Obviously, at the start of a dive, a diver wants to be negatively buoyant. That means a diver must change his volume by deflating his buoyancy compensator. Before he does anything, though, if you are in a wetsuit, you want to let water into your suit. You have to warm up that layer of water anyway and doing this at the beginning of your dive will force out any air trapped in the suit. Next, vent air from our BC’s and exhale. It is important to know where the BC dump valves are. If they are at the shoulder, the diver should start upright in the water, thereby forcing all the air to the shoulders of the BC, near the dump valves. He pulls his dump cords, holding them open long enough to vent all the air, and then as he starts to go underwater, he hugs himself to make a streamlined silhouette. Some BC’s are made with the dumps farther down the back of the vest so the diver can dump in a more horizontal position, but air can be dispelled from these BC’s even if the diver is upright, by holding the inflator hose above the head and depressing the vent at the end. As with all equipment, it is the diver’s responsibility to figure out how everything works.
After venting air from the BC and exhaling and assuming the diver’s weights are correct, he should float slowly beneath the water’s surface like a leaf, not a cannonball. At this point, only a foot or so underwater, if the diver takes in a big breath of air, he will pop right to the surface. This is where many divers start demanding added weight, which is only a mask covering poor technique. As we learned in our open water course while trying to perfect our fin pivot, good buoyancy can only be achieved with patience, so the key here is to slow down. Once a foot underwater, with his entire breath expelled, the diver should take a small breath in and then expel it as though blowing out birthday candles. To facilitate the descent, the diver should also keep his fins still. Remember that fins are powerful and if they are beneath you and in motion, they will work against the descent. I like to start with my fins pointing straight down (toes pointed), so I am like a pencil pointing to the bottom. The diver should also keep his arms at his side, or crossed in front of him, to stream line his profile and reduce drag.
As the weight of the water crushes the exposure suit and any air remaining in the BC, the rate of descent increases. This is the point where some divers are rolled onto their backs by the weight of their tanks. They start finning and flailing until they either work themselves to the surface or crash to the bottom, destroying the visibility. If the diver finds himself falling to his back, one solution is what Mike calls “Stop, drop and roll” (though the steps are not really in that order). Stretch out your legs and stop moving. Do a “log roll” so your tank is on top of you again and you are in a more horizontal position. The horizontal position will produce drag and therefore slow your descent. Then continue your “drop,” finning to the planned depth.
Another way to manage the this phase of the descent, is to start from the “pencil” orientation, then once the descent is underway, slowly bend your knees while leaning slightly forward and flair your fins wide and to the side. This creates some drag to slow the descent and gives me more stability. If you sense continued increase in descent speed, add a quick blast of air to the BC. Wait a beat or two, and if the descent is still too rapid, add another blast, and so on until you are descending at the rate you desire. The goal is to arrive at the planned depth neutrally buoyant.
The descent seems like it should be the easiest skill to master in the open water course, but to do it correctly, a diver must put some effort into perfecting it. It is really worth the effort to make your diving as enjoyable as possible. It will also go a long way to help a diver to conserve his air. As with all sports, however, the key is practice, practice, practice!
Tune in next week when I will be dissecting the ascent!