Fin blade rails, water channeling and tendon rails explanation

  • Some of this will be pretty basic, but I'm often reminded how many guys don't understand rails. There are two kinds of rails; water channeling, and tendon rails.


    Water channeling rails



    Water channeling rails are used to channel water down the blade, they eliminate the fin slipping off the water effect during the down stroke. This can be felt with many plastic fins which usually lack water channeling rails, and with some fiberglass and carbon blades where the manufacturer cut corners and didn't install good rails. Watch the beginning of this video of Sheri Daye using DiveR fins, notice the erratic movement of the fins on the descent. This is partially due to the DiveR being longer than the conventional 90cm, and is exaggerated because they lack water channeling rails, the fins become harder to control.


    [VIMEO]13734159[/VIMEO]



    Ideally the water channeling rails start where the footpocket tendons end and continue all the way to the front edge of the blade. The height of the rails should be about 1.5cm/a little over .5". The grade of rubber of the water channeling rails should be soft enough that they don't interfere with the flexing of the blade, but stiff enough to maintain a straight profile. Another function of these rails is to protect the side edges of the blade against impact, the edge is where fiber fins are most sensitive.



    Here is the schematic for the Pursuit water channeling rails.



    There has been little variation on the concept of water channeling rails. C4 came out with something they call the flap. Just as the name suggest it's a piece of loose rubber in addition to the normal rail height, that flaps around. I'm not sure what their reasoning for it is, but from what I hear people that get these fins end up cutting that part of the rail off. I never tried this flap but I miss it on my fins like I miss it on my ****. See the behavior of the rails when the blade is flexed, doesn't look efficient for water channeling.


    EDIT - I found out the flap name is in reference to the shape of the front edge of the C4 not the rail flap.



  • Foot pocket fin rails


    The first commercially manufactured fins were one piece with foot pocket and blade made of the same material. Advances in manufacturing processes allowed the foot pocket and blade to be made from different materials bonded together permanently; a hard plastic for the blade to achieve stiffness/propulsion, and a soft rubber foot pocket for comfort. This is still the case today with many plastic fins.


    Currently at the top end of performance we have fiberglass and carbon blades, which can be installed in standalone foot pockets from different manufacturers. This allows the diver to select an ideal foot pocket and blade combination for their foot shape, fitness level, and performance requirements/diving conditions. Foot Pocket Fin Rails are the conventional system used to install fiberglass and carbon blades in those foot pockets. The back edge of the blade is pushed into a deep straight slot on the bottom of the foot pocket. The small foot pocket fin rails on the side edges of the blade are pushed into a corresponding T shaped slot in the foot pocket tendons.



    The tendon rails are also in the shape of a T.



    The Speardiver Foot Pocket Fin Rails were designed to give the foot pocket tendons optimum holding power over the blades. For this the tendon rails need to be made of a stiffer grade of rubber than the water channeling rails. The outer edge of the T arms is rounded to be able to easily push the rail into the tight tendon slot. On the inside there is a sharp step which is what the foot pocket tendons grab.



    The foot pocket fin rails are glued onto the side edges of the blade over the distance corresponding to the T slot inside the foot pocket tendons. It is important to note where this slot begins and ends in different foot pockets.



    Cyanoacrylate is the best glue to bond rubber rails to the epoxy surface of blades, it creates an instant and permanent bond. Cyanoacrylate is commonly sold as instant glue such as Krazy glue. I prefer to use the viscous version not the more recently available gel consistency, because it spreads more easily through capillary action and very small amounts of glue are required. My preferred brand is Loctite with the sharp nozzle which allows precise application.



    There are many fin manufacturers that use poorly designed rails, that do not allow the foot pocket tendons to grip properly. This is why you often hear guys talk about gluing blades in foot pockets. When the tendon rails are good the foot pocket tendons grip them very well. There is no necessity for any kind of gluing of the blade into the foot pocket. Guys who use the black 5200 stuff have no understanding how this system is supposed to work. If the blade comes with good tendon rails, and you grab the foot pocket tendons on each side of the blade and attempt to force them away from the blade/rails you will not be able to pull them off, it's surprising considering how short the profile of tendon rails is. What does happen sometimes from the constant flexing and straightening is that the blade can work it's way forward and out of the foot pocket, with the foot pocket fin rails sliding forward inside the foot pocket tendons. This is the only thing that the screws some foot pockets use are needed for, to stop the blade from sliding forward, they don't need to be tight at all. Depending on whether the foot pocket installation kit has a nice backing plate between the screws and the blade, I sometimes prefer to use zip ties. Zip ties eliminate the need for a large round hole, which may potentially weaken a carbon blade.


