Introduction

Historically, swim fins, snorkel fins, and scuba fins are a category that has not changed much over the last six centuries, where for example Leonardo da Vinci, Benjamin Franklin, Louis de Corlieu, and Churchill US Pat 2,321,009 contributed to the art.

LEFT TO RIGHT: Leonardo da Vinci, Benjamin Franklin (hand paddles), Louis de Corlieu, and Churchill inventions

More recent designs include vented, split, scooped, or hinged blades. Until the advent of Truefin the user has had to choose between a flexible fin or a stiff fin according to the task to be performed. Truefin's innovative approach involves the introduction of artificial spines which enforce a distributed smooth curve flex of the blade during an efficient and streamlined angle of attack (no sharp 'hinged' bend or blade pivot which causes flow separation) which offers an improved user experience for this critical diving accessory. Truefin has the comfort and easy kicking of a highly flexible fin while kicking at slow speeds, while also having the high thrust of the stiffest technical fin while kicking at high speeds.

Swim fins are available in both open heel designs as well as closed heel designs. Open heel fins are generally more popular than closed 'full foot' fins due to the preference of many users to wear booties which are warmer in cold water, and booties are more comfortable on hot dive decks and also safer for shore diving while walking on rough shore terrain prior to entering or exiting the water during snorkel and scuba diving. Also, open foot or open heel fins may be adjusted to exactly fit the user's feet. Closed heel fins are usually less expensive and generally worn barefoot or with socks and have high comfort if properly fitted, as well as having less drag resistance than open heel fins while kicking or moving through water.

Efficiency of swim fins is a function of the average forward velocity in water and the power required to achieve that speed. The diver must overcome drag while swimming, where active drag (drag from a diver swimming through water) is generally greater than passive drag (drag from a diver being towed through water). In order to minimize active drag while flutter kicking for example, short kicks are advised over long kicks in order to minimize the projected area profile of the diver. However, to further confuse the matter, greater energy may be required to perform short kicks at high frequency versus long kicks at a lower frequency because moving a small mass of water rapidly is less efficient than moving a large mass of water slowly; so efficiency comparisons between long stroke kicking versus short stroke kicking are not apparent.

Truefin Model 110 has been under continuous development for over seven years and has been extensively tested for propulsion, efficiency, maneuverability, stability, comfort, and durability.

Although Truefin is generally referred to as a swim fin, technically Truefin may be considered to be a flipper, where fins have no true bones or skeletal structure within and are composed primarily of cartilage, whereas a flipper has a bone structure as well as cartilage, joints, and tendons.

BELOW: Structure of a dolphin pectoral flipper

Refinements of the advanced generations (up to Gen4) involved:

- Minimizing the mass of the artificial spines while maximizing strength - Establishing the optimum longitudinal articulation collision geometry of the artificial spines for most efficient kicking thrust - Establishing the optimum lateral articulation of the spine for the most efficient amount of 3D blade scooping - Determining what the neutral angle should be of the unstressed straight spine relative to the foot pocket - How to make the spine and the spine capture components self cleaning so dirt and sand can never damage the spine or the fin - How best to shape the trailing fin blade edge for greatest efficiency - How rigid and how long should the foot platform be while extending under the user's heel to minimize stress of the medial arch of foot - How thin should the Monprene wall be where it contacts the top of the user's foot (at the instep of the foot) in order to maximize comfort and durability - What should the target buoyancy be for best trim with wet and dry suits, and - How to make the fin tough enough to be able to offer a lifetime guaranty.

The user can explore these and other considerations and developments and review machine test data by browsing through this technical web site.

The result:

During high frequency flutter kicking Truefin performs exceptionally well and high swim velocities are achieved. Whereas during rapid kicking traditional (spineless) flexible fin blades collapse (or 'go flat' as it is known in the industry), and for this reason very stiff traditional fins have been chosen in the past in order to minimize the likelihood of the stiff fin blade collapsing during moderate to high kicking frequencies. Furthermore, a disadvantage with a traditional stiff fin is that muscle fatigue and relatively high oxygen consumption for the amount of speed achieved occurs during low kicking frequencies with a stiff fin, and as a result a traditional stiff fin is inefficient and uncomfortable to use when kicking at slow to moderate speeds due to abnormally high strain at the user's ankle for the low amount of thrust generated. Furthermore, at low kicking frequencies, with a traditional stiff fin much energy is lost with water spilling over the sides which increases resistance and produces no useful work. If a traditional fin is considered stiff enough that it can not be 'over kicked', then at low kicking frequencies the fin will perform unsatisfactorily and may cause muscle cramps.

