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What is the difference between a SMB and shooting Lift Bags in Scuba Diving?



I had a protege that I have been practicing with that asked me the difference in bag shooting at depth, what can be substituted for what?
So, on with the learning. . . .

Deploying a surface marker buoy (SMB) at depth.

Shoot a bag. Blow a blob. Toss a marker. These are all phrases for the same thing. That is to deploy an SMB.

First, let's distinguish the difference between a SMB and a lift bag. We will use SMB wording to include SMB which is a permanently inflated buoy and DSMB (Delayed Surface Marker Buoy) which is deployed underwater. So t
he SMB is for marking the location of something or someone. This could be marking the location of an object on the sea floor (DSMB) or marking the position of divers in the water (SMB). Lift bags are just that, they are made to lift objects. Many people use lift bags as marking devices, but as we will discuss, this is not the most optimal use for them.

Many SMBs will have different features. The biggest feature that a SMB can have is an over-pressure relief valve (OPV). The primary difference between a SMB and safety sausage is that a SMB will be deployed from depth. Whereas the safety sausage is deployed at the surface for signaling. Now we need to precise SMB and DSMB construction. SMB are sealed whereas DSMB suaully are not. Most of them are open bottomed, some are semi-sealed. As the DSMB rises from depth, the gas trapped inside will expand. As the gas expands it will stress the stitching that holds the SMB together. If there isn't some sort of mechanism to release this gas, then the expanding gas will rip apart the stitching of the DSMB and releasing all of the gas. Thus rendering the DSMB useless. So this cannot be possible for SMB.

Using lift bags for marking have many different problems. I will discuss the three of the biggest problems. First, lift bags do not stand out of the water as SMBs do (on the surface). This limits how boats/surface support can see the bag. Second, most lift bags are open at the bottom. As SMBs and bags rise to the surface, they do not go straight up. Instead the twist and turn as the gas moves/expands inside them. With the open end on lift bags, this can cause the bag to lose gas as it twists and turns on the way up. If it loses enough gas, it will sink back down or it will not provide enough lift stay at the surface. The third big problem of using lift bags for marking is that once the bag gets to the surface, a large amount of gas will cause it to ride high in the water. As it does this, if not enough tension is put on the line, then the bag can fall on it's side. This will cause the gas trapped inside the bag to escape out the open bottom. This can cause the bag to sink back down or not provide enough lift to properly mark the diver's location. SMBs are closed, that is they do not have an open bottom. Therefore, it is not possible to lose gas out of the marker.

Other features of SMBs that are important are oral/power inflate and radar reflective tape. The inflation mechanism for SMBs should be orally inflated or inflated with a drysuit/wing inflation hose. If the SMB does allow for LP hose inflation, there should not be a locking mechanism (like on BCD or dry suit inflation nipples) on the SMB inflation nipple. This way if the SMB quickly starts to ascend, it will not be locked to the diver causing a runaway ascent for the diver and the hose will just pull away from the inflation nipple.

Radar reflective tape is also important. When the Coast Guard looks for lost divers in the water (both by boat and helicopter) they will have their radar going. Radar reflective tape is a piece of tape on the top of the SMB that will reflect a radar signal back to the source. Thus showing up on the Coast Guard's radar screens. This will help the Coast Guard find the lost diver more quickly. This tape will reflect as well any torch light. Lastly, SMBs should provide enough lift such that divers cannot easily pull it back down to depth. Generally 25 lbs or more of lift is plenty.

As with many diving activities, deploying a SMB is a team effort. At a minimum, there should be at least one SMB per dive team. It is a good rule of thumb for every diver to carry a SMB. The steps to deploy a SMB as a team are below.

1) The dive leader will signal who should deploy the SMB.

2) The diver will signal to the team to watch them deploy the SMB

3) The diver shooting the SMB will pull the SMB and finger spool from their storage locations, generally in a diver's dry suit pocket.

4) If the spool isn't already attached to the SMB, the diver must attach it. Generally, most SMBs will have a small d-ring located on the bottom of them. Using the double ender of the finger spool as a weight, the diver will feed the double ender snap (with line attached) through the d-ring. Once through, the diver can pass the entire spool through loop of line at the end of the spool. Remove the double ender from the line and keep it on the right chest d-ring.

5) The diver should signal to a member of the team to look up to make sure that the SMB will not come into contact with another diver, boat, or other obstruction.

6) Once cleared to deploy, the diver should wrap up the spool, line and any access SMB material into one hand. Making sure that no materials, line, etc. are wrapped around fingers, dive gear, etc.

7) The diver should move into a slightly head down position. This makes recovering from a runaway SMB much easier as the diver will need to swim down to control the SMB.

8.) Holding the SMB and spool with the left hand, inflate the SMB till about half full. If you are orally inflating the SMB, take a normal breath, remove the regulator from your mouth with the right hand, blow a half breath into the SMB, repeat until the SMB is half full. You will want to avoid taking to deep of a breath or exhaling to much as it will change your buoyancy. If you are using a LP inflator, use the right hand to hold the LP hose and connect the SMB to the inflator hose. Pay careful attention to how fast the SMB is filling up so that you can remove the inflation hose to prevent the SMB from getting too full. I tend to avoid holding anything with the right had just in case I need to deploy the long hose in an emergency.

9) Once gas is in the SMB, hold the SMB in the left hand and the finger spool in the right hand. Verify that the knot/line attachment to the SMB (from the spool) is in place.

10) Let go of the SMB. As it rises, be careful as to not squeeze the finger spool to tightly. This could lead to a possible runaway ascent. The finger spool should gently unwind between the grasp of your fingers.

11) Once the SMB reaches the surface, extra tension should be put on the line in order to make the SMB stand straight up. To do this, wrap up the line on the finger spool 3 or 4 times.

12) You can lock the spool in place by removing the double ender snap from your right d-ring and clipping the line inside the double ender snap and through a hole in the spool. This will stop the line from unwinding from the spool.

As you move up the water column, you will need to wrap up the line on the spool till you get to the surface. Making sure you are keeping enough tension on the line to keep the SMB standing up out of the water, but not to much tension such that you pull the SMB back down. Due to the presence of line in the water. Diver awareness must be heightened to prevent any entanglements with the line. Also keep in mind that the diver who is managing the line now has only one hand free to respond to problems. So team awareness and communication is critical.

Knowing how to deploy a SMB is a critical skill for open water diving as this is a primary way to communicate with the boat/surface support. You should learn this as part of an Open Water class or Advanced Open water class. If you plan to do a lot of open water diving, such as in the Great Lakes or oceans, your proficiency with this skill must be excellent.