  • Good foot pockets that will readily accept foot pocket fin rails are: OMER Stingray and Cressi Gara Modular.


    These foot pockets also have clips at the end of the tendons that are supposed to not allow the tendons to come off the rails. It's a somewhat redundant system but it doesn't hurt. The area of the tendon that accepts the clip is thinner than the rest of the tendon, and results in the ends of the tendons being prone to tearing off off at that point after a few times removing and installing blades if not done carefully.



  • Foot pockets from different manufacturers have different tendon lengths, and foot pockets from the same manufacturer have different length at different sizes. For a fin to look like this, with the foot pocket fin rails exactly as long as the tendons, and the water channeling rails starting exactly where the tendons end, some careful planning and installation is required. It's basically a custom installation for every fin and foot pocket combination.



    Most manufacturers don't want to do this because it's too labor intensive, and have created cut corner solutions that are cheaper at the same time.


    In the case of the DiveR fins the small foot pocket fin rails are used throughout the length of the blade, which basically amounts to no water channeling effect. The result of this is an unstable fin and can be seen in the video in the first post.



    Leader fins went in the opposite direction and install water channeling rails throughout the length of the blade. Then cut down the height of the rails in the area where the foot pocket tendons grip. The resulting small rails appear like they're would do the job of tendon rails, but because they're not specifically designed for this purpose the rails are too thick to fit in the tendon slot properly. This results in poor grip of the foot pocket tendons over the blade, increasing the probability of the blade popping out of the tendons during a powerful fin stroke.




    Spetton uses a double purpose rail. This rail supposedly combines a water channeling rail on the inside with a foot pocket fin rail on the outside. What you actually get is a 0.5" wide strip of lost surface area on each side of the blade that is not being used for propulsion, a foot pocket fin rail that is too soft, a bulky water channeling rail that gets deformed when the blade is flexed, and an overall bulky less reactive fin.





  • When I was taking the FII class, one person was using DiveRs, and he was having a tough time controlling the blades. It was the same problem, they wanted to slip sideways on him. I mostly use my SpecialFin Hybrids, and they've got small rails (about 1 cm). They're not too hard to control, but they're not very forgiving if you use bad form. I also have some Beuchat Mundial Fibra, and while the blades don't seem to have much snap, I like the way the side rails are designed. The side rails have a variable height and they don't deform when flexed. Their side rails seem to be a good compromise for spearing.

  • When I was taking the FII class, one person was using DiveRs, and he was having a tough time controlling the blades. It was the same problem, they wanted to slip sideways on him. I mostly use my SpecialFin Hybrids, and they've got small rails (about 1 cm). They're not too hard to control, but they're not very forgiving if you use bad form. I also have some Beuchat Mundial Fibra, and while the blades don't seem to have much snap, I like the way the side rails are designed. The side rails have a variable height and they don't deform when flexed. Their side rails seem to be a good compromise for spearing.


    I like the beuchat fibra blade rails - prettiest ones I've seen on market. :D


    On completely unrelated note, snapped a pair of plastic beuchat blades saturday... spent most of day diving like Nemo.


  • This quote about the C4 is a classic:
    "C4 came out with something they call the flap. Just as the name suggest it's a piece of loose rubber in addition to the normal rail height, that flaps around. I'm not sure what their reasoning for it is, but from what I hear people that get these fins end up cutting it off. I never tried this flap but I miss it on my fins like I miss it on my ****."


    The "flap" is the extension at the end of the fin (an exaggerated curved out cut versus the standard "curved in" cut at the end of the blade). Pursuit offers a cut that resembles a "flap". DiveR comes with a "flap" cut. It has nothing to do with the rubber rail, and obviously it is carbon.


    Many folks are breaking the flap as it is exposed and cannot take too much abuse. When that happens they cut off the flap on the other fin and keep using them.


    C4 being a rather small fin, removing the cut is making it even smaller. I'm not a fan of the C4 - too small/soft to get me moving.


    As for the side rails - some folks in the PFI training team would not touch fins without or with small side rails. Considering they're making many 100ft drops per day and plenty between 100 and 200' per week I'm sure they know what they're talking. Watching Kirk dropping to 200' with C4 mustangs with cut-off flaps is something else.

