Technical Field [1] This invention relates to the safety of a ship. More specifically this invention is related to the prevention of extensive damage or inoperability of a ship upon collision with obstacles. Background Art [2] For a person steering a ship it is difficult to see obstacles in the water. Of course, a ship's depth sounder will alert the sailor if the water becomes too shallow, but a depth sounder measures the water under the boat, and not far in front of the boat. Therefore, a depth sounder does not warn for steep rising obstacles like for instance rocks or wrecks, or for floating or partially floating obstacles like for example buoys or containers or debris or ice. A collision between a ship and an obstacle in the water causes considerable damage to the ship's hull, endangering the water tightness of the hull, or to the keel, endangering the stability of the ship, or to the rudder, endangering the operability of the ship. In this application the terms 'ship, boat, and yacht' may be used as a synonym. [3] Ship's hulls, keels and rudders are made of a sufficient strength to withstand the forces of water. Water forces on the ship are divided evenly over de surface of the ship's skin. Obstacles in the water have a more solid structure than water and often cause a more localized impact than water, causing higher forces on a localized part of the skin. Therefore, although the ships part such as a keel, or a skin of a hull, or a rudder, are strong enough to withstand the forces of the water, they are often damaged, or deformed or punctured upon collision with an obstacle in the water. For the safety of a ship the integrity is important. In case a ship's hull is punctured or otherwise damaged, the integrity of the hull is lost and water will enter the hull. This may eventually lead to incapacitation of the technical means of propulsion and steering and to sinking of the ship. [4] Another important feature of a ship is its operability. Operability of a ship in this specification means that said ship is able to be propelled (by engine or wind), and said ship is able to hold its course and steer (i.e. manoeuvre), while being at least more or less upright in the water. It is clear that partial or complete flooding of a ship, or the inability to manoeuvre, decreases the operability. Therefore, operability in a preferred embodiment of the invention comprises manoeuvrability. Disclosure of Invention Technical Problem [5] It is a problem for a ship going through the water that a collision with an unseen or unexpected obstacle damages the hull, the keel, or the rudder. Technical Solution [6] An objective of the present invention is to decrease the damage of a collision of a ship with an obstacle by providing a collapsible and/or detachable provision of a part of said ship, wherein said provision collapses and/or detaches on impact of said part of a ship with an obstruction, thereby preventing at least in part damage to said ship and/ or flooding of said ship. The invention preferably maintains the operability of said ship. A provision of the invention is a technical feature comprising a collapsible and/or compressible and/or detachable and/or folding part that gives way under load, thereby absorbing at least part of the energy of said collision, preferably preventing at least part of the damage. A part of a ship means in this specification any part of a ship that encounters a collision with an obstacle. Preferable said part is a part of the bow, more preferably a part of the keel, more preferably a part of the rudder, more preferably a part of a rudder support brace. Of course, the forces resulting from a collision of a small boat with an obstacle are quite different from those resulting from the collision of a large boat with an obstacle. Notwithstanding these differences in mass and forces, the present invention discloses to a skilled person how to make and use the provision of the invention. It is in the art how to calculate for all different ships and sizes, a com¬ pressible and/or detachable and/or collapsible provision according to the invention. Advantageous Effects [7] The present invention prevents said rudder from becoming inoperable because it prevents said rudder from being stuck against the bottom of said hull or from puncturing said hull. In the above-described situations, a part of said rudder either collapses or is compressed or folded or is detached. For example, upon impact of an obstacle on said rudder in a forward moving ship, the upper distal part of said rudder blade is forced against the ship's bottom and is for instance detached or compressed or folded into said rudder blade. In another example, when a rudder is subjected to a collision from below, a part of the lower end of said rudder blade is for instance detached or compressed or folded into said rudder blade. In yet another example, when a rudder is subjected to a collision from the rear, the trailing edge of said rudder is for instance detached, or compressed or folded into said rudder blade. Therefore, the invention discloses a method according to the invention, comprising providing a rudder with a detachable or collapsible or foldable provision of a part of said rudder. [8] The invention also discloses a collapsible or detachable provision according to the invention, wherein said at least in part preventing damage to said ship maintains the water tightness and/or the operability of said ship. [9] The present invention also discloses a ship which, using the parts of the invention, can be designed and build in a much lighter, yet sturdier way than conventional ships. Said ship according to the invention is better safeguarded against collisions than a con¬ ventional ship, because only in those place were it is important to protect the ship from collisions, the hull or keel or rudder are provided with a provision of the present invention. Therefore, in another embodiment, the invention discloses a ship comprising a provision according to the invention, and it also discloses a rudder, or a keel, or a hull comprising a provision according to the invention. [10] Of course, a ship can now be fitted with the provision of the invention in all places that are subjected to impacts by obstacles, while the ship in other places, which are not prone to a collision, is build in a way that is only sturdy enough to withstand the forces of the water and/or the rigging on the ship. Examples of places that are damaged by obstacles and which are protected by the invention are for example: parts of a ship's bow, and of the front side and the bottom