Weight between double and singles on and off the boat

[Unbalanced]

Apologies for going over an old topic but I cannot find an explanation for someone (like) me that is a reasonably experienced recreational diver interested in adding a redundant air source - that isn't a weightlifter and diving in temperate waters.

The weight of the gear is becoming a factor out of the water and this is important for me to understand upfront (I'm not young).

I have asked quite a few folk with doubles what they assess the weight difference as - I am becoming sure that this advice is biased with people liking benefits for doubles so much they are perfectly ok to accept the net weight difference - and they are strong. Doubles have a lot of advantages.

I am worried about getting back on the boat - I find a single on the ladder not easy. I want to know how much more weight I will lug up the ladder.

I need to carry enough weight so that I am not buoyant near the surface with tanks (single or doubles) under the reserve ('ish).

Neutral Buoyancy Near the End of the Second Dive (for doubles)

Assuming the exposure suit is the same - my calculations to understand the difference between a single and doubles are:

With a 12.2 litre (100 cu ft) steel single - this is 13.2 kg empty (29 lb). Empty it has 1.1kg (2.4 lb) positive buoyancy.

A reserve of air is around 0.6 kg (1.3 lb). So the steel single is say 0.5 kg (1 lb) boyant with some reserve air.

So if I carry 2 singles as doubles - at the end of the dive I would need to take a little more overall weight to run double tanks

I currently carry around 12kg (26.5 lb) of lead weight - with doubles some would shift to the backplate and extra regs. But it looks like I would need to keep the same overall weight. People talking about reduce weight with doubles I think are talking about taking it from their weight belt to the backplate etc - not a reduction overall.

Out of the water - full a single tank is 13.2 kg + 3.37 kg (air) = 16.6 kg (approx) (36.6 lb). So that is the extra weight up the ladder after the first dive (not exact and not all the tank - but overall) (tank buoyancy doesn't make this intuitively obvious).

Do I have this correct ?

An option for redundancy would be to carry a secondary cylinder say 5.7 litre (40 cf) - this would weigh around 8kg full (17.6 lb) (around neutral when full). So overall another 8kg of weight (15 lb). So around half that of the double. If I want to get into technical diving I feel a gym course coming on

UB

 

[Nemrod]

Two tanks, doubles, will increase the weight considerably. Obviously you will double the weight of the cylinder and the manifold is heavy itself and then double regulators. Ancillary equipment will contribute. I know nothing of side mount except that waiting for my buddy who is side mount is painful even if it is not for him.

If you are having trouble mounting a ladder with a single then prepare for double the trouble with twins or maybe take the gear off and let paid help hoist it aboard or make your buddies suffer while you fritz around with side mount gear.

 

[Rilelen]

I have trouble climbing ladders in a single tank, esp in rough seas. I do not think I could get my BM doubles up a ladder in sporty conditions.

If you have a friend willing to let you, take their kit down to a pool and see if you can climb up the ladder with it. On the plus side: I wear absolutely no weight in doubles (and am in fact heavily overweighted the entire dive by tanks/plate/regs/bands/manifold etc).

 

[rx7diver]

Likely, you will strap (filled) back-mounted doubles on your back (maybe in the parking lot), stand up in them, walk in them down the pier, step off the pier onto the boat, back up to the bench, and sit down in them. You probably won't have your regulators attached, and you probably won't be wearing your weight belt when you do this.

For the dive, wearing your exposure suit, you will attach your regulators and ancillary gear to your doubles, sit down on the bench, backed up to your doubles, strap into them, and stand up in them. Depending, you might have your fins on, or you might have them in your hands. Likely, you will walk/waddle (wearing your doubles) to the "gate", don your weight belt, don your fins (if you're not already wearing them), don your mask after rinsing it, and giant-stride or back-roll off the boat into the water.

After you've surfaced, you will likely climb the stern ladder (either fins on or fins off, depending) still wearing your doubles, remove and hand your fins to the deck support person, remove and hand your weight belt to the deck support person, and walk/waddle back to your seat, where you will back up to the bench, sit down, and unstrap.

Whenever standing in your kit, you will not only be managing its full weight (as an example, full double Al 80's with regs attached weigh ~80 lbs, maybe), but you also will be balancing this mass several feet above the deck of a swaying/pitching, anchored/moored boat.

If you think you might have difficulty with all this, you might consider using, instead, a back-mounted single and taking a small pony bottle with.

I hope this helps.
rx7diver

 

[stepfen]

Welcome to scubaboard @Unbalanced

Empty it has 1.1kg (2.4 lb) positive buoyancy.

Are you sure about this?? 1kg positive when empty seems strange for a steel tank. Usually they are negative (even without the valve).

