Search and Recovery, SMBs and Night Diving — PADI IDC and Divemaster Theory

search-and-recovery-night-diving

Watch Will Welbourn explain search patterns, lift bag limits, the differences between SMB types, DSMB safety rules, and night diving signals and procedures.

Distance Estimation

Estimating distance underwater is most commonly needed when mapping a dive site. The level of accuracy required depends on the situation — a small site with poor visibility demands more precision than a large site where landmarks matter more than exact measurements.

Methods — least to most accurate

Method How it works Limitation
Kick cycles Count kick cycles over a known distance (usually 100 ft), then use that personal rate to estimate distance. Swim the known distance twice and average the result. Use your normal kicking style — frog kick, scissor kick, whatever you actually dive with. Current significantly affects accuracy — more kicks into a current, fewer with it
Elapsed time Note how long it takes to swim a known distance. Use that time-per-distance ratio to estimate distances on the dive. Affected by current in the same way as kick cycles
Tank pressure Measure air consumed over a known distance at a known depth. Extrapolate to estimate distance from air used. Least favoured method — many variables affect consumption
Arm spans Physically measure distance arm-span by arm-span. One arm span = a fixed, known distance regardless of current, depth, or equipment. Slow and cumbersome — only practical for small areas where accuracy is critical
Most accurate method Arm spans are the most accurate distance estimation method because they are unaffected by current. Use them when you are searching for a small, high-value object and accuracy is critical. For general dive site mapping, kick cycles are the most practical choice.

Search Patterns

The right search pattern depends on two main factors: the size of the object and the size of the search area. Bottom composition and visibility also influence the choice.

Before you search — historical or archaeological objects If you find something that may be of historical or archaeological significance, do not remove it. Note its location, photograph or video it if possible, and report it to the local authorities. Archaeologists collect extensive data about position, orientation, and surrounding sediment as part of any removal — information that is lost the moment an object is moved.
Search pattern Best used when Limitation
U-shaped (swim) Large search area, obstructed bottom (boulders, coral, kelp), larger object Less methodical — gaps in coverage are possible
Expanding square Smaller search area, good idea of where the object likely is — start the square there Less methodical — gaps in coverage are possible
Jackstay Small object, clear unobstructed bottom, high accuracy required Requires reels and lines — impossible on obstructed bottoms
Expanding circle Small object, open bottom, high accuracy required Requires reels and lines — obstructions such as dock pylons make it impossible
Decision rule Large object, large or obstructed area → U-shaped or expanding square. Small object, small unobstructed area → jackstay or expanding circle.

Lift Bags

For light objects, you can simply add air to your BCD to compensate for the added weight and bring the object up. But there is a clear limit to when this becomes dangerous.

Object weight Action
Up to 7 kg / 15 lb Can be lifted using BCD air — compensate buoyancy normally
7–45 kg / 15–100 lb Use a lift bag — recreational diver limit
Over 45 kg / 100 lb Do not attempt as a recreational diver
Memory anchor — the weight belt rule Anything heavier than your average weight belt (roughly 15 lb / 7 kg) needs a lift bag. If you drop an object that heavy mid-ascent without a lift bag, the sudden loss of weight combined with air already in your BCD could cause a fast uncontrolled ascent.

SMBs, DSMBs — Types and When to Use Each

Open-bottomed SMB

Inflated by directing exhaled air or alternate air source into the open bottom. Must be kept upright to retain air. Difficult to keep inflated in rough conditions. Not designed to be released from depth — if it surfaces while the bottom is above water, the air will spill out.

Oral / LPI inflate SMB

Inflated orally or via the BCD low-pressure inflator. A one-way nipple valve retains the air. Deflated by pressing the nipple and rolling the bag. Not designed to be released from depth — no overpressure release valve means the expanding air on ascent can burst the seams, weakening the bag permanently.

DSMB — Delayed Surface Marker Buoy

Specifically designed to be released from depth. Has a one-way valve on both the open bottom and oral inflate port, plus an overpressure release valve that vents expanding air safely as the bag ascends. Most useful when diving in areas with heavy boat traffic that does not respect dive flags.

Can be released during a safety stop or, better, as you begin ascending toward safety stop depth.

DSMB safety — slack line is the critical hazard When releasing a DSMB from depth, you are connected to it via a reel. If the line goes slack it can wrap around your equipment and drag you to the surface with the bag. Keep tension on the line at all times throughout the release and ascent.

Do not release a DSMB as a buddy team. Surge or movement between buddies will inevitably create a slack line at some point. One person handles everything — inflating the bag, holding the reel, maintaining line tension.

Night Diving

Lights — what to carry

  • Primary light — your main torch
  • Backup light — carried in case the primary fails or is dropped
  • Marker light — a glow stick or similar attached to your tank, so other divers and the surface can see you
If you drop your primary light Get out your backup light first — before doing anything else, including swimming down to retrieve the dropped light. Without a light, you do not know what you are swimming into. Once your backup is on, then retrieve the dropped light if it is safe to do so.

Light signals at night

Signal Meaning
Circular motion with torch OK
Rapid side-to-side motion Distress
Slow up-and-down motion Attention — "come and look at this" (not distress)

For hand signals at night, point your torch at your hands so your buddy can see them — standard signals remain the same, they just need to be illuminated.