side of a keel, and of the rear side, the bottom side and the upper side of a rudder blade, and of the front side of a keel support brace or skeg. A boat that is improved with provisions according to the present invention is a much safer boat that maintains its operability much longer than con¬ ventional boats. Of course, the invention is useful for all kind of boats, ranging from very small boats, to yachts and to large vessels. The invention is also very useful for catamarans. [11] In another embodiment, the invention provides a method to prevent puncturing of a ship's hull and/or to maintain the operability of a ship comprising providing a ship with a provision according to the invention. [12] As an example, the rudder is selected to further explain the invention. A rudder is particularly prone to damage by collisions. In many sailing yachts, the rudder sticks out of the hull only supported on a rudderstock. In other yachts, the rudder is supported by a rudder support brace or skeg. In a forward moving boat the collision force hits the rudder on the front or the side and bends the rudderstock backward. This backward bending results in an upward movement of the upper edge of the rudder blade, aft of said rudder stock, resulting in damage of the hull and/or inoperability of the rudder. In case of a collision of an obstacle like for instance a shallow bottom or a rock with the bottom part of the rudder, the bottom part of the rudder is hit in a more or less upward direction, damaging the rudder support and/or the rudderstock. In another situation, for example when the ship is moving backwards, or when a ship has a collision from the rear with a moving obstacle such as another ship, the rudder is bend forward or sideward. Bending of a rudderstock often causes said rudder to become stack against the ship's hull bottom. [13] More preferably a method of the invention is disclosed, wherein said collapsible part comprises a hard shell and a compressible core material. More preferably a method of the invention is disclosed, wherein said collapsible part comprises a cap fitting over said rudder blade. More preferably a method of the invention is disclosed, wherein said collapsible part retracts into a recess of said rudder blade. More preferably a method of the invention is disclosed, wherein said foldable part turns on a pivot and sinks into the main body of said rudder blade. More preferably a method of the invention is disclosed, wherein said detachable part is separated from said rudder blade along a region comprising a material of lesser density and/or rigidity. [14] Securing the water tightness is important for other parts of the ship as well, because the water tightness of a ship after a collision is maintained in that way. A provision of the invention, for example a compressible or collapsible provision of a part protects said part that is subjected to a collision by absorbing the collision energy and by forming a layer of protective material on top of the skin, thereby preventing the skin from being punctured. The space behind the parts that are provided with a provision of the invention is made watertight with respect to the rest o the hull, creating a watertight collision zone. The invention therefore also discloses a method wherein said rudder blade and/or said ship's part is a closed compartment. [15] Specific embodiments of the invention are set forth in the dependent claims. [16] Further details, aspects and embodiments of the invention will be described, by way of example only, with reference to the attached drawings. Description of Drawings [17] Figure 1. An example of a safety rudder with a collapsible or a removable part of the rudder blade. Part 1 is the rudder stock, part 2 is the main body of the rudder, part 3 is a collapsible or removable or foldable part, part 4 is the ships hull. [18] Figure 2. Some examples of a preferred embodiment for a collapsible part of a rudder (cross section). Part 1 is the main part of the rudder, part 2 is the collapsible part of the rudder, part 3 is material of lesser density than both parts of the rudder. 2A. Compressible part comprising a hard shell with a soft core. 2B. Compressible part comprising a hard shell with an air chamber and optional spring. 2C. Collapsible part retracting inside the rudder blade. 2D. Collapsible part folding inside the rudder blade. [19] Figure 3 . Some examples of a preferred profile of the adjoining faces of the removable part and the main part of the rudder blade (cross-section). [20] Part 1 is the main part of the rudder, part 2 is the removable part of the rudder, part 3 is material of lesser density than both parts of the rudder, part 4 is surface smoothing material. 3 A. One sided slope. 3B. Double sided slope [21] Figure 4. A schematic side view of a ship 10 provided with deformable provisions 11,12,13 on the hull 14, the keel 15 and the rudder 16. Best Mode [22] Example [23] A rudder is vulnerable for a collision with an obstacle. A rudderstock bends upon collision with an obstruction or an obstacle under water like for example a buoy or a buoy chain or another underwater object or in case of grounding of a ship. In a ship moving forward, a collision causes said rudderstock to bend backward, causing the rudder blade to move backwards, and the upper distal tip of said rudder blade to move upwards into the direction of the ship's bottom (figure 1). Said upper part of a rudder blade is normally made of hard material like for example iron or wood or some composite material like for example glass reinforced polyester or epoxy. Said hard upper part of said rudder blade then either damages the hull, possibly causing puncturing of the hull and even flooding of the hull, or it restricts said rudder blade in its movement because of the friction between said rudder blade and said hull bottom. In a round bottom or V shaped hull, said rudder blade is forced upwards on either side of the deepest part of the hull bottom and said rudder is fixed sideways which causes said ship to turn around in circles. In flat bottom boats there is a danger of puncturing the hull by upward movement of said hard upper part of said rudder. Because of the contact of said rudder blade with said ship's hull, the possibility to steer said ship is decreased. In the above-described situation said ship's safety is threatened, either by a punctured and leaking hull, or by the inability to steer said ship.. [24] Providing a collapsible or detachable provision to a part of said rudder prevents said rudder from being stuck or from puncturing the hull. [25] Collapsible