SMB as a backup depth gauge?

[dumpsterDiver]

I'm not so sure about this. As long as you and the line and the buoy just drift along, yes. But as soon as you swim against the current, the line will pay out more as the line will no longer be vertical. Interesting thought, though, thank you for bringing it up and getting me thinking about it!

Rule 7-B (c) of the offical scuba rule book clearly states, "Never swim against a current".

Actually, if you swim WITH the current the line will start to slant just as much (all things being equal, of course)

To bring this whole discussion back to reality, the knotted line would probably work pretty well in most conditions for an ascent from 40 feet on up. Wind, vertical heterogenity in the current or the dsire to swim, would make using the smb as a depth guage for anything much deeper pretty impractical for typical no-deco recreational diving.

 

[boulderjohn]

The currrent will have absolutely zero effect on the amount of line the smb will use or the angle of it (as long as the current is uniform from the surface to your depth). l

Current is rarely uniform from the surface to your depth. It generally diminishes as you get deeper. Not only that, it does not even go in exactly the same direction as you get deeper. It starts to sheer off at a fairly predictable angle. I can't remember the name for this effect right now, but I can look it up for you.

 

[boulderjohn]

Rule 7-B (c) of the offical scuba rule book clearly states, "Never swim against a current".

.

The official scuba rule book says that if you are going in one direction and back, you always start your dive into the current.

There are a couple of reasons.

The current is lighter when you are deeper at the beginning of a multi-level dive (see my last post) and swimming into it. You are also at your freshest and strongest. You will be returning at a shallower depth, when the current is stronger. You are also at the end of the dive and may be a bit more tired. Thus it is good to be going with the current when it is stronger and you are weaker.

Even if you are not doing a multi-level dive, you should swim into it when you are fresh and with it when you are tired.

 

[dumpsterDiver]

Current is rarely uniform from the surface to your depth. It generally diminishes as you get deeper. Not only that, it does not even go in exactly the same direction as you get deeper. It starts to sheer off at a fairly predictable angle. I can't remember the name for this effect right now, but I can look it up for you.

Possibly you are thinking about the Eckman Spiral effect, which is caused ONLY by a surface wind blowing for a long time. I don't think Coriolis Effect is going to do a whole lot, under most dive conditions at depths of 30 feet or so. I've spent dozens of hours doing freedrifting deco hangs with and without an SMB in the ocean, sometimes when the current is over 3 kts at the surface. At these shallow depths, the primary effect is the wind blowing directly on the SMB.

Tidal and wind driven currents can be very different from the surface to depth, especially when there is a thermocline, that is why I made sure to specify assuming a homogeneous current velocity distribution. My point was just because you are drifting in a current does not mean the SMB will expereince a signiifcantly different currentt regime as a diver at 30 feet or so.

 

[boulderjohn]

Possibly you are thinking about the Eckman Spiral effect, which is caused ONLY by a surface wind blowing for a long time. .

Yes, and the corresponding Eckman transport.

And many currents are indeed driven by prevailing winds.