Both devices hold line, but they solve different problems. A spool is compact, mechanically simple, easy to store in a pocket, and well suited to the amount of line needed to deploy a DSMB. A reel adds a handle and a winding mechanism that become useful when a diver needs to manage much more line or repeatedly lay and retrieve a guideline. Those features are valuable when the dive requires them, but they add little to an ordinary recreational DSMB deployment.
What a Spool Actually Does
A finger spool is little more than line wrapped around a compact center section. Depending on the design, you hold the outside of the spool, use the center opening, or use another gripping surface while allowing the line to unwind.
Your hand provides the tension and controls how quickly the spool turns. Afterward, you wind the line back onto it by hand. There is no crank, brake, ratchet, axle mechanism, or locking system required for the basic design.
During a DSMB deployment, the spool needs to allow line to leave cleanly while you maintain your depth, buoyancy, and awareness of what is happening around you. Removing mechanical parts also removes several ways for the device to jam.
A spool can still be mishandled. Loose line can wrap around equipment or a diver, an overfilled spool can shed line poorly, and careless winding can create a mess for the next deployment. Those are line-management problems. Adding a more complicated mechanism does not solve them.
Why I Prefer a Spool for DSMB Deployment
Deploying a DSMB already requires you to manage buoyancy and depth, inflate and release the buoy, control the line, remain aware of your buddy, and usually begin or prepare for an ascent. A spool handles the line without adding another mechanism to manage at the same time.
It also disappears into a pocket. Mine stays attached to the DSMB, so the two pieces I need are already together when I pull them out. I do not have a separate reel hanging from a D-ring or another relatively bulky piece of equipment outside my normal configuration.
The lack of a mechanical lock is useful during deployment. If the line fouls or something else goes wrong while the DSMB is ascending, I want to be able to release the spool. I do not want an inflated DSMB connected to me through a line and a device that has decided not to turn.
Keep the line away from your body and equipment, avoid allowing large loops of slack to develop, and never clip the spool to yourself while the DSMB is ascending. I also carry a cutting device somewhere I can reach it easily.
When a Reel Makes Sense
A reel becomes useful when the line itself is an important part of the dive rather than simply the connection between you and a DSMB.
Cave and wreck penetration are obvious examples. A primary guideline may need to cover substantial distance, remain under controlled tension, pass through restrictions and around placements, and later be retrieved while the team exits. Search patterns, surveying, locating objects, and other specialized work can create similar requirements.
A handle and winding mechanism help when you are managing hundreds of feet of line and expect to wind significant amounts of it back onto the device. The additional capacity and mechanical advantage are doing useful work.
A recreational diver deploying a DSMB does not have that problem. Buying a reel for that job gives you a larger piece of equipment with more mechanical complexity without providing a meaningful advantage.
Buy a 100-Foot Spool
For recreational diving, I recommend a 100-foot spool.
Most recreational divers who deploy a DSMB underwater are going to do it during the final part of the ascent, very often at or around the safety stop. A 100-foot spool provides considerably more line than those deployments require and still gives you room for current, horizontal separation, and imperfect positioning beneath the buoy.
DAN has recommended carrying at least 50 percent more line than the depth from which the DSMB will be deployed. Even using that conservative approach, 100 feet gives a recreational diver plenty of useful deployment depth.
There is very little reason to make your primary DSMB spool larger. More line means a larger spool, more line to manage, and more line to rewind after the dive.
If you have a specific reason to deploy from significantly deeper water, buy an additional longer spool for that purpose. That is a specialized requirement rather than something every recreational diver needs to accommodate with the spool attached to their everyday DSMB.
For normal recreational diving, 100 feet is enough.
Size and Handling Matter
The diameter of the spool, shape of the edges, center opening, and amount of line wound onto it affect how easily it can be used, especially with gloves.
A very small spool may feel fine with bare hands in warm water and become irritating with thick wet gloves or dry gloves. A larger center opening or an added handle can make cold-water operation easier. A spool wound completely to its outer edge can also leave less room for your fingers and make clean line control more difficult.
