TECHNICAL FIELD The invention relates to a marine propulsion/steering system particularly relates to category of propulsion units fitted on strut underneath ship's hull.

BACKGROUND ART According to common practice these units are of 360 deg. rotatable execution and usually arranged in pairs at the stern of the vessels entitled for high manoeuvring performances such as offshore support vessels, tugs, car and passenger ferries operating frequently in congested ports and similar.

However, there is a wide choice of vessel sizes and types which could benefit from the strut concept although exceptional manoeuvrability for such vessels is not priority. In other word, installation of pair of complex, vulnerable and expensive rotatable thrusters on vessels with operational profile comprising long distance runs with very rare calls at ports cannot be justified in terms of initial and maintenance costs.

DISCLOSURE OF THE INVENTION The intention of the invention is to substitute complex, vulnerable and expensive pair of rotatable (steerable) thrusters with less sophisticated and cheaper solution, without sacrificing propulsive efficiency at design speed.

Invented arrangement shown on Fig 1. comprises lower body (1) of streamlined form with pulling propeller (2) fitted on V-shaped struts (3). At the rear end of lower body (1) a rudder pintle or bearing (4) is fitted. Upper ends of V-struts (3) penetrate vessel's shell and terminate with upper gear boxes (5) each comprising horizontal input shaft (6), clutch (7) and pair of bevel gears (8).

Each strut (3) embodies one shaft (9) with the bevel gear (10) at the lower end, both bevel gears being engaged with gear (11) coupled to propeller shaft (15).

Described arrangement implies use of simple, low resistance, pram-type aft body sections having buttock angles of abt 15-17 deg. to the base line at the

position of patented system. Base plate (12) is common for both struts (3) and for steering gear (13) with upper rudder stock bearing and seals. Well spaced inclined struts are very rigid against side forces produced by rudder declination and effectively substitute for conventional rudder carrier (horn). As a rule, horizontal distance between upper gears'input shafts (6) should at least correspond to engine manufacturers recommendation for twin engine installations. Prime movers (14) are, basically, four-stroke non-reversible medium speed diesel engines.

The said invention is aimed to combine, in mechanically simple and sturdy construction, advantages of high efficiency contra-rotating propellers with well known benefits of a controllable pitch propellers and yet providing a certain degree of redundancy.