lift capacity needed for cold, open water diving with side mounted doubles

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rick00001967

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so i have been doing a substantial amount of reading online about different side mount rigs available and there are a few i am looking at. however, some of them are described as being for warm water diving only, due to the relatively low lift capacity. "they" say that due to the extra weight we need to carry for cold water (because of dry suits, steel tanks etc) that the lift capacity is too low. and everything i have read about choosing a proper sized wing seems to support this thought.

i am confused by this. let me explain why. then you can point out what i am missing and why i obviously have no idea what i am talking about.

i try to weight myself (no matter what type of tank or gear i am wearing) to be neutral (or slightly negative) at 10 or 15 feet with low gas (maybe 500psi if max is 3000psi) and my BC emptied. so at this point obviously the lift capacity is a non issue as i am neutral. so when do i need to add gas to the BC ?? and how much lift do i really need ?? i would need to consider that when full (as opposed to having 500psi) each tank would have maybe 5lbs more gas in it. if diving doubles, thats about 10 lbs. so i need at least 10lbs lift in my wing to counter act that extra 10lbs of gas at the start of a dive. i also need to consider that i want to be able to float comfortably on the surface with my head out of the water. i have seen it mentioned several times online that an average head with a hood on it, adds about 10lbs negative when above the water. so if i use that as a rough guess, that means i now need 20lbs of lift in my wing (10lbs gas + 10lbs for my head) at the start of the dive to be able to float comfortably. so if i had a wing with 20 to 25lbs of lift, i should be fine right ?? i have no intention (at this point) to carry added tanks beyond the two. i don't have to worry (as back mounted double divers do) about whether my wing will float my rig if i take it off. so what am i missing.

no matter whether i am in salt, fresh, diving wet, dry, cold, warm, using steel or allum tanks, i will always be weighted close to neutral with low gas in the tanks at my safety stop depth. the only lift i seem to need is the extra gas weight plus keeping my head above water. apart from that, i can see needing some extra lift possibly for wet suit compression (if i wore a wet suit), but i dive dry. so any compression will be offset by me adding gas to the suit.

now if i was looking at a worse case scenario, and had a flooded suit at my deepest part of the dive, i would be dumping lead anyway to hopefully do a controlled ascent. i would not rely only on my wing lift to get my back to the surface. and if my wing fails, well then the lift capacity becomes irrelevant.

so lets hear it. am i crazy or what ?? what am i missing ??
 
You're on exactly the right track. Properly weighted, most divers (side or backmount) don't need nearly the lift they think they do, even in cold water.

The rule of thumb starts with having lift in your trim device equal the weight of the gas in your tanks. For divers who use multiple stages, this can add up to a lot, and they legitimately need and use a lot of lift. For normal doubles, we're talking 10-15 pounds tops.

However, this rule of thumb presumes a "self-compensating" exposure suit like a drysuit. If you're diving cold water in a wetsuit, you'll need extra lift to counteract the suit compression. This can add up to a lot if you dive wet and deep. (10-15 lbs.) I have a super floaty 7/8/9mm which exceeds the limits of a smaller trim device past about 60 feet.

And finally, you will want that little "bump" to put your head that much higher when you're at the surface. I've found that effort in this department is better spent attaining the confidence that you're not going to drown if your BC floats you with your nipples below the water. +5 pounds to account for this is plenty if you're calm, composed, and properly weighted. The +10 you mention is generous IMO.

All that said, if you dive tropical wetsuit, or dry without stages, you can probably get away with a small trim device 20-25 pounds. Cold water with a wetsuit, or dry with stages, mid 30s. Heavy duty tech with stages and other gear, 40+.

Many of us in SoCal who dive dry (or shallow wet) with no stages and AL80s use the smaller devices, with lift in the low 20s.
 
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While drysuit diving you also need extra lift to accomplish buoyancy lost in massive drysuit failure. The volume of your undergarment while compressed.

- Mikko Laakkonen -

I love diving and teaching others to dive.
 
at least i know i am not completely off my rocker. i look forward to input from some others as well. thx fellas
 
Thats why I got the stealth. 30ish lbs of lift seems to encompass basically all diving situations i would do.
 
30 lbs of lift wouldn't be adequate for diving in cold water, with a drysuit and double steels ... not if you're in an area like me where you're going to be doing a lot of surface swimming ... not unless you're planning on doing that swimming while breathing off your reg the whole time. I base this not on theoretical calculations, but on trying it ...

... Bob (Grateful Diver)
 
Thats why I got the stealth. 30ish lbs of lift seems to encompass basically all diving situations i would do.

i was in a stealth for my intro to side mount. worked great. and yes i agree it would probably be great for me here. trouble is that i cannot buy one through my local dealer and i am required to do that now. my options are limited.

---------- Post added March 20th, 2014 at 12:11 PM ----------

30 lbs of lift wouldn't be adequate for diving in cold water, with a drysuit and double steels ... not if you're in an area like me where you're going to be doing a lot of surface swimming ... not unless you're planning on doing that swimming while breathing off your reg the whole time. I base this not on theoretical calculations, but on trying it ...

... Bob (Grateful Diver)

i appreciate the input of some real world trials Bob. but did you read my original post ?? i am curious about your thoughts on the theory i have used here.
 
Your theory is sound, if it's a balanced rig then if you have x amount of lift for x amount of static weight that would be sufficient. Now outside of the classroom, dive your setup using only the wing (suffer the squeeze) and see if it achieves your goals. For background, I dive SM steel 120's with a dry suit and 37 lbs wing, cold fresh water.


Sent from my iPhone using Tapatalk
 

i appreciate the input of some real world trials Bob. but did you read my original post ?? i am curious about your thoughts on the theory i have used here.

I prefer more lift, primarily because we do a lot of surface swimming here. When I was backmounting doubles, I found that about the minimum size doubles wing that worked for me was the DSS 49# Torus. It allowed me to carry the HP100 tanks, a couple of AL40 deco bottles, my camera, and a scooter. For deeper dives, using bigger tanks and maybe a stage bottle I'd switch up to a bigger wing (Halcyon Evolve 60).

Diving sidemount still puts me in the category of wanting comparable lift capacity. For just the 100's, I want something closer to 45-50# of lift ... like I get with the Nomad.

Now, when I go to MX, where I'm diving a skinny wetsuit, fresh water, and a couple of beer cans for tanks, I think a 30# wing is plenty ...

... Bob (Grateful Diver)
 
when I go to MX, where I'm diving a skinny wetsuit, fresh water, and a couple of beer cans for tanks, I think a 30# wing is plenty ...

And yet, Dive Rite sells their "Nomad LT" with just 22# of lift! They say it's only for wetsuit diving with AL80s (or Faber 85s). I dive a 5mm suit, I worry that it would not be sufficient lift if I went below 30m or tried to carry a deco bottle when and if I start taking some extended range diving instruction.
 
https://www.shearwater.com/products/perdix-ai/

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