I've done a lot of Blue Water diving and I'd be glad to answer any questions.
John Henie's book (Sea Grant) is a good start.
If memory serves there was a single incident, with recreationaly trained divers, where there were two on the rig, with a badly tied off downline that was heavily weighted and their tethers attached with snap clips to their weight belt (one case) and BC (another diver). The down line parted from the float and started to drag them down. One diver dropped his weightbelt making things much worse for the second diver who was unable to release the snap clip that was now being pulled very hard by about seventy total lbs. of lead. You only need a few lbs. for the down line and you should always use the tether system described in the book with the ring and spinnaker shackle.
We used the anchor line, double-tied to two bow cleats, and jon lines with SS carabiners clipped to the chest D ring and around the 1/2" nylon anchor rope. This worked, but was not ideal because carabiners on the rope could interfere with each other and the ones on the divers' D-ring would have been hard to unclip under a lot of tension.
Mike, why would you want to maintain a constant depth? What I found fun was going up and down watching the critters. To me, maintaining an appropriate depth range was fine -- just like on a wall where I'll go up and down within a set range. My depth gauge is sufficient for that.
Ummm, maybe I meant "Maintain a DESIRED depth range." As opposed to floating or sinking uncontrollably, or as Thal might call it, "Faith-Based Buoyancy Control".
Your individual tether is typically 10m long, that gives you a sphere 10m in radius around the pivot diver that you can work. Typically one will observe/photograph/collect one organism and then move onto another within that sphere. The pivot diver can change where the trapeze clips to the downline as needed.
Mike, do you have your depth gauge on your wrist? That helps a lot. We do a lot of practice of holding precise depth in midwater as training for staged decompression dives. What I have found is that, if I have NO visual reference (buddy, upline, or structure) and I begin to watch something in the water, I will be very likely to go where it goes, whether it ascends or descends. Because I have my gauge on my wrist, I don't get very far before the deviation gets corrected, but honestly, I would find an hour spent in entirely blue water to be extremely fatiguing.
When we did the black water dive off the Big Island, we were on tethers. Given that we were more than a mile offshore, that currents are possible there, and that the surface water wasn't very flat, I was glad of the tethers. I wasn't worried about getting excessively deep, but I was worried about getting separated from the boat.
Wrist mounted gauges would be a real help. We use Suunto Cobra console-mounted AI gauges on retractors for our normal rec diving. Great idea!
The tether system that we use provides a visual depth reference (the angle of the tether line going back to the trapeze) as well as a secure method that prevents going too deep or too shallow or getting separated from the boat. I've seen the single tether system that the folks here use and I do not care for it, it may work fine if you're out there for the experience of seeing the environment, but if you need to get some work done, I'll stick to our system.
Yes, the tether did give a reference, but we had sliding tethers, so the reference was ....variable. I can see the benefits of having a trapeze, but we would have to manage it ourselves because we wouldn't have a helper to do that. In our "system", we were prevented from going too deep or too shallow by the fact that it evidently just wasn't our day to die.
I love love love blue water diving! You find the most amazing critters under the flotsam and the water, even here in SoCal can have amazing visibility. To me, the key to having a safe and fun trip is good top-side support. Someone who is on constant bubble watch and ready to alert the divers as soon as conditions or or other circumstances change - and they can do so quickly out there. I like a single down line per dive pair with individual 20' lines clipped to it. Each diver MUST have good buoyancy skills and a SMB. Vertigo can creep in to new divers not accustom to diving where they can not reference something so new divers should always be buddied to an experienced blue water diver. Now, all I have to do is convince my wife to experience it. She has zero desire to dive blue water.:depressed:
Topside support is great if you can get it. We were fortunate.
My wife was apprehensive all night and all morning before the dive. She was tearing her nails, a symptom of high anxiety for her. But when we got to the kelp paddy and she saw the Mola Molas underneath, she pulled on her gear and was over the side before the other two divers could get our T-shirts off!
Just use the microscopic floating particles in the water as a reference. They dont move much up and down over a short time scale so relatively easy to use as a depth reference. You dont need a fixed depth reference diver or anything. Allow yourself a depth range as well.
Disorientating at first but you soon get used to it.
We were looking at Jellies and Salps, so we sort of had a reference, but there are a contnuous stream of critters coming at you, above and below.
A question, if I might, if you are tethered what use is an SMB?
Conditions were almost glassy calm, flat, little wind, gentle current. Our Photographer friend had a SMB with a finger spool, we did not. We did some free swimming under the paddy and he did a lot of free swimming and only tethered to the line towards the end of the dive.
We had friends on the boat to keep an eye on things. Still, in the open ocean, had anyone become separated from the boat, an SMB could have been a life-saver. I think in future we will carry SMBs as well.
