Background Art It is well known that boat hulls which remain in contact with sea, lake or river water for long periods become encrusted with algae, mollusks and other organic matter.

These encrustations are particularly undesirable in pleasure craft since they appreciably alter the hydrodynamic characteristics of the craft and hence worsen its overall performance, not to mention the permanent damage they cause to the bottom of the hull.

To prevent these encrustations from forming (or to remove them after they have formed), the boat must be periodically sent to a boatyard, dry-docked and, if neccessary, stripped, so that a suitable protective paint can be applied to it.

These operations are highly labour-intensive and require skilled labour and expensive machinery, besides having harmful effects on the environment since the protective paints normally used contain toxic and corrosive pollutants.

Disclosure of the Invention The aim of the present invention is to overcome the above mentioned disadvantages.

This aim is achieved by providing the boat hull with an effective protection against encrustations due in particular to water-borne biological organisms without using paints.

The first aspect of the invention envisages an element to protect the hull characterized in that it comprises sheathing designed to cover at least the submerged part of the hull and associated with means to position the sheathing relative to the hull.

The second aspect of the invention envisages a method to protect the boat hull against water-borne organisms by preventing such organisms from growing on the hull characterized in that the preventive action is achieved by covering at least the submerged part of the hull with sheathing designed to isolate the hull from the surrounding water.

The invention will now be described with reference to the acompanying drawings in which: - Figure 1 is a side elevation view of a boat moored to a quay, with the sheathing to protect the submerged part of the hull shown in a perspective view in the sunken position under the hull; - Figure 2 is a partial schematic cross section through the centre of the boat shown in Fig. 1 so as to better illustrate the bottom part of the hull and the sheathing underneath it; - Figure 3 is a view similar to that of Fig.l but showing the sheathing fitted over the submerged part of the hull; - Figure 4 is a cross section similar to that of Fig. 2 but with the sheathing fitted over the submerged part of the hull; - Figure 5 is a partial schematic cross section, through the stern, of the boat illustrated in Fig.3.

- Figure 6 is a perspective view of a boat with the protective sheathing showing the latter while it is being fitted to the hull when the boat is not moored to a quay but is, for example, riding at anchor in the roads.

With reference to Figs. 1 to 5, which illustrate a first preferred embodiment of the invention, the numeral 1 indicates a boat moored to a quay 2 in a harbour, the bottom of the hull 3, that is to say, the part below the waterline 4, being submerged in water.

The protection device disclosed by the present invention comprises a sheath consisting of a shell 5 made of metal, plastic or rubber, either rigid or flexible, essentially concave in shape

and designed to fit at least over the submerged part of the hull 3 so as to isolate it from the surrounding water. The back part of the shell 5, designed to fit over the hull 3 astern, is equipped with recesses 6 designed to accommodate the propellers 7 and the rudder 8.

The shell 5 is peripherally connected to positioning means comprising an inflatable element 9, preferably tubular, and long enough to run right round the hull 3 at the waterline and, once inflated, designed to keep the shell 5 against the hull 3.

The shell 5 and the inflatable element 9 are preferably equipped with spacers 10 made of metal, plastic or rubber running lengthways along the hull to keep the hull 3 at a preset distance from the shell 5 and from the inflatable element 9 in such a way as to create a gap 11 for the controlled forced circulation of a liquid used to wash the hull and the other submerged parts of the boat and containing anti-biological substances designed to inhibit the growth of water-borne organisms. This is especially convenient when the boat is used at sea since the washing liquid may consist of fresh water used to rinse harmful brine off the hull, the propellers 7, the outboard parts of the motors and the rudders.

The forced circulation of the washing liquid is carried out by a service unit 18 located on the quay 2, including a pump connected to the tank that contains the washing liquid and equipped also with a pipe 19 leading into a manifold that is connected to the shell 5.

The service unit 18 may also include a compressor used to inflate the element 9 through an appropriate pipe 9a shown with a dashed line in Fig. 1.

The bottom of the shell 5 is equipped with ballasting means, consisting, for example, of a container 12 filled with water or other ballasting substance to keep the shell 5 under the water in the mooring area when the boat 1 is not moored to the quay 2.

Buoys 13 are also envisaged, each being connected to the inflatable element 9 by a cable 14 and designed to mark the position of the shell 5 when sunken and to determine the sinking depth.

The spacers 10 have appropriate breaks in them in such a way as to form a system of transverse channels 15 on the surfaces of the shell 5 and of the inflatable element 9 facing the hull 3 so as to make it easier for water to drain off when the shell 5 is raised towards the surface of the water.

Fig. 6 illustrates a second preferred embodiment of the present invention particularly suitable for protecting the hull of a boat that is riding at anchor in the roads.

In this second embodiment, the inflatable element 9 is divided into a front portion 16 and a rear portion 17 which can be inflated separately: this facilitates the positioning of the shell 5 on the hull 3.

The protective device illustrated in Figs. 1 to 5 is used as follows. When the boat 1 comes into the harbour to be berthed near the quay 2, it is steered between the buoys 13 which mark the position of the shell 5, the latter being kept at a certain depth below the surface of the water. The inflatable element 9 of the shell 5 is then inflated and, if necessary, the water or other ballast emptied out of the container 12, so that the shell emerges from under the boat and the spacers 10 come into contact with the submerged part of the hull 3. Initially, the correct position of the shell 5 relative to the boat 1 is guaranteed by the buoys 13 which constitute a set of reference points to enable the boat 1 to be moored in the correct position over the shell 5. Then, when the inflatable element 9 starts emerging, it tends to move naturally into position round the hull without requiring complicated manoeuvring of the boat since the perimeter of the inflatable element is substantially the same in shape as the cross section of the hull at the waterline.

To use the protective device illustrated in Fig.6, it is necessary first of all to lower it into the water from the boat 1, with the inflatabale element 9 deflated so that it will sink. The front portion 16 of the inflatable element 9 must be inflated using the service unit installed on board (not illustrated) and, once the front portion 16 has emerged, the boat 1 can be steered forward into it in such way that the bow of the hull 3 comes into contact with the front portion 16. This manoeuvre is facilitated

by the "V" shape of the front portion 16 which is designed to accommodate the bow of the hull. In this way, the shell 5, still partially sunk is correctly aligned with the hull 3. It is then possible to inflate the rear portion 17 of the inflatable element 9 so as to bring the back part of the shell 5 and the related recesses 6 into contact with the back part of the hull 3.

In both the cases described above, the positioning of the shell 5 against the hull 3 of the boat 1 is quick and easy. The hull 3 of the boat 1 is protected easily and effectively against encrustations due to algae, mollusks and other organisms since the shell 5 isolates the submerged part of the hull 3 from the surrounding water during the periods when the boat is not used which are usually those when the encrustations develop.

To make the shell 5 stable under water, it can be equipped with safety stays anchored to the sea bed (not illustrated).

The service unit 18 may be equipped with a remote control to start inflating the inflatable element 9 while the boat is approaching its berth.

Moreover, to facilitate the emergence of the shell 5 once the element 9 has been inflated, a number of single-acting valves, collectively labelled 5v in Fig,. 4, may be fitted to various parts of the shell in order to allow the water to flow out of the hollow parts of the shell.

The invention described can be subject to modifications and variations without thereby departing from the scope of the inventive concept. Moreover, all the details of the invention may be substituted by technically equivalent elements.