two d-rings either side of the harness along with following the tips above I don’t think you’ll be too far off. The only other tip I’d have is to avoid putting weights on the tanks, it’s much better on your body/harness. If you are struggling without using tank weights is normally a gear configuration problem, tank buoyancy is dynamic and will change through the dive, using static weights to counteract this is not a good solution, body positioning is a much better technique.
In my opinion, the SMS50 is a bad harness, obviously everyone has their own opinions but for aluminium tanks I think the Stealth 2.0, Razor 2 or UTD Z-Trim (only the small 9kg lift, not the Z-plus or Alpha delta) are much better options - just in case you have that option!
Hope that helps!
Karl