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

[bada3003]

I adjusted trim/weighting before and after a cavern course recently. Here's what worked for me:

1. Wetsuit. Wore 7mm, Hollis SMS 50 Sport, two shoulder pouches holding 2 lbs each, 3 lbs each on weight pouches attached to camband of AL80 cylinders. At deeper waters (80+ feet), neoprene compression does require inflating the BC aggressively (SMS 50 has only 23 lbs of lift). If using front D-rings to counteract AL80 cylinder buoyancy, it may be possible to optimize further.

2. Drysuit. Trilam, 2 lbs each on shoulder pouches, 2 lbs each on side pouches (the SMS 50 Sport has two integrated side pockets but lacks the neck pouch in the SMS 50), 4 lbs each on cylinder cambands. With neoprene hood, heavy duty rock boots. I have yet to dive this configuration in deeper waters but I suspect will need most of the SMS 50's lift when tanks are full. Of course, with drysuit there's backup just in case.

at least i know i am not completely off my rocker. i look forward to input from some others as well. thx fellas

 

[Jim Lapenta]

I have not side mounted yet in a dry suit. But in a 5 mil my Nomad Lt Tech is enough when carrying a pair of LP75.5 steels. It is rated for al80's wet or LP85's when diving dry. The 75.5's seem to be somewhere in the middle and I could go either way as even wet I need to add a little weight on a belt to get me down and allow me to hold a stop comfortably. This year I plan to do most, if not all open water diving sidemount. If not in the LT then in my HOG rig as soon as it gets here. I have the following tanks set up and ready to try a bunch of combinations - LP 75.5's, LP 85's, LP 95's, and Al 80's. Plus stage and deco bottles. A big reason I am getting more training in SM by taking adv wreck and maybe deco procedures in the configuration.

 

[rick00001967]

I adjusted trim/weighting before and after a cavern course recently. Here's what worked for me:

2. Drysuit. Trilam, 2 lbs each on shoulder pouches, 2 lbs each on side pouches (the SMS 50 Sport has two integrated side pockets but lacks the neck pouch in the SMS 50), 4 lbs each on cylinder cambands. With neoprene hood, heavy duty rock boots. I have yet to dive this configuration in deeper waters but I suspect will need most of the SMS 50's lift when tanks are full. Of course, with drysuit there's backup just in case.

i am going to assume since this was a cavern you set your weight so with the min pressure (1000lbs each tank) you were neutral with that 16lbs (4 on shoulders, 4 on waist, 8 on tanks). so would be aprox 8lbs heavier at the start with full tanks.
i am also going to assume that the sms50 worked just fine ?? did you have any issues at all as far as lift ?? i wouldn't think so.

 

[bada3003]

For the cavern course, I dove with the 7mm wet suit. As I had spent a good amount of time/effort on trim/buoyancy during the course (in addition to preps prior), the rigging outlined above to achieve better trim were the result of that process.

As to drysuit trim in general, obviously a lot depends on what you're wearing. For the config described, I wore my thinnest undergarment (100g fleece) whose buoyancy contribution is less than my two thinsulate undergarments. In general, I would be hesitant to dive the SMS 50 with drysuit in overhead environments because of its small lift as noted earlier, even with the drysuit acting as a backup buoyancy control device. In addition, the SMS 50 Sport has the inflator valve/hose exposed in the neck region (hence no weight pouch as in SMS 50) which, for those interested in entering constricted areas, would not be advised.

As to being 8 lbs heavier with full cylinders, it depends on buoyancy. For Catalina AL80s, I believe the tanks are negatively buoyant by a tad less than 2 lbs when full, positively buoyant by about 3 lbs at 500 psi. At about 1000 psi (following rule of thirds), the positive buoyancy to compensate for would be closer to 4 lbs in total. Also, my instructor likes to leave 200-300 psi in addition to the 1/3-rule (he's been diving caves for a long time so I'm following his recommendation) so all my practice practice dives were completed with 1200-1300 psi remaining. For cavern diving there were additional variables that came into play but are outside the scope of the OP's question and present discussion.

i am going to assume since this was a cavern you set your weight so with the min pressure (1000lbs each tank) you were neutral with that 16lbs (4 on shoulders, 4 on waist, 8 on tanks). so would be aprox 8lbs heavier at the start with full tanks.
i am also going to assume that the sms50 worked just fine ?? did you have any issues at all as far as lift ?? i wouldn't think so.

