Split Fins

[Split fins][]
Split fins are a relatively recent invention, developed in the late 90's. They are easily recognizable as fins that are, well, split down the middle. Before we get into them, let's talk about basic fin design.

The purpose of fins is to convert the up-and-down motion of a kick into forward thrust through the water. The more efficiently it does this, the less energy it requires for you to move, since a higher percentage of kicking energy is converted into thrust. This efficiency is generally what makes one fin considered "better" than another.

Most full fin designs have some sort of ribs on the edges and possibly in the middle, creating channels along the fin. This directs water along the fin, ensuring that most of it gets pushed backwards, in turn pushing you forward. The ribs prevent water from "spilling over" the edges of the fin.

This is straight-forward, but things start getting fancy with split fins. Split fins are in principle similar to a propeller, which uses hydrodynamic concepts to turn rotational motion into forward motion (i.e., the blades spin and the plane / boat moves). As you kick, the fins angle inward, forcing water down a narrow channel. This creates a pressure differential that, along with the water running down the channel, helps propel you forward. (Note to physics geeks: see Bernoulli's Principle)

Ok, sure, but what does it mean? Basically, that you get more motion for the same amount of kick. Split fins are more efficient and thus, "better" by some definition.

It's not all good, though. This description only applies for the usual flutter-style kick. If you are a frog kicker, split fins will actually be less efficient than ordinary full fins. For this reason you won't see split fins on a lot of wreck or cave divers.

Split fins are also expensive, costing 2-3 more than regular fins. It's up to you, whether the extra cost is worth the increased kicking efficiency. Besides, fins are probably not the most expensive piece of scuba equipment you'll buy (even split fins), so when you factor in their lifespan, the extra cost becomes less significant.

I use these split fins from Apollo. Although I haven't had much experience with other fins, I really enjoy them.

In the end it comes down to the importance you place on kicking ease and your budget. However, next time you shop for fins, at least check them out and see if they work for you.

[Split fins]: http://thedivingblog.com/uploads/2010/02/spit_fins.jpg

Easy Color Balancing of Underwater Pictures

In previous tips for underwater photography, we covered the importance of using flash to supply the color absorbed at depth. Sometimes you can't use it, though. For example, in scenery type photos where there is nothing to get close to and use a flash. These pictures are going to be very blue, and its up to your camera mode to adjust the color balance, or you have to do it manually afterwards.

Manually color-balancing photos is a lengthy topic, and one we'll get to someday. But today, I have a super-quick way for you to restore color that ...

Fish Identification: Bar Jack

[A bar jack][]
A fish commonly encountered during Caribbean reef diving is the bar jack. Other English names include blue-striped cavalla, red jack, neverbite, passing jack, skipjack, and pointnose, although I believe bar jack to be far more common.

Physical description

Bar jacks have a horizontally symmetric shape, and bear a remarkable resemblance to the fish symbol . They have a forked caudal (tail) fin, which implies they are quick, continuously-moving fish. They have dorsal, anal, and pectoral fins, although these are generally hard to see until the fish is close. They are moderately-sized fish, usually a little over a foot long (40 cm ...


Skip-breathing is briefly holding your breath between inhales / exhales. Theoretically, it could cause hypercapnia, or excess carbon dioxide in your blood. Serious complications from skip-breathing seem unlikely, nevertheless it is a bad habit that should be avoided. Why would someone do it?

  • Unconscious activity

    Sometimes we do things without even thinking about it. Any habit formed during basic certification could easily persist without any conscious effort. Force yourself to become conscious and it won't take long to break the habit.

  • Trying to save air

    The less you actively breathe the less air you use, right? Wrong. Holding your breath ...

Why is the Recreational Diving Limit 40m / 130ft?

Basic certification in most organizations permits diving to depths of 20m / 60ft. Secondary, advanced certification extends this depth to 40m / 130ft--the recreational diving limit. Past this and you enter the realm of technical diving.

Where did this limit come from? What happens at 130ft that makes it so special?

There are two factors that led to this number:

  • No-stop limits

    40 meters is about as deep as one can dive on air while still having somewhat of a bottom time (albeit less than 10 minutes, with a safety stop strongly recommended) before decompression stops are required. Even brief moments at ...

Diving Equipment in Carry-on Luggage

Dive trips are exciting, but can be a real chore when it comes time to pack for the flight. There's the advantage of having all your own equipment and not using questionable rentals versus the task of getting it all to your destination.

Dividing equipment between carry-on and checked luggage can ease the burden slightly. Then the problem becomes, "What equipment should I check?" Here are four considerations to help with this decision:

  • Legality

    If you're not allowed to have it on a plane, then the decision has been made for you; so don't even bother with ...

Location of Sound Underwater

It can take a while to get used to hearing sound underwater. You are constantly receiving visual input through an entire dive, so your brain learns to compensate for refraction and other visual properties of water. However, audible input is not constant, so each time a boat drives by it catches you by surprise. What makes sound different underwater?

The largest difference is speed. At the surface, sound travels at about 340 m/s. In sea water that increases to about 1500 m/s, over four times faster! You constantly use the speed of sound to distinguish the audible world ...

How To Do a Simple Buoyancy Check

Previous posts have discussed the importance of [proper buoyancy underwater][]. Here I describe how to do a simple weight check in the water:

  1. Initial weight

    If you don't know where to begin, take about 10% of your body weight. If diving in tropical waters with a thin wetsuit, subtract 4-6 pounds; if diving in cold water with lots of exposure protection, add 4-6. This will give you a starting weight to tweak.

  2. Enter the water

    Begin at the water surface with full diving equipment and an inflated BCD.

  3. Hold a normal breath and deflate your BCD

    At this point ...

Sharing Air: Which Regulator Second Stage?

PADI training dictates it is acceptable to share your primary second stage or your octopus with a buddy in need of air.

It is a good idea to work this out with your buddy ahead of time. Should the need arise, it could lead to an awkward underwater shuffle if a buddy needs air and doesn't know which to take; trust me, this is not the time where you want to be doing underwater choreography. This frustration may lead to panic---a bad situation for everyone.

So take a quick second and decide which second stage is for sharing. Your ...

Adjustable Second Stages

Some regulators come with adjustable second stages. This is usually in the form of a knob to rotate or a lever to slide. First of all, this serves two purposes:

  1. It allows you to adjust the sensitivity of the air flow, making it harder to free flow. If you are in a strong current at the surface, for example, your second stage might normally be inclined to free flow.
  2. As the service date approaches, the performance of regulators decreases slightly. This may mean it requires more energy to pull air from the second stage. An adjustable second stage allows you ...