The fin blade flex characteristics of Truefin equipped with Blue '412 spines is twenty degrees (20°) in the 'toe up' direction, and sixty degree (60°) in the 'toe down' static flex direction (45° 'toe down' effective dynamic flex) and is determined by the articulation limit between each of the collision sites between a series of artificial vertebrae. With Blue '412 spines the articulation limit between successive vertebrae is four degrees (4°) in the toe up direction and twelve degrees (12°) in the toe down direction (note Blue spine part numbers XX412).

Below are examples of the ‘412 vertebra:

Truefin’s blade flex characteristics are also affected by the elasticity of the pass core bands, where as the blade flexes the bands tighten against the vertebrae socket forks which results in a slight 'springiness' when the blade is fully flexed. Note that when the spines are removed from the fin, negligible blade flex 'springiness' is exhibited when the blade is flexed 'toe down' to sixty degree (60°) because the spine vertebrae socket forks are not present to create tension in the pass core rail bands. As a side benefit, with the spines removed, the fin may be bent to a 'U' shape during transport in order to minimize luggage volume.

As further background in this regard, based upon machine testing by Truefin, the elastic spring ribs or rails (beams) of traditional fins have not been demonstrated to improve efficiency as compared to the ribs or rails (beams) of Truefin. This may be because if you consider the full kick cycle, and regardless as to how much rail spring bending resistance the fins has, the user of traditional fins first has to load up the 'rail spring' at the beginning of the kick stroke which takes energy away from the diver, followed by the 'rail spring' returning energy to the diver's kick at the end of the kick stroke, which is at best a ‘net zero’ in overall energy expenditure in a given kick cycle while the rail spring is loaded and then unloaded. As pointed out by Pabst (D. Ann Pabst in Springs in Swimming Animals, Biological Sciences and Center for Marine Science Research, University of North Carolina), "For any spring-mass system, there is a natural frequency at which the system will oscillate (reviewed in Alexander, 1988 and Farley et al., 1993). At this frequency, the elastic restoring force of the spring and the inertial force due to acceleration of the mass, are equal in amplitude, but in opposite direction - thus these forces cancel." Pabst investigated how parallel geometry of "skins, tendoms, and skeletons of axial locomotors can function as springs", where the 'parallel geometry constrains the spring to be non-linearly elastic" and "muscle power is diverted to load the spring only when swimming muscles are not capable of producing maximal hydrodynamic thrust". In practical applications of traditional swim fins utilizing passive rails (springs), the restoring force at the end of a flutter kick power stroke is always less than the initiating force at the beginning of the flutter kick power stroke due to hysteresis power loss from internal friction of the elastomeric rails, as well as hydrodynamic drag losses of the blade moving through the water, so ‘net zero’ can never actually be achieved.

Furthermore, although biological evidence suggests that fish use mostly anterior muscles for steady swimming while the caudal part of the body acts as a carrier of energy, relying on rails or ribs of swim fins having flexural stiffness which may be approximated as thin elastic beams according to Euler Bernoulli's method is only a mediocre approximation of nature inspired swim fins for two reasons: (1) the first reason is that Euler Bernoulli's method is a simplification of the linear theory of elasticity, and in the case of Pabst's investigation the dolphin 'spring' is nonlinear, and (2) a dolphin's kinetic energy may be temporarily stored and actively released in internal elastic structures, versus the passive structure of traditional or conventional spineless swim fin rails. As noted during Truefin machine testing, the propensity of Truefin to readily form an optimum angle of attack at all kicking frequencies is attributed to the low bending resistance of the Truefin fin rails up until the artificial vertebrae collisions occur and the angle of attack is rigidly enforced, and this is perhaps the primary reason Truefin consistently performed best in thrust as a function of both kicking frequency and/or muscle exertion. (Refer to INNOVATION/COMPARISONS (PDF) graphs). As Pabst noted, to date (2020) no study has integrated data on spring mechanics, kinematics, force output, and metabolic energy use in any swimming vertebrate, and the role of springs in vertebrate swimming is apt to remain controversial until such data exist.

