Learning Curve For Doubles

[Gilldiver]

As others have said, a lot will depend on your size and how you have them rigged. One thing I would suggest is that you put everything on them, BC, regs, just everything that you normally use and put them into the pool or shallow confined area with full tanks.

Use a fish scale and see just how negative the full rig is – write it down.

Now swim around with the full tanks, do head down, head up, rolls, back flips, just do everything that you have ever done with a single, get used to the full tanks.

Now go back to the shallow area, take them off, and blow the tanks down to 250 to 500 psi – whatever you will feel is the minimum you want to let the set get to in normal diving and see how they float. I bet they will be just about neutral but with a pretty good righting moment trying to lift the bottom of the tanks. The rig will just want to float bottom up.

I am 6'2" and over 250 pounds now and my 72's can still move me around on a DECO stop if I let them.

Now try strapping a 2 or 3# weight between the tanks as far down as you can, long wire ties work well. The idea is to get the tanks as close to neutral but floating horizontal and a 3# weight should do it Use the fish scale to weigh the trimmed out rig again. Write it down.

Now:
1) Add the amount of trim weight to the first measurement for full tanks. This is you trimmed rigs start weight.

2) Subtract the trimmed weight for empty tanks form #1 and you now have the exact swing weight for your set, with your regs, and any other toys you have on your system.

You should now have enough air left in the tanks to run through all of the positions you did with full tanks and see what the difference is.

You now know:

How negative your are at the start (Trimmed Start weight)
How negative/positive you will be at the end (Trimmed end weight)
The amount of swing between the start and finish

You also should have a good idea of how the tanks will move you and how you will move them at both ends of the spectrum. Steel 72's are easy and you should find that there is very little adjustment between a trimmed set and a single.

The final thing that you will have to find out for yourself is how much they will move you around getting into and out of the water. You have at least twice the mass on your back now and it can be a challenge on a rocking boat or surf entry.

 

[mwilding]

I just dropped of my regs for service and the guy at the LDS told me about the DSAT intro to tech class which is basically a how to dive with doubles class. Since I have been out of the water so long (4 years since my last drysuit dive), I think I am going to take the class. I should come out of it with my gear sorted and the with a fair ability to manage my trim and bouyancy with the gear.

Without the class, I'd have to find a buddy willing to dive with me while I flounder around getting everything worked out.

Thanks for the input all!

 

[String]

Valve drills can be harder with drysuits/thick undersuits and thick gloves but a lot of that is down to size of suit and so on.

Most technical instructors will offer an "intro to twinsets" course where they'll do things like harness setup, valves, trim and so on to get you started as well and its worth having one of those if you have problems. In fact most recommend you take that course BEFORE buying so you know exactly WHAT to buy.

Regarding weighting - id recommend weighting to be able to hold a stop at 3m with completely empty tanks (assuming a worse case scenario. The last thing you want in a bad moment is to finish that nightmare by being too light to stop). You can do this weight check with a known quantity of gas in the tank and simply look up how much that amount of air weighs and add that to the total needed.

1 litre of air at sea level density weighs about 1.2 grams.

If i have 50 bar in my twin 12l tanks that means i have 50 x 24 = 1200 litres of air in that tank.
So if its 1.2g per litre i have 1.2 * 1200 = 1440grams. Thats 1.44kg.

So if you weight yourself to be neutral with not much gas in the suit/wing at 3m with 50 bar in your tanks, add 1.44kg to that figure and thats the correct weighting you need for those tanks.

 

[BlueDevil]

As others have said, a lot will depend on your size and how you have them rigged. One thing I would suggest is that you put everything on them, BC, regs, just everything that you normally use and put them into the pool or shallow confined area with full tanks.

Use a fish scale and see just how negative the full rig is – write it down.

Now swim around with the full tanks, do head down, head up, rolls, back flips, just do everything that you have ever done with a single, get used to the full tanks.

Now go back to the shallow area, take them off, and blow the tanks down to 250 to 500 psi – whatever you will feel is the minimum you want to let the set get to in normal diving and see how they float. I bet they will be just about neutral but with a pretty good righting moment trying to lift the bottom of the tanks. The rig will just want to float bottom up.

I am 6'2" and over 250 pounds now and my 72's can still move me around on a DECO stop if I let them.

Now try strapping a 2 or 3# weight between the tanks as far down as you can, long wire ties work well. The idea is to get the tanks as close to neutral but floating horizontal and a 3# weight should do it Use the fish scale to weigh the trimmed out rig again. Write it down.

Now:
1) Add the amount of trim weight to the first measurement for full tanks. This is you trimmed rigs start weight.

2) Subtract the trimmed weight for empty tanks form #1 and you now have the exact swing weight for your set, with your regs, and any other toys you have on your system.

You should now have enough air left in the tanks to run through all of the positions you did with full tanks and see what the difference is.

You now know:

How negative your are at the start (Trimmed Start weight)
How negative/positive you will be at the end (Trimmed end weight)
The amount of swing between the start and finish

You also should have a good idea of how the tanks will move you and how you will move them at both ends of the spectrum. Steel 72's are easy and you should find that there is very little adjustment between a trimmed set and a single.

The final thing that you will have to find out for yourself is how much they will move you around getting into and out of the water. You have at least twice the mass on your back now and it can be a challenge on a rocking boat or surf entry.

Sounds overly complicated to me!

I just went down to my local shallow pier, strapped on the doubles and jumped in. Had my wife on the pier to hand me extra weights if need be. Only took a few minutes to work out roughly how much weight I needed, then fine tuned it a bit over the next few dives I did. Maybe I was lucky - I never have had any trim issues with my doubles, they just worked fine straight away.

 

[Walter]

They are easier than a single tank.