Water-jet Propulsion

Abstract

An underwater jet propulsion nozzle includes means for injecting air into the jet stream issuing from the nozzle, to give increased thrust when stationary or at low speeds. The nozzle is mounted for swiveling movement on a fixed jet pipe, and the plane of the swivel joint is inclined downwards in the direction of forward motion.

Description

This invention relates to jet propulsion units for boats. These units impart a propulsion force to the boat as a reaction to a water jet.

Such propulsion units can be made very efficient, but it has been found that full thrust is not reached until the boat has gained some speed. This is particularly so where the jet nozzle is underwater.

It is an object of this invention to overcome this difficulty with underwater jets.

Accordingly, the invention comprises an underwater jet propulsion unit having an outlet water-jet nozzle including means to inject air into the jetstream as it issues from said nozzle.

These means may consist of a tube projecting above the water level and feeding air to an annular cowl around the nozzle.

An exemplary embodiment will now be described with reference to the accompanying drawing, which shows in section a jet nozzle of reversible type.

A jet pipe 10 is fixed to the stern of a boat (not shown) and extends rearwardly and downwardly to a swivel joint 11 by which it is connected to a curved underwater jet nozzle 12, which may be turned from the "forward drive" position shown in full lines to a "reverse" position, shown dotted.

The nozzle 12 ends in a short cylindrical section 13, which is surrounded by a cowl 14 forming an annular space 15 therewith.

The rear edge 16 of the cowl 14 lies a small distance (say, 2 cm. or 1 cm.) beyond the outlet edge 17 of section 13.

Space 15 is connected to pipe 18 (shown as a flexible tube) which extends above the water level 19 and is therefore open to the air.

In operation, the water jet issuing from section 13 exerts suction on space 15 and draws air from it and from pipe 18, so that the jet is surrounded by an annular air envelope at least for a short distance beyond edges 16, 17. It is found that this provides full thrust from the jet stream even at low boat speed. The Provision of an air control valve 20 at a convenient position will demonstrate this, as well as enabling the amount of air admitted to be regulated. If valve 20 is turned off, low-speed thrust drops considerably to rise quickly as valve 20 is turned on.

With ample air available from pipe 18, the arrangement is to a large extent self-regulating. As the jet speed increases, more air is drawn in and close to optimum conditions occur.

The air injection may take various other forms. For example, a short rigid pipe (corresponding to pipe 18) may project vertically up above water level 19. In this case, the pipe will lie under water in the reverse position, but this is not of much importance in some circumstances.

Alternatively, the cowl 13 may be dispensed with and air injected directly into the jet stream within nozzle 12. The air then adopts a generally annular relation to the jetstream and the same effects are produced.

The invention is particularly useful in larger jet boats such as fishing trawlers or tugs where it is necessary to haul nets or other tows at low speed.

Various other changes and modifications may be made in the arrangements described without departing from the invention claimed.

Claims

What I claim is:

1. An underwater jet propulsion unit having an outlet water-jet nozzle, said nozzle being mounted for swiveling movement on a fixed jet pipe, the plane of the swivel joint being inclined downwards in the direction of forward motion whereby the nozzle may be swiveled for forward or reverse thrust, an annular cowl surrounding the discharge end of said nozzle and defining therewith an annular space, means to inject air into the jetstream as it issues from said nozzle comprising an air supply pipe communicating with said annular space between said cowl and said nozzle, and an air control valve for regulating the airflow through said pipe thereby to provide full thrust even at low boat speed.

2. The unit of claim 1 wherein said cowl projects beyond the outlet edge of said nozzle.