Posted by MHK on September 26, 2000 at 09:24:25:
In Reply to: DIR Question posted by Gerry Smith on September 25, 2000 at 17:46:47:
Posted by Gerry Smith on September 25, 2000 at 17:46:47:
I want to understand why DIR calls for a ss backplate in open water diving. I've always felt that to maximize ditchable weight was a good thing. I see the difference with an OE dive, but I don't see how it applies once you have direct access to the surface.
Let me see if I can explain it. DIR suggests that you pay particular attention to your weighting package taken as a whole. By that we mean you should wear no more or less than necessary and then it should be distrubuted to afford the best possible trim and to allow for enough to be ditched in an emergency, but provide for some stabilization so as to prevent a rapid ascent to the surface if you need to ditch.
So for example, and I'll use my personal rig as a way of example, I'm 6'2' and weigh 220 lbs. I generally use twin AL 80's ( which each tank swings from 4 lbs positive to 3 lbs negative when depleted ). I have found that in my desire to keep proper trim that it is better to distrubute the weight as opposed to having it all on my hips. With it all on the hips the natural tendancy then would be to have the lower portion of your body lag and therefore increase the drag.
I therefore use a SS BP ( which is 6lbs ), I use a 8 lb V-weight, and then my light cannister ( 5 lbs ), if I need to ditch I will drop the cannister and still be able to surface but not rapidly ascend. Terry, Troy and myself have all done this from 100'+. We tested this over a period of dives wherein we each handed off our cannister and then ascended simulating a failed bc. Each of us has no problem doing a controlled ascent without using our bc..
What you are trying to accomplish is to weight yourself for the lightest point of your dive, which should be when you tanks are nearly empty and you want a 10' or 15' stop. Conversly, you want sufficient lift capacity to lift yourslef at the heaviest point of your dive , which is when you first descend and your tanks are full..
The SS BP allows us to evenly distribute the weight, stabilize your trim and prevent a rapid ascent if you need to ditch. Bear in mind that if you are in the Carribean or some like environment, you could also use a AL BP which weighs 1 lb..
Hopefully this helps..
Regards