Additionally, with regard to fin rails of traditional spineless fins, as mentioned above with regard to the hysteresis power loss or self dampening of rubber or thermoplastic elastomers (TPE) rails, Truefin has negligible self dampening characteristics and the angle of attack rapidly changes upon fin blade reversal. Truefin does however exhibit a very minor rail spring rate due to flex of the blade away from the neutral profile of the blade, and as the rail bands are loaded in tension against the forks of the artificial vertebrae immediately prior to vertebrae collisions, and this is considered a benefit at very low kicking frequencies.

An addition comment should be made with regard to what is commonly referred to as 'snap' of a fin. Snap is technically the relaxation modulus (Re) of a material. Based upon testing by Truefin, a popular argument that a fin of traditional length should have a high relaxation modulus in order to deliver a 'snap' of propulsive thrust toward the end of the kick stroke again is not demonstrable when machine testing for efficiency because Truefin simply does not have any 'snap'.

FREEDIVING FINS

Having said that, very long fins such as freediving fins are a different category and may benefit from both elasticity and 'snap'. Long freediving fins have a large surface area which displaces more water while typically forming a long 'S' shape during both up and down kick strokes thereby potentially offering more thrust, and the elastic 'snap' may be more noticeable and beneficial. The elastic 'snap' or relaxation modulus (Re) of the blade material is highest with carbon fiber blades. Fiberglass, plastic, and rubber have decreasing moduli and consequently less 'snap'. 'Snap' is a property that defines the response rate or elastic rebound rate, and is independent of blade stiffness. Typically, the relaxation modulus (Re) of a material is measured by holding the material at a given strain, and then measure the rate at which the internal stress of the material decreases with time as it relaxes.

The flexible blade of Truefin in combination with the artificial spines results in a new type of swim fin that performs very well during scuba and snorkel activities due to overall comfort and efficiency. Truefin also has applications when spearfishing under certain circumstances because Truefin is highly maneuverable, and the artificial spines prevent the blade from collapsing when shore diving and during fast acceleration when in surf, or while in rough seas with big swells and current around reefs. However, freediving fins (with blade lengths between 31-40 inches for example, and overall lengths 43-52 inches which includes a typical foot pocket) may be more efficient then Truefin and certainly all traditional scuba fins in general, so spearfishing is typically performed with long, flexible freediving fins, and most often in deep open water where the diver is not concerned about critical maneuvers, and it is unlikely that collisions will occur between the fin blades and underwater obstructions. Truefin has not conducted efficiency tests of long freediving fins, and until Truefin has more information, if you exclusively use long freediving fins during diving activities, and particularly if the freediving fins have a full foot pocket and you tend to kick more with your knees rather than at your hips, then Truefin is not recommended as a superior replacement.

Definition of freediving: Freediving - Surface diving or underwater swimming without the use of an artificial breathing apparatus, especially the sport of surface diving as deeply as possible on a single breath of air.

Note: As of 6/2022 Truefin has not conducted machine efficiency tests with freediving fins so efficiency of freediving fins compared to Truefin is not known.

Paddle Blade

More will be discussed about vented blades in Efficiency and Geometry, but based upon prior research and subjective testing as well as machine testing by Truefin, the benefit of a vented fin blade is not supported and in fact may disadvantageously reduce the volume of water directed rearward at all kicking frequencies. However, for relatively stiff prior art technical fins, blade venting may offer a benefit in order to reduce kicking resistance at low kicking speeds when the optimal 'angle of attack' is not possible and it is desired for water to flow or spill through the blade vents (in an inefficient manner) in order to reduce low frequency kicking resistance. Note that with the design of Truefin the optimum 'angle of attack' readily occurs at low kicking frequencies as well as at high kicking frequencies so such spilling of water through the blade is unnecessary and reduces efficiency.