Device for a ship propeller Untreated immobile surfaces on ships in the sea will in time be overgrown with for instance acorn barnacles. Such"biofouling"occurs quicker at increased salt content and at increased water temperature. All biofouling, increases the water resistance with the consequence that the fuel consumption increases at a given speed compared with the surface that had no biofouling_and as a result had less water resistance. Also without biofouling the surface structure is of importance for the friction against the flowing water.

A common method of counteracting_ biofoutingis appiymg poisonous paints on the surfaces to be protected. Commercial shipping often uses TBT paints (Tributyltin) and pleasure boats in most cases copper based paints. Increased environmental consciousness of the risk of spreading these very poisonous paints in the sea has resulted in local prohibitions against usage. Besides a global restriction against the use of TBT paints for commercial shipping is expected to be in force in the year 2008.

The effect of the poisonous paints decreases in course of time and new paint must be applied after a number of years, which means docking for big ships.

Ship's propellers are in most cases made of bronze or of stainless steel. On surfaces against which water flows very rapidly the paint is quickly worn away. The water flow can be very quick at the fringe areas of propellers. Therefore big ship's propellers are not painted in most cases. If, after a long stationary period, there is biofouling it can be removed more or iess, dependinon how firmly. the growth is attached to the foundation.

Also entirely without biofouling the water resistance depends on the friction between the water and the propeller surface.

The purpose of the invention is making an environment friendly and hardwearing surface layer on ship's propellers, which can give the propeller blades little friction against the water for a long,. time.

The invention will be explained in detail in the following, with the help, of figures where design examples are shown. For reasons of clarity certain measures have been exaggerated.

Figure 1 shows a three-bladed propeller seen from behind.

Figure 2 shows enlarged a profile A-A from Figure 1.

Figure 3 shows enlarged a profile B-B from Figure 1.

On the figures the boss of the propeller is indicated by 1 and the three propeller blades by 2,. 3 and 4. : Att the propeller blades can be cast together with the boss or each blade can be fixed to the boss with screws. The blades can also be adjustably fixed to the boss. The propeller blades can be of any material,. for instance bronze or stainless steel.

The propeller blade 2 is without arrangement, that is as a traditional propeller blade.

The outer edges of this blade iust as the outer edges of the other blades are indicated by 5,6 and 7. On blade 3 the foils 8 and 9 have been glued, whose outer edges reach outside the edges 5, 6 and 7 of the blade. The foils have one common outer edge and they are fixed on to each other in the area 11 between the outer edges of the blade and the outer edges 10 of the foils. The edge of the foil against the boss is indicated by 12.

Since the blades are thicker towards the boss, the foils must be somewhat stretchable.

Onto the propeller blade 4 the foils 13 and 14 have been fixed, whose outer edges, 15 and 16 do not reach as far as the outer edge 7 of the blade. A U-formed strip 17 encloses the outer edge 7 of the blade and the outer edges 15 and 16 of the two foils.

The strip is fixed to the outer area of the blade with glue. The edges 19 of the foils closest to the boss are covered on each side of the blade with strips or similar material 20 which are fixed to the blade by gluing_, or with screws. The foils have an outer side of polyfluorethene of which there are variants called PTFE, PFEP, PCTFE and PETFE. A common trade name for PTFE (potytetraf) uorethene) is Tefton. The foils can be solid and only consist of polyfluorethene. They can also contain filling. Their insides can be etched and with self-. adhesive glue or glue can be applied on their insides or on the propeller blades immediately before the foils are pressed on. The foils can, instead of being etched, have a fibre layer or an elastic fabric on their inside to which clue can be applied. It is possible to start from a foil which contains a fabric and where both sides of the foil have a smooth surface of polyfluorethene. On the surface which later is to be glued, the surface layer can be ground away so that part of the fabric is uncovered while other parts of the fabric are part of the remaining-foil. Suitable glmes can be polyurethan