johndiver999
Contributor
I think the correct answer (bottom or deco) is determined by two primary environmental factors, which are current velocity and sea state.
In reality, launching an smb from deep (for me that would be over maybe 150 ft) is actually easier than a shallow deployment and it is probably safer too. I generally use a 5-6 ft tall smb, not a lift bag or something ridiculous. When deep, you don't need to add much air, since the expansion of the gas will inflate the bag before the surface. I think that entanglement with an inflated SMB at depth is a considerable risk; it is not that likely (hopefully) but the consequences could literally be fatal (i.e., explosive decompression).
Since the stakes are kinda high here, I much prefer a situation from which I can recover from my own personal error. For example if I am 160 feet, I probably only need 10 lbs of lift worth of air in the bag at depth - a little more than a gallon. If I release and get entangled, chances are I can exhale, dump air from the BC and completely stop the ascent, grab the bag/smb, pull it down, dump it and sort out the tangle. Even if I am slow on the recovery, at considerable depth, the air in the bag is NOT going to expand quickly -in fact, the expansion in 10-15 feet, is more or less negligible. Worse case, I have time to use a knife and just cut the line and allow it to go up without me.
If you wait until you are at 30-40 feet, you probably need to put a lot more than a gallon of air in the bag and if you screw up and get tangled with it, there is a much better chance you are going for a very quick ride as the larger volume of air in the bag expands much, much quicker. This type of accident might be unrecoverable.
Perhaps I am paranoid, but I have gotten caught in a lift bag (not an smb) in 60ft and did take a quick ride to the surface, dragged by an improperly secured octopus, if I remember correctly.
So this factor alone points me in the direction of minimal inflation facilitated by a deep launch.
However, as already mentioned, the two primary factors are current and sea state. If there is a strong current like 2-3 kts, the captain has to idle into the current (and the wind) in order to hold position over, say the wreck. If the bag does not come up near the wreck, he may not see it and if there is a 4 kt current, if he is looking the wrong way for 60 seconds and the sea state is a little bumpy, then the smb and diver will very quickly be out of sight as they move down current. If the diver is delayed (or ascends early) for ANY reason at all, then the captain is further handicapped because he may not know exactly when the smb should arrive on the surface and he does not know precisely when he should stop looking for the smb to arise near the expected location and begin the search down current. In other words, if you deploy deep, then the capt should always know where the bag will come up, he just might not know when.
This is a huge problem and is compounded, if there are more than one team in the water, one may come up early and one may come up late and they can be spread out over a distance that exceeds easy visibility. This is a crappy situation, makes driving the boat a lot less fun and it can very easily result in lost divers who may be left drifting for miles until they are found.
If there is zero current and the diver were able to ascend vertically over the agreed upon location, then the smb CAN be launched shallow and it will appear on the surface where the capt. expects it to be. Regardless of whether the deployment is early or late and/or deep or shallow, the smb will appear exactly where the capt is expecting it to be.
Perhaps for additional clarification, if the diver drifts for 7-10 minutes in a screaming current and then deploys the smb when he reaches deco depth, he could be half a mile or more from the dive site.
Also, the argument that deploying the smb (on a reel) delays the ascent is a bunch of nonsense. It is not a big deal to ascend while deploying and filling the lift bag. It can be a distraction, but the process should only require about 30 seconds of the divers attention, and if you have a skilled buddy team... the two divers can discuss who will deploy and task the other diver with staying close and setting the pace of the ascent (what else does he have to do anyway)?
If you do that, it is very easy to just stay with your buddy while you fiddle with the smb and reel and inflation... and you never have to look at gauges, or the bottom or focus on particles in the water column to make sure you are not sinking or floating up while doing the 30-second deployment. If you are deep, the bc air does not expand that quick anyway, so you should not be unstable with respect to buoyancy, even if you lose focus for a few moments.
Also, being being dragged further downstream during the ascent because of the deep deployment of the smb, is not an issue .. as long as the captain sees it hit the surface and knows to follow it. Drifting off for an unknown period with no smb over your head (being invisible) is worse, not better, in terms of being picked up by the boat.
