Buoyancy and Trim Control
When you’re drift diving, the current is your taxi. You’re not fighting it; you’re going with the flow. This makes buoyancy control the single most important skill, and your tank choice is a huge part of that. A standard aluminum 80-cubic-foot tank is negatively buoyant when empty by about 3 to 4 pounds. Now, imagine that weight shifting on your back as you breathe down the air. With a strong current pushing you, any imbalance can cause you to porpoise (move up and down) or spin, breaking your trim and turning a relaxing ride into a constant battle.
A small diving tank, like a compact 50-cubic-foot or even a 30-cubic-foot pony bottle, presents a distinct advantage here. Its reduced physical size and lower air capacity mean a smaller overall weight and a significantly lower swing weight. This makes it easier to maintain a horizontal, streamlined position. You can fine-tune your trim with less weight on your belt, and the tank’s smaller profile creates less drag in the water, allowing you to move more efficiently with the current. For longer dives where you need more gas, technical divers often opt for twin sets, but these are balanced and manifolded to act as a single unit, specifically to avoid buoyancy and trim issues.
Gas Management and Dive Planning
Drift diving often covers more ground than a typical shore dive. You might start at one point and end up a kilometer or more down-current. This directly impacts your gas planning. The rule of thumb for a standard, non-drift dive is to plan to be back on the boat or shore with a reserve of 500 PSI (about 35 bar). For drift diving, you need to be more conservative. You should plan to surface with a reserve of 700-1000 PSI (50-70 bar) to account for any unexpected delays in pickup or slight changes in current strength.
Using a smaller primary tank forces a more disciplined approach to air consumption. It makes you acutely aware of your breathing rate from the very beginning. However, the real consideration is redundancy. A small tank as a primary source is suitable for shorter, shallower drifts. For deeper or longer planned drifts, a small tank’s primary role shifts to that of a redundant pony bottle. This is a critical safety tool. If you were to have a primary regulator failure or run out of air at the furthest point from the boat, your small pony bottle is your guaranteed way to make a safe, controlled ascent and safety stop while drifting.
Here’s a quick comparison of common tank sizes and their suitability for different drift dive scenarios:
| Tank Size (Cubic Feet) | Volume (Liters) | Working Pressure (PSI/Bar) | Typical Use in Drift Diving |
|---|---|---|---|
| AL 80 | 11.1 L | 3000 / 207 | Standard primary tank for most recreational drift dives. |
| AL 63 | 8.7 L | 3000 / 207 | Good for shorter dives or divers with low air consumption. |
| AL 40 (Pony) | 5.5 L | 3000 / 207 | Ideal redundant bailout bottle for dives to 30 meters/100 feet. |
| HP 100 | 13.1 L | 3442 / 237 | High-capacity primary for longer drift dives. |
| LP 50 (Pony) | 6.9 L | 2640 / 182 | Less common, but a larger volume redundant option. |
Maneuverability and Exit Strategy
Getting in the water is usually simple: a giant stride off the boat into the current. Getting out is where the considerations for tank size get really practical. In a typical drift dive, the boat will follow the divers’ bubbles and pick them up at the end. You need to be able to surface, get the boat’s attention, and efficiently handle your gear while floating in the water. A large, heavy tank can be cumbersome when you’re trying to hand it up to a crew member or climb a ladder back onto the boat, especially if the sea is a bit choppy.
A smaller, lighter tank is significantly easier to manage during this exit phase. It’s less weight to support at the surface, making you more buoyant and stable. This is a major advantage for divers who may have less upper body strength or are diving in conditions where a quick exit is desirable. Furthermore, if for any reason you need to swim against the current briefly at the end of the dive to reach a specific pickup point, the reduced drag and weight of a smaller system is a welcome benefit.
Dive Duration and Depth Limitations
This is the trade-off. The physics are simple: a smaller tank holds less air. This directly limits your bottom time, especially as you go deeper. Your air consumption rate increases with depth because the density of the air you breathe is greater. At 30 meters (100 feet), you consume air four times faster than at the surface.
Let’s put some numbers on it. A diver with a respectable air consumption rate of 20 cubic feet per minute at the surface would have a Rock Bottom Gas Reserve for a dive to 30 meters. This is the minimum gas needed to get two divers from the deepest point of the dive to the surface safely in an emergency. For a 30-meter dive, that calculation might be around 40 cubic feet. If your primary tank is only a 50-cubic-foot cylinder, your usable gas after accounting for this safety reserve is minimal. Therefore, using a small tank as a primary source dictates a much shallower dive profile. It’s perfect for drifting over a colorful reef at 12-15 meters (40-50 feet), where you can enjoy a longer dive time without the gas constraints of a deep dive.
Equipment Configuration and Redundancy
How you rig your small tank is a key consideration. If it’s your primary tank, the configuration is standard. But if it’s a redundant pony bottle, its placement is critical for drift diving. It must be securely mounted to your main tank with a bolt-on bracket, not just bungeed loosely. In a strong current, a poorly secured tank can swing around, causing entanglement hazards or simply being a nuisance. The regulator on the pony bottle should be configured with a long hose, ideally 40 inches or so, stowed neatly. This allows you to donate it to a buddy in an out-of-air situation while still being able to manage your own ascent.
Another piece of essential gear that pairs with your tank choice is the Surface Marker Buoy (SMB) or delayed Surface Marker Buoy (dSMB). You should always deploy an SMB at the end of a drift dive to signal your position to the boat. Practicing this skill with your chosen tank configuration—whether it’s a single large tank or a main tank with a pony bottle—is non-negotiable. You need to be able to inflate and deploy the SMB while maintaining neutral buoyancy and managing your gear in the current.
Choosing the Right Tank for the Conditions
There’s no one-size-fits-all answer. Your choice depends on a clear-eyed assessment of the dive conditions and your own skills. Ask yourself these questions before you gear up:
What is the predicted current strength? A mild, gentle drift is more forgiving of a larger tank’s handling characteristics. A raging river-like current makes the streamlined benefit of a smaller system much more attractive.
What is the planned depth and bottom time? Use a dive computer or planning software to run the numbers. If your planned dive at 20 meters requires 60 cubic feet of gas and you want a 20-cubic-foot safety reserve, an 80-cubic-foot tank is the logical choice. A 50-cubic-foot tank would rule that dive out.
What is my air consumption rate? Be honest. If you’re a newer diver or naturally have a higher breathing rate, opting for a larger primary tank gives you a bigger safety buffer and more time to enjoy the dive without anxiety. A small tank is best suited for very efficient breathers on appropriately planned dives.
What is the boat crew’s preferred procedure? Some operations are set up for handling standard-sized tanks efficiently. Using an unusual, very small primary tank might slow down the process. It’s always best to discuss your gear choices with the dive guide beforehand.