Ditchable weight

[TSandM]

Doubles present a different set of challenges. Big steel doubles in a wetsuit are problematic. If it's a thick wetsuit, you add the buoyancy loss of the suit at depth to the weight of the gas and you can run into problems swimming the gear up. If it's a thin wetsuit, it never had the buoyancy to offset the tanks from the beginning. This is one of the reasons that GUE doesn't like steel doubles with wetsuits at all.

If you have a setup where you have nothing ditchable AND you have the potential of being unmanageably negative in the event of a wing failure, you need a redundant buoyancy source. This is most easily and often accomplished by using a dry suit. The likelihood of a suit flood AND a complete wing failure (which is hard to accomplish) is small.

Note: most people new to diving doubles are head heavy and would be better off skipping the V-weight and using a belt to put whatever lead they need "south".

One of the recent "duh" moments I've had was when I realized that a tail weight put weight much further south than my weight belt does. If you are very short in the torso, a weight belt may basically just put weight at your center of gravity, and not help counter head-heaviness much at all. I found that the evil 72's that I couldn't balance, I could dive if I put a 7 lb tail weight on them, whereas heaping weight on my belt had done nothing effective.

 

[rjack321]

One of the recent "duh" moments I've had was when I realized that a tail weight put weight much further south than my weight belt does. If you are very short in the torso, a weight belt may basically just put weight at your center of gravity, and not help counter head-heaviness much at all. I found that the evil 72's that I couldn't balance, I could dive if I put a 7 lb tail weight on them, whereas heaping weight on my belt had done nothing effective.

This is why Melody has to use a tail weight. A woman's waist can be too high relative to their CG to make weight belts effective at correcting trim. On her, waist = CG so I'm not surprised you're similar. Of course in MX there's alot less lead and so it gets harder and harder to counteract the weight of the valves and crowns of AL80s with lead towards their rear.

She's diving dry here and there so having redundant buoyancy vs. ditchable lead isn't an issue.

 

[OkiMike]

Jax,

Your instructor should have pointed out that ditchable is relevant on any dive where you don't have a redundant buoyancy source in the case of wing failure and you're too heavy at depth (mostly relevant at the start of the dive when you're at your heaviest, but it could also be at any point if you're still negative).

The classic example in the slide presentation is always steel tanks and wetsuits for reasons that have already been pointed out, however, whatever your setup, if you are STILL somehow excessively negative and your wing fails and you're not wearing a drysuit (your obvious second possible source of buoyancy), you could still possibly ditch backup lights, a primary light, etc. that will at least take you a few pounds in the positive direction.

In class, the point is usually made about how you wouldn't want to have to ditch those expensive forms of weight, hence the need to ensure a balanced rig prior to diving.

But yes, your intuitions are correct: ditchable is only necessary if you are overly weighted and need to get back to your entry, but can't.