Fig. 3 Dependence of an ECL viscosity from a temperature
In a number of cases even serviceable seals tolerate the oil leakages from a board up to 10 litres per day, and at damages they are increased up to hundreds litres. Similar pollutions kill the all alive, especial in northern areas, where the life of animals and plants is restored through tens and hundreds years. For this reason the whole groups of vessels (the ice breakers. all river fleet) do not use the oil lubricated SPS. Besides that existing propeller shaft seals for the oil are very complicate and very expensive. The numerous firms which manufacture propeller shaft seals consume much money for their update. The oil utilization systems are created, the multi-cascade principle is used, the new achievements of tribology are applied. However the absolutely reliable decision is not found more than for 100 years. By virtue of a conservatism of the fleet as spheres of economic activity the progress in the field of ecological safety of SPS is characterized by slow rates, which can not suit nor state environment departments, nor shipowners compelled to pay more and more sanctions.
If to try briefly formulate a problem as a whole, it will look as follows: "SPS has unsufficient safety and it is one of the basic sources of water areas pollution, that hurts both a shipowner, and Nature".
Strangely enough, neither scale of a problem, nor obvious losses, nor the ecological consequences have not put forward its (problem) into the priority category. In Russia it is caused by economic problems, in advanced West and East - owing to wide cooperation and collision of commercial interests, at all together - owing to the mentioned conservatism of fleet. Besides that today we don't see any technical decision, which has removed all the listed problems, would be clear, understandable and accepted by the sea experts and did not require of the large investments in research works.
Nevertheless there are the sufficient reason for an optimism. It is based on the significant scientific and technical potential that is accumulated in world shipbuilding in last years with reference to SPS problems. The complexity consists that there is a plenty of the local developments which have been not connected by the general technical decision. The global task is not formulated. There is no one organization interested in progress of the system decision of such task.
So the purpose of the creation of the "ideal SPS", in which the advantages are assembled and the disadvantages of water and oil systems are eliminated, can be declared. It should satisfy the following basic requirements:
a) Only the ecologically clean liquids (ECL) should be used for lubricating and cooling of propeller shaft bearings;
b) ECL should be minimal corrosive deleterious to the carbon steel (a material of the propeller shaft);
c) ECL should have low-pitched characteristic of the viscosity change in range of temperatures from 0 up to 80 centigrade degree;
d) Heat- and wear-resistant antifriction materials capable to work with ECL. without swelling should be used for propeller shaft bearings;
e) Stern tube seals should be of a simple design to provide a minimal leakage of ECL into outer water and to prevent the water penetration into ECL;
f) The seal between propeller and propeller shaft should be absolutely reliable and prevent the water penetration into ECL.
Let's try to estimate a practicability of the similar technical decision.
It is obvious, if ECL is used, the problem of the ECL ecological safety disappears by itself. However a practicability of such "simple" decision is not too obvious. First of all numerous liquids widely used in an industry (mineral and synthetic oils, emulsions, acids etc.) are completely excluded from a consideration, as not appropriate to the requirement of ecological cleanliness. Outer water or a water condensate of a ship power system are an ideal ecologically clean liquids. however it cannot be used in "ideal SPS" because it is an agressive liquid for a carbon steel of the propeller shaft. Besides that the metal antifriction materials (babbits) are practically nonoperable if they are lubricated by water, and the traditional unmetal materials are characterized by high speeds of wear process, which reach 0.2 - 0.5 mm per 1000 hours, that can not suit the shipowners.
Thus, for an items (a) and (b) execution it is necessary to have several special ECL, developed for working in SPS conditions. Now the conventional ECL for SPS are not exist, however it is possible to speak about some first developments.
The more detailed analysis of ECL for SPS has shown, that the following additional properties are desirable. It should have a relative density more than fresh and sea water and completely be dissolved in water during immersing onto a bed. In this case not only technical problems of water pollution by ECL leakages are excluded, but also psychological. Outflowing from the stern tube located on a some depth from a water surface, ECL is not rise to a water surface and is not visible to the observer, so ECL is immersing onto a bed and thus is completely dissolved. If the ECL is not toxic, the problem of SPS ecological safety, can be considered solved.
