RECOVERY AND SALVAGE
After locating the object of your search, if it is small enough, an immediate retrieval and recovery can be initiated. It is a good idea to carry a mesh type "Goodie Bag" to assist in transporting the object to the surface. However, should the recovered object be of evidentiary value in court, special handling may be required. Larger objects will require that additional measures be taken. It is recommended that a marker buoy be attached to the object prior to beginning recovery. This will serve to accomplish two things:
1) Your surface support will be aware that you have located the object, and;
2) Should you later lose sight of the object, you will avoid repeating the search. (There have been instances wherein divers located an object, failed to mark it, lost it, and were unable to locate it again to recover it).
Plan ahead with respect to the mode of recovery to be used. Decide what you will do with the object once it is at the surface. Depending upon the size of the object, you may have to use either an apparatus to buoy the object to the surface or alternatively a surface line secured to the object. The use of a surface line is probably the safest method, however, a surface line requires the use of a watercraft capable of carrying the added weight of the object. Either block and tackle or a power winch system can be used. As a precaution, it would be best to first ascertain the weight of the object (out of water) to ensure that the lifting apparatus and it's mounts are sufficient in strength to handle the load. There are several cases on record where would-be salvers have "winched" the stern transom right off their boat, or instead of hauling an object up, have pulled the bow of their vessel underwater, which resulted in a new dive site for the wreck diver or underwater photographer.
Inflatable devices are the most commonly used and most practical method of underwater recovery. This technique involves attaching containers to the object to be raised, then filling them with air until the positive buoyancy of the container equals the negative weight of the object. The object will then rise to the surface. Although commercial lift bags and 55-gallon drums are commonly used for this type of recovery, any number of devices may be used including air spaces or inflatable compartments in the object itself. One must take care, however, to ensure all lifting devices be equipped with either a line or chain strong enough to handle a load of at least three times the lift
capacity of the device. When using a lifting device be sure to choose an appropriate one for the size of the object.
A fairly common mistake made by novice salvers is to attempt to "ride" the lifting device either during descent or ascent. This is extremely dangerous and can cause serious injury or death to the rider. During a descent, divers who attempt to ride a device to the bottom can rupture eardrums or sinuses, due to inability to equalize during the rapid descent. In addition, the diver can become entangled in the chain or line, which can cause injury or damage to diving equipment. During the ascent, an embolism could occur due to expanding air in the lifting device causing rapid acceleration toward the surface. Avoid an uncontrolled lifting ascent by bleeding air from the lift bag dump valve or "burping" the bag during ascent.
Overfilling a lift device can also be extremely dangerous. An out of control device might break the surface, rise completely out of the water, dump its air, and cause the object to descend rapidly, which could strike or pin any diver beneath the object to the bottom. It is generally better to use two smaller devices as opposed to one large one. Prior to commencement of filling the lift device(s), clear the area above and around the object. Leave a marker buoy attached to the object, in the event that during ascent, should the object "break loose" it can be easily relocated. Once the lift device begins its ascent with the object, clear the area below the object.
In order to lift an object, the size and weight of the object to be raised must first be determined. This can generally be ascertained by checking known sources. For example, if a diver is interested in raising an outboard motor, check with a local dealer, manufacturer's representative, or find a similar article and check its weight. For an "on the spot" salvage attempt, however, a different method of determining the approximate weight will need to be applied.
1) Simply make a "blind guess" at the approximate weight, and then provide extra flotation equipment to over buoy the object.
2) Calculate the weight by using a basic formula: Weight = (volume) (specific weight).
As an example: If it is known that the specific weight of iron is 490 pounds per cubic foot, and estimate that the iron object you wish to lift contains a total of 3 cubic feet of material, then ...weight = (3 cubic feet) times (490 pounds) = 1,470 pounds gross weight.
Engineering manuals are available that list the specific weights of materials; and if the approximate size of the object can be determined in terms of a cubic measurement, the approximate weight of any object can be determined. You can then concern yourself with the device(s) necessary as a lifting force.
Assume that you have located a section of steel pipe that you wish to raise, and the estimated weight of this pipe is 850 pounds. The question would now be, "How many 55-gallon fuel drums will be needed to effectively lift this pipe section to the surface?"
There is one other piece of information needed to solve this problem: the depth of water from which the pipe is to be raised. As the depth (pressure) of the water will have a direct bearing on the volume of gas required to fill the lift device to positive lifting capacity.
You find, as an example, that one (1) 71.2 cubic foot cylinder compressed at 2,475 p.s.i. (2250 + 10%) will completely fill nine and three fourths 55-gallon fuel drums at the surface. This same cylinder will only fill four and seven-eighths drums at a depth of 33 feet, (sea water). A 55-gallon drum contains 7.33 cubic feet of gas at sea level.
To calculate the volume required for lifting devices, use the following formula:
Number of gallons capacity - 7.5 (the number of gallons in a cubic foot) = cubic feet of air contained.
To determine the gas requirement "at depth", apply Boyle's Law to the cubic feet of gas required at the surface.
Now that you can calculate the amount of gas required to fill a given device, how do you determine the actual lifting capacity of that device? To begin, one cubic foot of air will lift 64.2 pounds of dead weight in sea water. A cubic foot contains approximately seven and one half (7.5) gallons. One gallon of air will lift eight and one half (8.5) pounds of dead weight in sea water. There are 1,728 cubic inches in a cubic foot.