in this specification means that a part of said rudder blade can move downward and/or sideways and is thereby reduced in volume. Said reduction in volume of said collapsible part of said rudder blade is preferably caused by compression. [26] Therefore, said collapsible rudder part comprises a compressible part such as for example foam (Fig. 2A), or a hollow space (Fig. 2B), or a space filled with water. In another embodiment, said collapsible part of said rudder for example comprises a hollow shell in the shape of said rudder part, said hollow part attached to said main part of said rudder, or comprises for example a hollow shell as mentioned before held in place by an attachment on the front side and with a spring inside to ensure a proper placement of said collapsible part on the main part of said rudder. [27] In a preferred embodiment, said foldable provision of the invention is pushed into a recess of said rudder blade (Figure 2 C and figure 2 D). [28] In a more preferred way the invention also discloses that said removable or detachable part is separated from said rudder blade by downward or sideward forces. Removable or detachable is this specification means that a part of said rudder blade detaches from the rest of said rudder blade and breaks away. In this embodiment of the invention, said removable part is lost, but the rest of said rudder blade is still relatively undamaged and steering is not hampered. This is preferably achieved by positioning a layer of softer material between said removable part and the rest of said rudder blade. Sheering forces in a sideway direction separate said removable part from the main part of said rudder blade through said layer of softer material. Said layer of softer material may be any material that has a lesser density or integrity that the material of the rest of said rudder blade. Said rudder blade is for example made of wood, or wood with a covering of glass fibres in epoxy or in polyester resin, or is for example made of a foam core with a hard glass fibre-reinforced shell. Therefore, in another embodiment, the present invention provides for a method to protect the operability or steerability of a ship comprising replacing the upper part of a rudder blade by a removable or collapsible or foldable part, wherein said removable part is separated from said rudder blade by a material of lesser density and/or rigidity. It is preferred that the remaining rudder blade is not open to the water, once said removable or collapsible part of said rudder is removed or collapsed. This is preferably achieved by sealing the border of said rudder blade at the junction with said removable or collapsible part with a suitable sealant, for example epoxy resin or polyester resin Or any other watertight sealant. More preferably, said sealant is capable of bonding to glass fibres, resulting in a glass reinforced seal. [29] Therefore, in another embodiment, the present invention provides for a method to protect the steerability of a ship comprising replacing a part of a rudder blade by a removable or collapsible part, wherein said rudder blade is a closed compartment. Preferably the profile of the parts at the junction of said removable part and said main part of the rudder blade assists in a clean and easy breaking away of said removable part when the sheering forces are sufficient. This is achieved by for example joining the smaller removable or detachable part and the main part of said rudder by a band of material of a lesser density like for example foam, or by cutting the adjoining faces of the two parts under an angle, and gluing said adjoining faces together with a layer of material of a lesser density like for example foam in between (Fig. 3 A) or by for example cutting a ridge on each said adjoining face and filling in the open spaces with material of a lesser density tike for example foam. In a more preferred embodiment, the ridges of said adjoining faces are not opposite of each other, but are just next to each other (Fig. 3 B). The frontal edge of said removable or collapsible part of said rudder may be adhered to the main body of said rudder blade by material of a lesser density like for example foam or another easily detachable material. In another embodiment, the separation line of said removable or collapsible part is a straight line from aft of the rudderstock, downward in an oblique direction to the trailing edge of the rudder blade, as is for example depicted in figure 1. In another embodiment, the separation line is near the lower part of the rudder blade (or near the trailing edge of said rudder (figure 1). [30] In a preferred embodiment, said outer layer of the junction between said removable or collapsible part and said main body of the blade are treated with a filler compound or a rubber compound to seal the junction and smooth the surface of said rudder blade. Preferably, said materials are not an obstruction for the action of collapsing or removing of said removable or collapsible part of said rudder blade. Of course, the same Mnd of provision is disclosed in other parts of the rudder, as for example the bottom part or the trailing edge part of the rudder blade, or the rudder support brace or skeg, and/or the front side or the bottom side of the keel and/or the bow of the hull. [31] The example is one of many working examples, and the invention is not limited to the example described above. Of course, with the invention disclosed by this spec¬ ification, a skilled person is given guidance for various applications of this invention to make and use a detachable or compressible or foldable provision to a part of a ship. With modern electronic technology, it is for example within the art to place electronic contacts in the parts with a provision of the invention that warn the sailor which part of a ship has undergone a collision and how the status of said part is after the collision. For example, an electronic device gives information about the extent to which a part has collapsed, or is detached. Such electronic devices also warn the sailor for localized penetration of water into the hull or its appendages. Now the present invention is disclosed, it is also within the art to combine more than one of the provisions of the invention. For example, a part of a ship is provided with the provision to collapse, and also with the provision to break away. Said part thus first collapses or is compressed upon collision, and then breaks away if the forces of the impact are sufficiently large. Any combination of said provisions in a ship's part are herewith disclosed.