The Air Lift is often employed to work a shipwreck site, especially where mud, sand, gravel and small stones need to be cleared away, or where visibility or other conditions dictate that the sediment be lifted to the surface for screening. This is especially useful when the items being sought cannot be detected with any sort of detector. A good example is the “Emerald City” cite of the Atocha. Air lifts are used to bring the bottom sediment to the surface vessel, where it passes through various screens, and then is visually inspected by people with a trained eye for spotting emeralds.
Air lifts come in two types; both work on the same principle. The simplest type acts like a vacuum cleaner, sucking up the bottom sediment, sand, shells, small rocks, and sometimes small treasure artifacts. It has a discharge tube, which can be as short as ten feet or may be much longer, with or without a flexible section at the discharge end. The debris is spewed back into the ocean at the discharge end of the tube and the current carries away the fine particles and sand. Generally, two or three divers work this type of airlift, one manning the airlift tube, one watching for artifacts where the airlift is being utilized and removing those items too large to enter the airlift, and one diver at the discharge end of the tube watching for coins or other artifacts that got sucked up by the airlift. Implementation of this type of airlift requires some visibility to be useful and productive.
In the absence of visibility or in other conditions that warrant lifting the “vacuumed” sediment to the surface for examination, a more complicated airlift may be utilized. This implementation requires more sophisticated positioning and anchoring of the suface vessel since the discharge tube will carry the sediment to the top and actually out of the water and onto the vessel (usually into a mesh basket or screening mechanism). The discharge tube is generally constructed out of a combination of rigid and flexible sections of tubing. This type of airlift has limitations on the amount of movement of the surface vessel that can be tolerated without disrupting the airlift operation. Usually, some type of small anchor blocks are required to keep the discharge tube from being swept away by the current. So, the airlift with topside discharge is more complicated and more expensive to implement and use, but both type work on the same principle as explained below.
Air lifts generally work well where there is 30 feet or more of water. They can be utilized in as little as 15 feet, but their suction ability is greatly diminished in the shallower depths. Air lifts work on the principle that compressed air released under water will rise towards the surface, and while rising towards the surface, the water pressure decreases and the volume of air increases as it expands. An air compressor on the surface vessel pumps compressed air down to the airlift “vacuum tube” (which may be any tube with a diameter of 2 inches to 12 or more inches depending on the situation. A standard airlift tube is generally 4 to 6 inches in diameter). The compressed air is fed into the airlift tube near the bottom and immediately begins to rise towards the surface, expanding and filling the tube as it does. This creates a vacuum effect at the bottom of the airlift tube, and sand, rocks, and other small items in the sediment are lifted along with the air and water in the tube. The deeper the airlift is implemented, the stronger the suction because the air is rising more distance and expanding more as it does. Divers at 60 to 90 feet of depth have reported airlifts sucking up 12 to 16 pound cannonballs with no problem. As explained above, this suction force is enough to lift the bottom sediment up out of the water and deposit it on a boat.
The diagrams below illustrate the two types of airlifts.
Drawing by Jennifer Gunn ©WUAG (modified by William K. Seliger)
Drawing by Jennifer Gunn ©WUAG