  • oops, I thought flap was referring to the loose rubber rail part. Thanks for the correction Stephan. Nevertheless my reference to guys cutting off the floppy rubber rail is still correct.


    The round cut on the Pursuit C100 blades only. Haven't had a front edge break.



  • oops, I thought flap was referring to the loose rubber rail part. Thanks for the correction Stephan. However my reference to guys cutting off the floppy rubber rail is still correct.


    In my experience, the rails on the c4 work very well for vertical diving. Great for pure depth freediving, not as good for hunting. The rails are so large that it makes any lateral movement slightly more difficult. You can physically feel the strain on your ankle as you try to turn the blade to change direction. Trade-offs in performance. When I used mine, I left the rails & just adapted to it. After a while, I got used to it & never thought about it again.


    In shallow reefy areas, those big rails get shredded pretty good... but then again, most rails do over time.

  • Great for pure depth freediving, not as good for hunting. . .


    That seems to be what I have found with most fins that are being produced in the last couple of years. It's all fine and good until one actually shoots a fish and has to try and keep if off the bottom or keep it from going into structure.

  • Different ideas John, fuzz found them lacking in maneuverability, your issue is a matter of stiffness. You can't have your cake and eat it too, if you use a stiffer blade you're going to be less comfortable swimming for extended periods of time. I find medium to be a good compromise. Of course I don't regularly find myself shooting 50lb fish around wrecks. Still just this weekend Rolo shot this +40lb permit on a wreck, and they're hard pullers. Not the first time either and I never heard him say that the medium stiffness blades were lacking. I suspect any fin would be briefly overpowered fighting a bigger fish.



    Back to rails. One other thing that should be mentioned is that there are very few foot pockets which don't have a T slot in the tendons but rather a straight slot. These foot pockets do not require tendon rails to be glued on the blades, the blades are glued directly into the tendons using the same Cyanoacrylate glue. This results in a very compact fin with less loss in the power transfer from legs to blades since there isn't even a little movement of the blades inside the foot pockets. The down side is that the installation is permanent. It's very difficult to separate the blade from the foot pocket if the need arises without damaging the foot pocket to the point where it's not usable. And I've yet to see a good foot pocket that uses this system, Pathos foot pockets are shit IMO.


  • The Hell I can't!!!:D


    One of the best overall blades I have ever used was the Omer Pegasso in medium stiffness. I think the tapered construction really made the difference. Thinner in the front for easier and more efficient finning while on the surface and diving; stiffer in the back for power on demand. The down side to these, was just as you explained in your initial post. The tendon rails and the glue jobs were very poor. Mine came off in a matter of a few weeks.


    Gluing the water channeling rails back on was a complete failure for me. The tapered design made it very difficult for the rail to stay put and glue down evenly, because the blade got thinner and thinner as I worked toward the front.


    The blade also had a tendancy to split over time because it was so thin in the front. Great blade initially, but it just did not hold up over time.

    The post was edited 1 time, last by John Hanson ().

  • Side note, my C4 falcon blades have the tendon molded onto the actual blade, it is not a separate tendon glued onto the blade, they are a bit smaller then most tendon slots though.

  • Side note, my C4 falcon blades have the tendon molded onto the actual blade, it is not a separate tendon glued onto the blade, they are a bit smaller then most tendon slots though.


    by the tendon do yoou mean the strips for channeling water?

    Scupper Pro Gives You Wings!

  • by the tendon do yoou mean the strips for channeling water?


    No I mean where the foot pocket attaches to the blade, I've noticed most blades like Dans are a flat piece of carbon with a small tendon attached that slides into the footpockets slot, the C4 have a tendon molded out of CF on the actual blade

  • I think I understand. What you're saying is that instead of tendon rails glued onto the blade, the blade has tendon rail incorporated, like the plastic fin blades. I'd be interested to see a pic of that.

    Ask and you shall receive, IMO great idea but poorly executed. I noticed their new blades have a rubber tendon instead of the molded one like these.


  • I was taking pics of the Speardiver C100 Fins and a wave crashed over them. The water running off the fins really put into perspective the work water channeling rails perform. As small as the rails are the effect is pronounced, you can see it in the pic. A lot of water was directed down the fin along the left side which is what caught my eye. I guess the same thing could've been demonstrated with a garden hose but I think it's more meaningful like this because it was spontaneous.


Create an account or sign in to comment

You need to be a member to leave a comment.