 

[jborg]

It seems roughly reasonable, excluding valve. We can calculate the buoyancy of the tank itself since we know it's weight, composition, and internal volume.

Steel weighs about 8 kg per L. For a 13.2 kg tank, we get a steel volume of 13.2 / 8 = 1.65 liters of steel. That means the external volume of the tank is 12.2 L + 1.65 L = 13.85 L. Water weighs about 1.02 kg per L, so total displaced mass of water is 13.85 * 1.02 = 14.1 kg. Assuming the tank is indeed empty we get a buoyancy of 14.1 - 13.3 = 0.9 kg (positive). Then comes the valves and whatnot, of course. All in all I guess it should be about neutral with valve, and a bit negative at minimum gas.

(My 12x232s are a little heavier than this and indeed slightly negative even when empty. I guess this is an American low-pressure tank, 12.2x200?, "HP" notwithstanding.)

In any case, double 12s are heavy out of the water

 

[stepfen]

It seems roughly reasonable, excluding valve. We can calculate the buoyancy of the tank itself since we know it's weight, composition, and internal volume.

Steel weighs about 8 kg per L. For a 13.2 kg tank, we get a steel volume of 13.2 / 8 = 1.65 liters of steel. That means the external volume of the tank is 12.2 L + 1.65 L = 13.85 L. Water weighs about 1.02 kg per L, so total displaced mass of water is 13.85 * 1.02 = 14.1 kg. Assuming the tank is indeed empty we get a buoyancy of 14.1 - 13.3 = 0.9 kg (positive). Then comes the valves and whatnot, of course. All in all I guess it should be about neutral with valve, and a bit negative at minimum gas.

(My 12x232s are a little heavier than this and indeed slightly negative even when empty. I guess this is an American low-pressure tank, 12.2x200?, "HP" notwithstanding.)

In any case, double 12s are heavy out of the water

Indeed it seems possible with a low pressure steel tank to be slightly positive when empty (sorry - I forgot low pressure tanks even exist, as they are very uncommon where I dive).

OP's main concern though is clearly the (roughly) doubling of the tank weight. Not the ~1-2kg extra ballast he might need.

That's one of the main reasons I don't do doubles, although I am still (relatively) young. Instead, I sling a 3lt or 7lt pony tank (aluminum) when redundancy is needed (solo rec diving down to 30m/100ft).

 

[Wibble]

Yes, a twinset/doubles is heavier than a single tank.

However, due to the additional weight you will adopt different techniques when moving them around than with a single tank, you'll ensure you put it at sitting height as you simply cannot don the twinset from the floor unless you like emulating a turtle on its back.

Strangely, when I used singles I used to get a lot of back pain. Moving to twinsets forced me to lift them properly and think about how to handle it before putting them down. I never got back pain again!

Getting back onto a boat with a twinset is heavier, but you get used to the techniques required. Walking around with a twinset on your back is not difficult as you're upright. Arguably a twinset fits better as you will have a backplate and harness that holds the weight closer to your back and prevents it from moving around.

The size of a twinset matters.
Two 12 litre steel cylinders (no idea what US size that is) are "standard" twins. This will enable you to do two 'normal' dives without a fill.
You can dive with a pair of 7 litre cylinders which would give you the gas redundancy and will only be a "bit more" heavy (second regulator, manifold) than a single 15 litre steel. You would need a gas top-up between successive dives though.
10 litre cylinders are the compromise. Lighter than 2x12s but a lot more gas than 2x7s. Probably enough gas for two dives but you'd need to be careful.

Don't forget that when you move to a twinset you will need additional skills for knob twiddling drills. Also setting up your kit. Could easily do that in a single day as a mentoring session with a skilled twinset diver, or you could do it as a "intro to tech" course.

You will need to be able to twiddle the knobs behind your head -- shutdown drills. You must understand how pressures work (e.g. why did my pressure not drop over the past half hour).

Aside from that, it's a better way to dive opening up decompression diving so longer and a little deeper.

 

[Lostdiver71]

I have not been able to find any steel hp100’s that are positively buoyant when empty with a valve. The closest you get is a Faber that is .59 pounds negative and remember they will never be completely empty. You are also looking at an additional 34-45 pounds to lift with doubles before you fill the tanks. I would go with the 5.7 liter pony bottle which you can hand up to the boat crew at the end of a dive unless you are getting into technical diving.

 

[happy-diver]

Yeah my single tank Faber 125s with a little extra pumped in weigh 24kg = 53lbs
Yeah my manifolded Faber 55s, with a little extra pumped in weigh 24kg = 53lbs

The twin 55s are far more manageable on land than the much more bigger ones

I find that a good fitting quality wetsuit assists in firming me up for the big climb
stops my legs from exploding not those that become see through when bending