BACKGROUND ART It is well known that propulsive efficiency of contra-rotating propellers exceeds those of two separated propellers at the same conditions and also considerably exceeds efficiency of single propeller. However, majority of know contra- rotating arrangements is based on set of bevel gears engaged with two concentric shafts.

When reversing the direction of rotation theese gears suffer due to considerable inertial forces caused by retardation and acceleration of the system during manoeuvring.

In fact, it is only reversible two stroke diesel engine or reversible electric motor that can be coupled to contrarotating propellers system as described above.

According to available statistics, portion of four stroke installations sold on the market tends to increase while number of two stroke installations shows decreasing tendency.

The proposed invention is supposed to be viable solution for application of medium speed four stroke engines for high efficiency propellers in contra rotating arrangement.

DISCLOSURE OF THE INVENTION The invention combines advantages of variable pitch propeller and contrarotating propellers, former being well known as advantageous in terms of soft thrust reversing, engine load control and overload protection, possibility of maintaining constant RPM for an PTO generator drive and cleaner environment and later being known as propulsive device having proppeler efficiency far

exceeding those inherent to single propeller, jet recuding the overall diameter to abt 80% of single propeller diameter.

Basically, pitch control concept of each one of the propellers put in contra rotating relation, uses well known mechanic developed by any one of the major CPP systems manufacturers.

Pitch actuating servo unit can have cylinder arranged at a number of usual position. such as, within gearbox, at gearbox front side, forward or aft of bevel gear within strut supported body of rotatable or transverse thrusters. on the shaft itself, or in propeller hub. For the sake of compactness hub integrated cylinder version is described within the scope of invention, although any one of the said arrangements can be used as well.

In the heart of the patented mechanism there are two yokes (1 and 2) fitted on the pitch controling rod (3) thereof one (1) belonging to"inner"propeller hub (6) being in conventional arrangement while another (2), entitled to acts on "outer"hub (7) propeller blades, is provided with both sides axial bearings (4) to allow transfer of longitudinal forces while active ring, (5) fitted to pitch controlling rod (3), rotates in oposite direction, together with"inner"propeller (6).

Although pitch controling rod (3) drives both yokes in the same longitudinal direction desired pitch change of the propeller blades of"inner"and"outer" propeller is attained by blade cranks (8) being fitted at opposite sides of the blades flanges (121, producing therefore blades (9) rotation in opposite directions as well.

As it is evident, zero pitch of both propellers is at central position of pitch controlling rod and pitch change, arising from the momeyent of the rod to any direction. will produce thrust of both propellers in the same desired direction.