Perfect fins. Monofin swimming.
When examining functional characteristics of fins we should consider physical system swimmer-fins-water
regarding its energy conversion. Muscular energy of a swimmer turns into potential energy of fins resilience which acts upon water in the form of fins movement, thus creating thrust power of a swimmer according to the impulse theorem. Propeller repels air, jet engine repels gases as for a fin - it is water.
Thus, for a fin as an energy conversing element the most important property is its ability to take over the energy in a form of reversible flexural strain in the most efficient way and to transfer that energy to water. As in a jumpers pole or a bow the accumulated energy is transferred to another object. The best material for such energy system is the material which is able to accumulate maximum energy while having minimum mass. On the assumption that the specific index of flexible strain for aluminum alloy mass unit is 1, the similar indices for titanium, carbon and fiberglass are 2, 4.6 and 17 correspondingly. Therefore, it is desirable that fins for active swimming to be made of fiberglass. When fins are used as a rudder the most important criterion of their efficiency is the efficiency of control i.e. rigidity and flexural strength. For that purpose carbon is the most appropriate material but carbon fins would surely lose its driving efficiency.
Fiberglass has an advantage not only because of higher power-consuming ability but also due to its other properties. Carbon fins is characterized by high rigidity and low breaking deformability or brittleness which means that low deformation (especially local deformation) would result in fibers desintegration. Carbon fins have a comparatively low breaking point (when deforming at bend) than fins made of fiberglass. The swimmer in fact does not reach the breaking point when swimming but any casual handling (e.g. blows, extreme deformations, nicks, large scratches and cuts) will bring about quick destruction of carbon fins. Low rigidity fins made of thermoplastic materials do not ensure high power consumption because of their poor strength, high thermal effects on their flexibility, creepage index. The latter can be clearly traced at incomplete return to the initial state after large deformation. Besides, such fins have one more very essential shortcoming - high internal demping or rubbing. It results in absorption of elastic vibrations energy and its transformation into material heating. Therefore, certain amount of the swimmers energy is wasted for nonproductive heating of water. The share of elastic energy transferred to the environment for some types of rubber comes to 15 - 25% and this percentage increases as the temperature falls. Besides, any straps in the form of rubber ribs which hamper water flow also increase nonproductive waste of energy due to internal demping which stultifies any improvement of hydrodynamic characteristics.
When examining materials to be used for fins production one should not forget that the name of one or another material does not produce complete list of its characteristics. That is one type of fiberglass is not similar to another type of fiberglass. Bad material can differ from good one ten times and more, especially when considering specific energy consumption. There could be different fiberglass and carbon: firm and not firm, rigid and soft, cheap and expensive. Materials of highest quality are used in the aerospace industry where special requirements should be fulfilled as to correspondence of its weight and firmness or rigidity, disregarding its price. One kilogram of some helicopter structures costs as much as one kilogram of gold. Space engineering structures are even more expensive. As for general goods, as a rule the manufacturers use materials of middle solidity. Characteristics of such materials generally meet customers requirements. However, sports and special purpose goods are usually made of high quality materials.
Energy transformation in the system swimmer-fins-water is carried out under the influence of periodic actions of a swimmer. Fins, set in motion by a swimmer, change their form and in certain time release accumulated energy. In the air this system would sway quite a long time but in the water under strong hydraulic resistance the fins quickly (within one swing) return back in the initial (zero) state. From the physical point of view the fins represent a swinging (oscillating) system with periodic excitation and high damping. In any swinging system index of the energy output corresponds to the resonance, that is coincidence of frequency of internal impacts (swimmers actions) and their own elastic swinging.
Each swimmer has specific and particular technique of swimming, swing frequency, physical power therefore the concept of perfect or best fins is particularly personal. In other words, each swimmer should use own characteristics set as for solidity, rigidity and weight of fins as well as geometrical characteristics. Those who like fast swimming would enjoy rigid fins, those who practice power swimming - soft fins, strong swimmers of great endurance need large area fins. Examination of material the fins made of does not give an idea of their quality. Energy consumption of the fins may depend on different geometrical parameters to a great extent. The most important geometrical characteristics are the following: shape, trend of changing thickness of fins blade, angle in the butt. A shape is the most important factor which determines hydrodynamic properties of fins concerning transfer of energy in the water. Theoretically the fins are tending to be longer with gradual increase from the butt and following narrowing. But the longer fins are the more their weight and frequency of own swaying. The most efficient technique of swimming tends to slow movements, not typical for a man.
The angle of a fins base is meant for improving natural imperfection of a human foot - its bend. Without the bend angle a swimmers resistance rises while swimming efficiency comes down.From the functional point of view absence of the bend angle is equivalent to lowering of the efficiency coefficient of the system swimmer-fins-water. Depending on the a swimming technique (diving, speedy swimming) this parameter has quite determined optimal characteristics. Sometimes, the fins are made by milling of thick plate of plastic to make the fins ending thinner. Such a technology does not provide for use of the bend angle because fibers in plastic would be cut. But the most significant disadvantage is opening of internal pores. The point is that thermosetting binding agent produces and lets out volatile components when hardening. Certain amount of these volatile components remain in the material in a form of very small bubbles. Their dimensions and quantity depend on the binding agent, hardening technique, pressure and vacuumization level in the process of hardening and a quality of drainage. The greater part of bubbles are inside the material.
Later they will create conditions for desintegration of the material, promote water absorption and ageing of plastic. Milling of plastic opens pores, therefore, increases water absorption and accelerate ageing of plastic. Painting does not prevent plastic from water penetration since apart from water penetration physically the same penetration occurs on molecular level (osmosis).
Manufacturing of high quality fiber-glass fins is linked to solution of a number of technological problems.
In order to avoid the above mentioned problems manufacturers often use pre-formed plates - glass-reinforced material.