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Side mounting AL80's how do you trim your tanks?

Discussion in 'Sidemount Diving' started by eternaljonah, Sep 11, 2013.

  1. DevonDiver

    DevonDiver N/A

    # of Dives: 5,000 - ∞
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    duplicate
     

    Attached Files:

    Last edited: Sep 12, 2013
  2. eternaljonah

    eternaljonah Captain

    # of Dives: 5,000 - ∞
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    the good news is that i can go and try some stuff :)
    below are the tanks at 140 bar :) a little better :)

    side3.jpg
     
  3. DevonDiver

    DevonDiver N/A

    # of Dives: 5,000 - ∞
    Location: Subic Bay, Philippines
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    Here is my lightweight rig (good for 3x AL80). You can see the initial attachment point on the 'off-set D-rings'. When the tanks get floaty, they go onto the forward D-rings on the waist strap. These D-rings sit on the front of my hips.

    View attachment 165865

    You can swap to steel tanks to mask the issue, but that won't leave you many options when travelling with the rig. It's hard to get steel tanks in many tourist destinations. Then you're back looking like 'Mr Floaty-Tank-Noob' again...

    [​IMG]

    Sidemount-aluminum-cylinder-attachment.jpg
     
    EdwardHall likes this.
  4. a878bob

    a878bob Manta Ray

    # of Dives: 1,000 - 2,499
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    Andy is right. My sidemount partner Walt has a Hollis 50. And his tank trim and body trim are absolutely perfect. The butt plate rails are useless for al cylinders. 4 low profile d-rings to start. First pair against the wing and the other in front of your hips. Even in the 3rd picture, cam straps are too low on the cylinders. The first thing I did on my rig was to go with 1/4" bungie. Makes all the difference in getting hooked up.
     
  5. Karl_H

    Karl_H DIR Practitioner

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    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.

    11111 trim.jpg

    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.

    Tank Wraps.jpg

    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.

    Double wrap attachment.jpg

    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.

    Harness with bungee.png

    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.

    111 - Full Tanks.jpg 111 - Bouyant Tanks.jpg

    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.

    Cam Band Positioning.jpg Cam band.jpg

    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
     
    Last edited: Sep 13, 2013
  6. victorzamora

    victorzamora Solo Diver

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    I really liked your post, Karl....but there were a couple things I found wrong.

    Buoyancy of the tank has nothing to do with the first stage. Buoyancy of the assembled scuba system (tank+regs) IS influenced by the weight of the first stage....but in precisely the opposite manner in which you described. A heavier first stage will make the unit LESS buoyant (more negative) and a lighter first stage will make the unit MORE buoyant (less negative)....regardless of the weight or buoyancy characteristics of the tank. I think what you're saying is a lighter first stage will cause less "rotation"....but I also fail to see how that would be the case, using physics (Aerospace Engineer and Fluid Dynamacist...I've tried the math and I just can't find anything to support your claim).

    While I agree that there are better systems, I think the SMS50 is a fine harness. I believe that all harness systems have their failings, their strengths and their weaknesses. I think the SMS50 has a lot to offer over the Razor (the only one I have experience with of the other three you mentioned) in that the wing is fixed to the harness. The reduced beach-ball effect and the fact that it stays on your back are huge benefits, imho. As for the door rails, I think they're the perfect place to clip SMBs, reels, spools, etc. I do agree, however, that a few low-profile D-rings on the waist would go a long way to making it a really good rig. Andy Davis has his students looking flawless in it. Like anything else, though, it's not a matter of buying the harness and calling it good. It's the diver that makes the harness.
     
  7. Karl_H

    Karl_H DIR Practitioner

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    I think it's a terminology thing, by buoyant tanks I mean the floaty bottom principle and not necessarily the tank being buoyant as a whole. A heavy first stage at one end (the top) will make the other end (the bottom) pop-up quicker when held horizontally. It's the same principles we apply to trim, i.e. floaty feet - A see-saw is the probably best example.

    seesawmain.jpg

    The SMS50 is not the worst harness, but sadly it's down there, I'd call it the jacket BCD of sidemount; it's simple, easy to use and mass-produced. Agreed the diver make the difference, not the harness, but given the choice I'd rather have both the diver and the harness.

    As a standalone harness itself is hard to beat, the weakness of the Razor 2 by far is the BAT wing imho. That said, the beach ball effect is slightly over rated for me - we often teach in razor 2 harnesses and if using two Al80 tanks the diver is getting a beach ball then they are either very overweighted or have the wing set-up incorrectly. The weight used by the diver should only compensate for the air in the tanks, in an Al80 that's typically around 1.5kg per tank or 3kg/7lb total negative buoyancy as a worse case - 7lbs of lift doesn't cause a beach ball. For me, the biggest weakness is the inability to 'trap' air on one side of the wing, if the tanks are unbalanced (or you are carrying a stage/deco) it's a little unstable.

    The only way I can see a beach ball becoming a problem is if an equipment failure occurs, like a flooded drysuit, otherwise is should be easy to manage. It's a nice harness when set-up correctly for recreational/techreational sidemount dives, and I'd but it head and shoulders above the SMS50. Everyone has their own opinions though so it's often down to personal choice.

    Spools on the door handle is one use but a central d-ring is better, it's hard to reach spools with either hand when using the door handles? I imagine they are easy to knock off there too (as they are from a central d-ring also)? I think I just have an irrational hatred for butt plates on warm water harnesses in general, binning it may be a little harsh, it could make a great coaster or mini-drying rack :)
     
    Last edited: Sep 13, 2013
    sologuy likes this.
  8. eternaljonah

    eternaljonah Captain

    # of Dives: 5,000 - ∞
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    Thanks guys had a try with some extra d rings on the waist harness today and it was great when i was facing down but when i rolled to swim on my back as i was looking at students it was a little less comfortable. I live in india and gear is not easy to come by and try, even good bungee is a little hard to get :) but i will eventually get my hand on t :) lots of good input to take out there and try so i guess i have some fun dives ahead of me

    i did have a try too at adding a second clip to the cam band and then the tanks were locked on the back and front, that was super comfy and tanks were super in place but have to play with it a bit more before i'll know :)

    thanks again every one :) will let you all know once i get somewhere :)
     
  9. victorzamora

    victorzamora Solo Diver

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    A seesaw is a DRASTIC over-simplification of the concept....but I think I have figured out the math that would support extra tail-up trim using a heavy first stage. It has to do with the center of buoyancy and the center of gravity shifting further apart, creating a large moment arm about which the tail can swing.

    As for reaching door handles with both hands: I just remember what goes where. In a cave, my safety and my primary are on my left while everything else is on my right. In OW, my SMB and spool are on my right and nothing is on my left. My tail d-ring is hard for me to reach, so I ignore it almost completely. Overall, I agree that a buttplate isn't necessary on a warm-water rig....and that is NOT a rig optimized for steels (23# of lift is pretty low for steels. Manageable, but low).

    BTW, you would need to compensate for more than just the negative buoyancy of your tanks. You would also need to compensate for the POSITIVE buoyancy of your empty tanks. Overall, you'll need to compensate for the total buoyancy swing of the tanks you'll be carrying. An AL80 has a 4.864# buoyancy swing (assuming 77ft3 actual capacity, usage from 3000psi-500psi). That means you'd need over 9# of lift for two AL80s.
     
  10. MSargeant

    MSargeant Nassau Grouper

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    Last edited: Sep 13, 2013
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