Footpocket tendon stiffness

Is there actually a difference between tendon stiffness of various footpockets? If so what does this translate to? I hung a 6oz lead weight off the tendon of a waterway footpocket (a clone of the Sporasub footpocket) and off the tendon of a soft tendon footpocket. The soft tendon footpocket has slightly longer tendons, the weights were placed at an equal distance from the back of each footpocket which apart from the tendons are fairly similar. I believe what accounts for the difference in tendon stiffness in this case is:
1. How much rubber material is in the tendons, the waterway have a significantly taller profile throughout their length.
2. The rubber material itself, the waterway footpocket tendons are made of a harder rubber material.

You can imagine that if a tiny 6oz weight has this effect, what effect the force of a leg stroke pushing against water will have on the soft tendons. It will be like they're non existent.

There will probably be a slight difference but not to the extent that you see in the pics. The tendons of the soft footpocket in the pics are very soft to begin with, that's why I used it to demonstrate the difference. They are so soft that I will not sell them. They negatively affect the performance of the fin.

My premise in this thread is to dispel some ideas about the benefit of soft footpocket tendons. This/my conclusion is based on having experimented with various footpocket/blade combinations and actually having felt the difference. Here I'm just trying to bring the idea across and the reasons for it. It's not easy to put in words but if you were to actually try the footpockets with the soft tendons you'd immediately see what I mean.

In my testing I used 3 different stiffnesses of carbon blades:
soft - medium soft (didn't have) - medium - medium hard
I used 2 different kinds of footpockets:
Waterways (Sporasub clone, normal tendons) - Other brand (soft tendons)

** I tried all pairs of blades in both kinds of footpockets, all pairs of blades with each blade in a different footpocket (at the same time), and combinations of different blades and footpockets for example a medium blade in a normal tendon footpocket on the right foot and a medium hard blade in a soft tendon footpocket on the left foot.

The first step to understanding the dynamics of the fin is to understand why correct blade flex is important. Compare the two drawings of the fin profile at the middle of the down stroke.

A blade flexing in this manner is not doing a good job of moving the diver forward. This kind of shape results from overpowering any blade. Overpowering the blade happens much more easily when the blade or the blade/footpocket combination is not stiff enough. When the fin acquires this shape it moves through the water easier resulting in reduction of kicking effort, but at the same time it propels the diver forward less.

This picture demonstrates better flex. Only the tip of the blade is curved up. Most of the blade surface is still pushing water moving the diver forward. At the end of the down stroke the blade will snap back into straight quicker giving the diver some extra forward push.

The ability of the blade to maintain correct flex depends on a few factors. First there's the structure of the blade itself which includes two factors, stiffness and incorporated flex. These two factors are really one but for purposes of discussion I'm going to separate them. Stiffness is easy to envision as in the case of a blade that is made from one uniform piece of fiberglass. In such a case the blade can be as stiff as you want it and will theoretically flex in a perfect arc because of its uniformity. It's been demonstrated that an arc is not ideal for propelling the diver forward therefore flex is incorporated into the blade. The way this is done is by varying the thickness of the blade from thickest at the back to thinnest at the front. The front being less stiff will then flex first/more than the middle of the blade if proper kicking form is maintained.

The other factor in maintaining correct flex is the footpocket.

IMO the most accurate way to control blade flex is by tooling the blade. Tooling means that layers of fiberglass material are sliced off in a specific pattern creating a gradual change in stiffness from the back of the blade to the front. This slicing of material is computer controlled and is very precise creating very gradual variations in flex that are impossible using other methods.

The alternative to using tooling to create a gradual decrease in stiffness from back to front is through the laying of the fiberglass material itself. With the DiveR blades this effect is created by incrementally receding the layers of fiberglass towards the back as seen in this pic with the light shining through the blade. This method creates a gradual enough decrease in stiffness so as to create good flex over a long blade.

The Edge blades try to achieve this with a criss cross pattern of laying out the fiberglass. This method does not create a gradual enough decrease in stiffness from back to front. The blade bends in an arc if pressure is applied at the tip. This blade would have very poor flex characteristics had it not been for the footpocket factor. The footpocket and tendons stiffen up the back and middle of the blade in a gradual enough fashion so as to give the total fin acceptable flex.

It is clear then that the footpocket adds stiffness to the blade, and that in terms of overall stiffness and flex the blade and footpocket need to be considered together. The central argument to which I'm responding in this thread is "If a blade with perfect stiffness and flex characteristics can be obtained why not remove the footpocket factor from the equation?" or in other words why not have a footpocket with tendons so soft that they do not at all affect the flex of the blade itself?

To be continued.

I know I never finished this writeup, frankly I got tired of arguing about this point. But I thought I'd throw out the following bit of information. The only known "soft" tendon footpocket in production is the Pathos footpocket. It is the footpocket referred to in the medfish forum as the one that exemplifies the idea you're promoting, that soft tendon footpockets are better. Well, we're now communicating with Pathos to see if we can sell their footpockets. I recently spoke with the owner and was told that they are now manufacturing a redesigned footpocket. Redesigned with stiffer tendons in response to the feedback they've been getting. Their old footpocket tendons were too soft.

I will grant you this though, some footpockets can have tendons that are too stiff. I realized this after switching to the Dessault footpockets from waterways. In this case though it's not that the Dessault tendons are soft, they're normal, and it's the waterways that are a little too stiff. Really it doesn't matter because the stiffness of the tendons changes the overall stiffness of the fin. Some waterway footpocket and blade combination may be just the right overall fin stiffness for someone. I do believe though that rather than jump around and mix and match blades and footpockets, it's better to pick a good comfortable footpocket with normal tendon stiffness, stay with it and try to achieve the right fin stifness for you by varying the blade.

But have you ever tried such soft tendons :frustrated2:

I'm glad you said that, it was one of my points. Remove the added support from the tendons and to maintain correct flex you must compensate in the blade by making the base thicker. This will make the blade very stiff at the base. In such a case all the power you put into the stroke will translate into stress at the only point of connection between the blade and your foot, which is the sole of the footpocket. This area will suffer deterioration and you will feel the base of the blade poking through the sole of the footpocket. The whole reason for tendons is to distribute the stress evenly.

Sure different guys will prefer a different stiffness. But there are limitations imposed by our physiology and most people fall within certain parameters. I don't think the tendons in our ankles are designed to handle the kind of stress created when using a stiff fin. However there will be people who are an exception.

As far as selecting stiffness; having a blade that is too stiff is like starting to ride a bicycle in low gear. It's hard to start but once you get going you go faster. Conversely if you start in high gear it's easier to get going but you then put too much effort to keep up speed. It's exactly this way with fins. I find that spearfishing it's always stop and go. Rarely do I have to maintain a constant pace for very long. As such I choose a fin that is relatively easy to get going.

There are longer fins but they're generally considered strictly freediving fins. Our carbon fins can be ordered longer but 75-80cm is considered ideal for spearfishing. I don't think that a person with less mass needs to look at softer fins. I think smaller guys make much better use of a fin of any stiffness. It's a misconception that big guys need stiffer fins.