 

[rick00001967]

thx for extra info on that.

btw......to be clear.....when i said 8lbs for the extra air, i meant for both tanks. 4 lbs each. i usually use a rough estimate of aprox 6lbs of air in an 80cf tank. if you test for neutral buoyancy with 1/3 full tanks then you have aprox 2 lbs of air in each of them. at the start of the dive when they are full, they will each be aprox 4lbs heavier. makes no difference what type of tank. 80cf weighs what it weighs.
thx for the input

 

[EireDiver606]

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.

Cold water with a wetsuit, or dry with stages, mid 30s.

Are you talking about doubles or singles? I assume single.

 

[J-Vo]

Dry with heavy undergarments plus thick gloves & hood for 45* fresh water. I'm natural sands tanks & regs with 20lbs lead. My Luxfer AL80s + Apex regs are neutral at about 400psi. This means my wing only needs to compensate for 8 to 10 lbs of air. I'm using a 2x22lb Deco wings. One for primary & one manual inflation for a contingency. 22lbs has been more then enough. I like using the AL tanks because my rig is balance with and without tanks and I would rather carry the extra weight on my harness anyway. Added bonus is redundant lift in the Caribbean with a thin wet suit. Just have to adjust the shoulder straps.

 

[sea_otter]

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.

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.

I can't speak for sidemount, but I disagree with this from a backmount perspective. Having 10 or 15 lbs of extra lift for the surface is a good thing, especially when you are floating your head, valves, and regs out of water in rough seas. Took a class recently with a lot of sitting at the surface in a rough ocean chatting. My gas (LP85 doubles + AL80) weighed about 22 lbs full. I was perfectly neutral with tanks near empty. My wing had 40 lbs of lift which worked well, but I was glad I didn't have anything smaller. Without a full wing (18 lbs extra lift) and a bit of gas in the drysuit, I would have had a lot of water splashing in my mouth.

 

[stuartv]

At about 1000 psi (following rule of thirds), the positive buoyancy to compensate for would be closer to 4 lbs in total. Also, my instructor likes to leave 200-300 psi in addition to the 1/3-rule (he's been diving caves for a long time so I'm following his recommendation) so all my practice practice dives were completed with 1200-1300 psi remaining. For cavern diving there were additional variables that came into play but are outside the scope of the OP's question and present discussion.

I may be misunderstanding what you have posted, but it sounds like you are weighting yourself to be neutral with an empty wing when you have 1000 psi in each of your tanks. Is that right? If so, what will you do if you ever happen to stay down longer than planned and end up with a deco obligation that requires you to breathe your tanks all the way down to empty before surfacing? With weighting for 1000 psi in 2 tanks, that means you would be 4 # positive before the end.

 

[bada3003]

I may be misunderstanding what you have posted, but it sounds like you are weighting yourself to be neutral with an empty wing when you have 1000 psi in each of your tanks. Is that right? If so, what will you do if you ever happen to stay down longer than planned and end up with a deco obligation that requires you to breathe your tanks all the way down to empty before surfacing? With weighting for 1000 psi in 2 tanks, that means you would be 4 # positive before the end.

This is an old thread but you make an important point. There's a lot of emphasis in recreational diving about not being overweighted which has its place. In technical diving, not being underweighted is important for the reason you mention. I rarely dive wet suit anymore, but weighting to be neutral with 1000 psi is not recommended. With a dry suit, you could open your neck seal to partially flood in an emergency but it'd be better to weight for neutral at nearly empty pressure.