Re: Backpacks and bouyancy


Outer Bamnks diving on the Great Escape Southern California Live-Aboard Dive Boat

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Posted by Chris on March 10, 2005 at 00:28:45:

In Reply to: Backpacks and bouyancy posted by Dick Analog on March 09, 2005 at 16:21:33:

1a) I compensate for buoyancy by varying both depth of breaths and breathing duty cycle. I do it almost without thinking. One of the things you find when removing the BC is that much of the buoyancy change you encounter is do the residual air in the BC and BC exhaust and inflator hose.
1b) You are correct that you gain buoyancy as you use air, this change in buoyancy of the tank sets an upper size limit of tanks that can be dove sans BC. Twins have too much buoyancy change and require wings. I find that with a 120 cf. tank is the biggest tank I can use.
1c) The amount of wetsuit compression varies with wetsuit material and the of dives on the wetsuit. I get a special low buoyancy material for my wetsuits so the change is minimized. Thick buoyant wetsuits tend not to work well for backpack diving, but some manage it.
1d) Yes there is a problem with shallow depths at the end of the dive. However this depends on the weight used. With my deep belt I must find a kelp stalk or anchor chain or trap float line to do my 15 ft. stop on. If that is not available I grab a “ballast rock.” Lacking that then I will do my safety stop at 20-25 ft. It is fort this very reason that a backpack sans BC is inappropriate for decompression divining.
I use a 13 lb. Belt for dive depths of 30 ft. or greater. This belt is usable up to 25 ft and down to any depth beyond.
I use a 16 lb. Belt for the 50ft. to 15ft. depth range. I can do a 15 ft. stop with this belt, yet still be just buoyant on the surface at the beginning of the dive.
For very shallow dives I add a 2 to 5 lb. weight to the cross strap on the backpack. This allows me to dive in the shallows, but does make me negative at the surface. By having the extra weight on the backpack cross strap, it can be dumped separately from the weight belt to gain surface buoyancy.
1e) I remove the scallops from the shell in place and only take the meat. The meat does not weigh much when submerged. Also if one is in need of ballast for a safety stop, a scallop can work well.
Also lobsters do not weigh much when submerged. I have had no problem doing a long surface swim with 45 lbs. of bugs, but I kept the entire bag submerged.
2) The main advantage of the backpack sans BC is the incredible mobility it offers. As Eric S. mentioned it almost as if you are free diving. When properly weighted you cover far more territory with less effort, and less air consumption than with a BC.
Even a well-designed backplate and wings still have a significant amount of drag when compared to a backpack sans wings. The backpack is also easy to don and doff when one needs to crawl into a tight spot to get a bug. I prefer the old fashioned kind without a crotch strap for just this reason. I also personally find the old fashioned backpack more comfortable than a tech backplate for single tank diving.



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