Choose a spool that works with the exposure protection you actually dive in. I care more about whether I can hold and wind it cleanly in my normal gloves than whether it has the absolute smallest dimensions.
Bright line is also easier to manage around your hands and equipment, particularly in darker water. I do not care much about a specific line color as long as I can see it easily.
Buy Delrin, Not Aluminum
I prefer Delrin because it solves the spool problem extremely well. It is durable, corrosion resistant, inexpensive, and far less negatively buoyant than aluminum. A good Delrin spool can live in a pocket for years with almost nothing to maintain or damage.
I do not see a valid reason to buy an aluminum finger spool.
Aluminum is substantially denser, so when you drop an aluminum spool underwater it heads down quickly. That is exactly the opposite of what I want from a small piece of equipment that may need to be released unexpectedly during a DSMB deployment.
It also costs dramatically more. High-quality Delrin spools are available for a fraction of the price of machined aluminum versions. The additional money buys machining, anodizing, styling, and sometimes additional grip features. It does not buy a DSMB capability that a good Delrin spool lacks.
Aluminum is strong, but strength is not a meaningful advantage here because a quality Delrin spool is already far stronger than this application requires. A spool needs to hold line, survive normal diving abuse, and remain easy to control. Delrin does all of that without the extra weight or cost.
Basic molded plastic spools can also work, although I prefer Delrin for its durability and feel. If I am buying a spool that I expect to use for years, Delrin is the material I want.
The Bolt Snap
Most recreational spools come with a stainless double-ended bolt snap. I use it to secure the wound spool and keep the line from unspooling while the equipment is stored.
Learn how the line and bolt snap on your particular spool are intended to be secured. The double-ender should keep the stored equipment together without turning the deploying DSMB into something physically attached to you.
During the actual deployment, I want the option to release the spool immediately if something jams, catches, or becomes entangled. A spool is replaceable.
How I Carry Mine
I carry a DSMB on every dive. Mine is a closed-end design with an oral one-way inflation valve, with a 100-foot spool already attached. The complete assembly stays in my left thigh pocket.
Keeping the spool attached to the DSMB removes a step underwater. When I need the buoy, I pull out the complete assembly rather than retrieving two separate pieces of equipment and connecting them while maintaining depth.
For an ordinary fun dive, that is normally all I carry. If the dive is more involved, I add a backup spool in my right pocket. The backup gives me another usable length of line if the primary spool is lost, damaged, deliberately released, or needed for something else.
If I expected to need significantly more than 100 feet of line for a particular dive, I would carry a longer second spool for that specific purpose rather than putting an oversized spool on the DSMB I carry every day.
I do not carry a reel as redundancy for a spool. A second simple spool provides the redundancy without adding another type of device or more bulky equipment.
Manage the Line, Not the Hardware
Keep moving line away from your body, hoses, valves, lights, clips, and other equipment. Do not allow coils of slack line to collect around you, and pay attention to where the line is running rather than staring only at the DSMB.
If the spool stops feeding cleanly while the DSMB is ascending, do not let the buoy determine your depth. Release the spool if necessary and maintain control of your ascent. Recovering or replacing equipment comes after keeping the diver under control.
Wind the line neatly after use. Check it occasionally for knots, abrasion, cuts, or sections that have been wound badly. A spool has almost no mechanism to maintain, but the line itself still needs to be in usable condition.
Practice matters more than buying elaborate hardware. A diver who regularly deploys a DSMB with a simple Delrin spool is better equipped than someone carrying an expensive device they rarely use.
Spool or Reel?
For recreational open-water diving, use a 100-foot Delrin spool with your DSMB. Make sure you can operate it with the gloves you normally wear, keep the line under control, and practice deploying it before you actually need it.
Skip the aluminum spool. Skip the reel unless your diving has created an actual requirement for one.
If you eventually start doing dives that require longer guidelines, deeper DSMB deployment, search work, surveying, cave penetration, wreck penetration, or another specialized application, buy the appropriate line-handling tool for that job.
For ordinary recreational diving, a 100-foot Delrin spool is the answer.