The invention concerns a marine propulsion system which makes it possible to obtain a propulsive lift through a bi-convex oscillating body, to exploit the cynetic energy of the threads moving past the two sides of the oscillating body by deflecting them into a one-way jet, and thus to directly convert an oscillatory motion into a propulsive thrust in water without any intermediate mechanism.

Its underlying principle is found in the formula expressing the thrust T of a fluid in motion:

T = w/g Ve + (Pe - Pa) Ae which reaches its maximum value when Pe = Pa, i.e.:

T = w/g Ve and in Bernoulli's theorem.

If a biconvex body (axonometric, plan and cutaway views in figures 1 , 2 and 3) is made to rotate around its axis (O) in a fluid medium, the fluid flowing past its two sides must increase its speed so as not to alter the overall energy balance. But the speed varies at each of the body's cross sections, increasing with the latter's distance from the axis of oscillation. Thus, the pressure on an oscillating body's surface decreases not only as a function of the curvature of its cross-section, but also with the latter's distance from the oscillation center, and the body is, as it were, sucked in by the medium.

Tank tests carried out with a small-size model of the type shown in figures 1 , 2 and 3 have indicated that this "centrifugal lift" is low when the oscillation axis is [0), high when it is (O').

Since the tests were made under the same conditions, the difference in behaviour is to be attributed to the one variable introduced: the distance between the body's geometrical center and its axis of oscillation.

An oscillating body displaces a mass of water determined - all other parameters being equal - not as much by its volume as by its longitudinal cross-section. Therefore, in order to assess the water displaced, rather than the body it is possible to take into consideration the area of its longitudinal cross-section (active surface) and the distance of its geometrical center from its axis of oscillation (momentum).

Assuming that there are no efficiency losses, the power required to do so in a given time is measured by the average weight of water displaced in the unit of time.

Of course, all propulsion systems, using either a rotary or oscillatory motion, are subject to efficiency losses, due to induced currents, turbulence, sliding frictions etc..

Attempts at reducing such losses have constantly met with number of problems, many of which would disappear if in building propulsion systems there were no limits of size.

But in a propeller screw, the dimensional constraints entailed by a rotary motion in a transversal plane perpendicular to the direction of the craft's motion do obviously dictate limits.

In a foil oscillating throughout its length, these limits are less strict and, as a result, an oscillatory motion makes it possible to obtain greater power and a better adjustment of the expansion ratio.

Now, a non-flexible foil inserted lengthwise at the tip of the body

shown in figures 1, 2 and 3, where the water flows at maximum speed, causes both an increase of the body's active surface and a deflection of the threads, thus generating a more powerful thrust in the direction opposite to their motion.

The different nature of the phenomena occurring on the two sides of the foil - one being explosive, the other implosive in character - causes strong turbulence at the exit edge, and therefore lesser efficiency; in order to eliminate this drawback, their effects must be kept apart and also somewhat damped.

The first condition was achieved by adding two lateral containment walls abutting on the foil's rim, and a few shallow directional grooves on its surface; the second, by replacing the non-flexible foil in fig. 4 with a flexible element freely bending throughout its length, except for a segment at the basis where its maximum swing is limited by the span of a V- opening made at each wall's end.

The flexible foil will bend, where free, according to the varying difference in pressure at its sides, and - just as it happens with an oscillating rope - the system will go into resonance and develop an undulating motion, with its nodal points where the difference in pressure is nil.

The pressure around the hydrofoil profile thus becomes practically constant at all points, except for the local alteration in the streamline caused by its passage.

As an alternative, this condition may be achieved by a foil split at its leading edge into one or two elements of suitable length, hinged together and free to swing within a predetermined span, as shown in fig. 5.

Figures 4 and 5 show a marine oscillatory-motion propulsion system of the hydrojet type. It looks like a scuba diver's fin, but actually the two perform in a quite different way.

In a fin, the active surface is conditional upon the elasticity of its structure, which varies according to the forces acting on its sides. When these forces become greater than the fin's structural strength, it collapses and its active surface rapidly decreases.

Nothing of the sort happens in a hydrojet, whose active surface undergoes no change,

In a hydrojet, for a given amplitude of oscillation, the thrust is a function of the oscillation frequency.

In the tank tests, the hydrojet evidenced a higher efficiency than a propeller screw and an almost constant thrust-to-power ratio. This performance was obtained with two small models operating in parallel and in the phase opposition mode.

The sea trials, carried out with a flat-bottomed, 60 cm long boat model, equipped with a 12- watt engine and two hydrojets mounted in parallel and in phase opposition, gave more than satisfactory results. The craft sped away, gliding on water and leaving no wake swirls but just a long transverse wave lowering at both sides, caused more by the moving hull's weight than by the hydrojets. Her motion was linear and steady.

An oscillatory-motion propulsion system has an additional advantage over the propeller screw: the pulsating action of an engine can be applied to the system, or to a component solidly attached to it, without driving mechanisms.

As a result, it is possible to convert a hydrojet into a pulsehydrojet, where the propelling action can be developed either inside or outside its body.

This double option differentiates pulsehydrojets into two categories, A and B.

Fig.6 outlines an /4-category pulsehydrojet, whose basic structure comprises a central butterfly-shaped body (4), rigidly attached to the hull by two wings (5) and (6), inserted in two cylinder sections (5') and (6') which are housed inside the hydrojet, so as to enable it to freely oscillate on the supporting necks (C) and (C).

Fig. 7 outlines a S-category pulsehydrojet, whose basic structure comprises a combustion chamber and a compression chamber, housed in a monobloc (7) attached to the hull. The oscillating component is the central butterfly body, connected to the hydrojet by a shaft (8).

Fig. 8 and fig. 9 show, in vertical cross section and plan view, a snorkel version of the pulsehydrojet in fig.6. Fig.10 shows another version of the pulsehydrojet outlined in fig.7.

The operating cycle of an >A-category pulsehydrojet involves two stages.

In the first stage (see fig.9, plan view) the combustion gases in section (A) impinge directly on the wall (A'), compelling the hydrojet to rotate; in the second stage, the combustion gases in (B) act upon the wall (B') and force the hydrojet to invert its rotation. The result is an oscillatory motion.

Actually, the powerplant is composed by two combustion chambers and two precompression chambers, whose volume varies as a function of

the oscillation amplitude of their housing structure.

As ajready indicated, fig. 8 shows the basic structure of an A- category poweφlant, using a snorkel-type feeding system for the powerplant, which can operate as a two-stroke engine or also in a way very similar to a jet engine.

To this end, the exhaust gases must be accelerated so as to create in the emptied space a pressure drop which sucks them back, still burning, into the combustion chamber.

A β-category pulsehydrojet can be operated also in a simpler way, by the reciprocating motion of a conventional engine's pistons, as outlined in fig. 10.