One cubic inch of air will lift approximately four one hundredths of one pound (.0372) of dead weight in sea water. One cubic foot of air will lift 62.5 pounds of fresh water, 3 percent less.
With the 55-gallon fuel drum as an example, you can figure an approximate lift capacity of just under 420 pounds. To arrive at a lift capacity figure for any given container, you must have the following figures:
1. The liquid capacity of the container (expressed in gallons);
2. The fact that one (1) cubic foot of liquid = 7.5 gallons;
3. The specific weight of water (62.4 pounds per cubic foot for fresh water and 64.2 pounds per cubic foot for sea water);
4. The weight of your container (weight at surface).
With this information, you can now supply the following formula:
L.C. = (V) - (7.5) X (ws fluid weight) - weight in air.
In this formula, we find that L.C. = lift capacity, (V) = volume (in gallons of liquid) of the lift device, (7.5) = the figure used to convert "gallons of liquid" into cubic feet (since cubic foot is equal to 7.5 gallons), ws fluid - the specific weight of water (62.4 pounds per cubic foot for fresh water and 64.2 pounds per cubic foot for sea water) and weight in air minus the weight of our container when it is weighed empty at the surface. (The weight of the average 55-gallon drum is 53 pounds).
If you now put this formula to work, you find that L.C. = (V) gallons - (7.5) X ws fluid) minus 53 pounds. So the lift capacity of the 55-gallon drum is found to be approximately 418 pounds in sea water (408 pounds in freshwater).
Returning now to the hypothetical salvage problems of the 850 pounds section of pipe, you find that we will need two (2) 55-gallon drums, and that at a depth of approximately 33 feet in sea water you will need slightly less than the contents of one half of a standard diving cylinder (71.2 cubic feet at 2,475 p.s.i.).
When deciding on the lifting device to be used for any particular salvage attempt, one must keep in mind the fact that if the device is filled with compressed air "at depth" that volume of compressed air is going to expand as the device ascends to the surface (Boyle's Law). With this in mind, one must be certain that the expanding air can escape, or the lift device may rupture before it reaches the surface.
This "venting" or escape of air can easily be provided for by leaving the openings or plug out during the ascent, and then inserting the plug at the surface.
The reason more lifting devices are not needed is because there is a specific gravity differential between the sea water and the steel pipe of 64.2 pounds per cubic foot of steel, which must be subtracted from the weight of the steep pipe in salt water.
So assuming the steel pipe contained approximately 2 cubic feet of steel at 450?? pounds per cubic foot, on the surface it would only weigh 772 net pounds in sea water....Archimedes Principle.
A sinking object will displace its material mass or size in a liquid. The amount displaced is a working force that can be deducted from the material weight of the item in a liquid when determining the lifting force necessary to make buoyant that sunken object. But to lift the object from the sea, the lifting device used in this case should be designed to handle the surface weight of the object.
Do not partially fill a container! Fill each container totally, starting with the smallest and finishing with the largest.
Install the largest lift in the center and balance lift from both ends. Tie a bowline onto the container and a double turn and two half hitches to the item to be lifted.
Tie downs can be attached to a lift device by using thick netting, cable, or ropes tied around the outside. However, a much simpler and easier method is to weld chain to a piece of angle iron or strap iron of dimension that will allow the strap to slip into the mouth of the 2-inch bung. Insert the strap iron in the bung and it will turn crosswise in the inside of the barrel with the chain and tie-on ring extending below barrel, to be used to attach the object desired to be lifted. This method is secure and will allow barrels to tilt as necessary with very little, if any, loss of air.
The most useful knot for tie-on lines underwater is the bowline. It holds fast, and is easy to untie later.
FIGURES & FORMULAS PERTINENT TO SEARCH & SALVAGE
- A cubic foot of salt water weighs 64.2 lbs
- A cubic foot of fresh water weighs 62.4 lbs
- 1,728 cubic inches equals 1 cubic foot
- 1 Gallon weighs 8.5 lbs in seawater (indicates the lifting ability of air)
- 1 cubic foot is equal to 7 1/2 gallons of lifting capacity
- 1 gallon will lift 8 1/2 pounds
- 1 cubic inch of salt water weighs 0.0371 pounds per cubic inch
- 1 cubic inch fresh water weighs 0.0361 pounds per cubic inch
- Volume of a cube = width X height X depth
- Formula for determining the lift capacity of an inner tube in ocean:
D = Diameter
(0.7854 X D X NC) X 0.371)
NC = Neutral Circumference
- Formula for the lifting capacity of a cylinder:
L = Length
(0.7854 X D X L) X 0.0371
(Gives lift in salt water less the weight of the container).
- Formula for lifting capacity of a sphere:
(0.5236 X D ) X 0.0371
- Gallons divided by 7.5 = cubic ft Air
- 1 ea 72 cubic ft tank w/2150 PSI fills 7 ea 55 gallon drums at sea level or 2 & 1/3 drums at 100 feet
- 1 ea 71.2 cubic ft tank at 2250 PSI = 10 @ 70 degrees F. will fill 9 3/4 (55 gallon) drums @ sea level and 4 7/8 drums @ 33 feet (ocean)
- 1 ea 55 gallon drum lifts 420 lbs (less weight of drum)
- 55 gallon drum holds 7.35 cubic foot air at sea level
(Lift capacity) LD = (V) X ws fluid = (weight in air)
L.C. = Lift Capacity
V = Volume (in gals liquid of lift device)
7.5 = Gallons of liquid (cubic feet)
ws fluid = Specific weight of water (salt vs fresh)
