The present invention relates to a raft arrangement which is to be towed after a vessel, especially in connection with seismic measurements of the sea bottom, where the raft is provided with a water flow passage between two side hulls and an upper deck and in an opening between the float members, at a level beneath the normal level of the sea surface, is also provided with guide surfaces for defining longitudinal rearwardly directed water guide ducts.

In NO 156 717 a raft of the afore-mentioned kind is illustrated and described. The present invention repre¬ sents a further development of the solution according to said NO patent. In rafts of said kind problems can occur in main¬ taining the tow buoy on the correct course and in the correct position relative to the mother vessel and also in the desired stable position on the surface of the sea, especially in heavy wave movements and in strong winds. With the present invention the aim is a solution where heavy movements of the raft on the sea surface are avoided, so that the raft can assume a relatively stable position in the sea even in strong winds and with rela¬ tively heavy movements on the sea surface. In other words the aim is to make the solution as known from the afore¬ mentioned NO patent 156 717 still better suited for the purpose.

The arrangement according to the invention is characterised in that on the under side of the side hulls from a distance behind their bows to the rear end of the raft longitudinal water guide ducts are arranged for lead- ing separate water currents parallel to the water flow passage between the side hulls, that at the front end of the raft a middle hull is arranged which divides the water flow passage between the side hulls into two current por¬ tions, and that substantially in alignment with the middle hull, at a distance behind the rear portion of the middle hull, a middle, elongate water guide duct is arranged for collecting parts of the two current portions for a course through the middle water guide duct.

By means of the said water guide ducts arranged sideways and the rear, middle water guide duct one has the possibility to maintain the raft in an effective, sub¬ merged contact with the water on the under side off the raft on opposite sides of the raft and at the rear portion of the raft while the side hulls ensure the intended buoy- ancy of the raft. Correspondingly with the middle front hull extra buoyancy can be ensured at the middle, front portion of the raft, while the middle water guide duct can ensure that the rear portion of the raft is maintained in contact with the water immediately behind the middle, front hull portion.

In order to ensure a sufficient supply of water\' in a current to the rear, middle water guide duct, the front middle hull portion is provided on the under side with a middle, front water guide duct which has an outlet immedi- ately in front of the rear, middle water guide duct.

In practice the water guide ducts can be designed on the under side of the side hulls and on the under side of the middle, front hull as relatively narrow keel-forming water guide ducts which provide effective side steering at the same time as they provide minimal buoyancy.

It has been customary in rafts of the afore¬ mentioned kind to convey electric current via cable

between vessel and raft. This solution is both complicated and expensive and in addition unreliable during use in various practical conditions at sea, such as during the occurrence of heavy wave movements and the occurrence of strong winds. Battery operation has also been employed, but such a solution is also complicated, expensive and unreliable.

With the present invention the aim is to equip the raft with a separate current generator, which provides a relatively simple and inexpensive and reliable solution. A special objective is to be able to utilise the tension of the vessel for transferring energy to the current gener¬ ator. More specifically the aim is to utilise the energy in the water flow passages which are formed between hulls of the raft for the operation of the current generator.

In this connection the arrangement according to the invention is characterised in that the raft is equipped with a drive means submerged in the sea for an electrical current generator, which by means of tension which is supplied to the raft, can transfer energy for the oper¬ ation of the electrically driven equipment on the raft, the drive means of the current generator being placed in the rear, middle water guide duct at the stern of the raft. By means of the afore-mentioned solution, energy can be transferred by simple means from the vessel to the current generator of the raft by utilising the tension of the vessel. A particularly favourable positioning of the drive means of the current generator is obtained by arranging the drive means of the current generator at the stern of the vessel in connection with the rear, middle water guide duct, the latter duct being able to provide the basis for a reliable and effective operation of the current generator under the various conditions, even in significantly heavy seas and in significant winds. Corre¬ spondingly a particularly favourable placing of the cur¬ rent generator is achieved by receiving this in the front,

middle hull, which is arranged immediately in front of the middle, rear water guide duct.

This can be further guaranteed by fastening the tow line or the tow lines from the vessel to fastening points arranged at relatively low lying levels in the raft, in order thereby to ensure that the stern of the raft can be sufficiently tilted backwards/downwards the whole time so as to safeguard the placing of the drive means of the current generator in a submerged condition in the sea, in order thereby to maintain a constant flow of water to the drive means of the current unit even under difficult con¬ ditions on the sea.

Further features will be evident from the following description of a preferred embodiment having regard to the accompanying drawings, in which:

Fig.l shows a raft according to the invention viewed from the side.

Fig.2 shows the raft according to Fig.l viewed from the stern. Fig.3 shows the raft according to Fig.l viewed in perspective from the stern.

Fig.4 shows a section of the raft illustrated from the front and viewed in perspective .

Fig.5 shows a section of the raft illustrated from above/from the rear.

In Fig.1-3 there is illustrated a raft 10 which comprises two side hulls 11 and 12 and a front middle hull 13 for reception of a current generator (not shown fur¬ ther) together with a middle, rear water guide duct, in the form of a separate rear housing 14, for the reception of drive means 15 of the current generator.

In the illustrated embodiment the drive means 15 consists of a three bladed propeller, which is fastened to a relatively elongate drive shaft 16, which extends coaxi- ally of the current generator in the middle hull 13.

Alternatively various other types of rotatable or back- wardly and forwardly oscillatable drive means can also be

employed, without concrete embodiment of these being pro¬ vide herein.

The side hulls 11,12 are constructed from two elon¬ gate, pipe-shaped, sealed containers 11a, 12a which are preferably filled with floating material. The container

11a (and 12a) is provided with a front plate lib (and 12b) deflected obliquely downwards and obliquely sideways and an astern plate lie (and 12c) deflected obliquely down¬ wards. Each container 11a, 12a is provided on the under side with a keel-like, sideways arranged water guide duct 17. The duct 17 extends over approximately 3/4 of the length of the container to just behind the stern of the container. During towing of the raft in the sea the duct is flowed through by water, something which has special significance for stabilising the raft after movements of the sea and to keep the raft in the intended, correct position in the sea. The keel-like duct 17 is defined between two relatively high side plates 17a, 17b and a relatively narrow bottom plate 17c. The duct 17 is illu¬ strated with a constricted rear outlet 17d. There are shown first fastening points 18 of the tow line on the inner side plate 17a of the ducts and alternative second fastening points 19 on the outer side plate 17b of the ducts, both fastening points being shown approximately midships on the raft at the lowermost portion of the raft.

The aim is that the equipment on the raft has a centre of gravity, which substantially coincides with the transverse axis through the fastening points, and that the raft, as a consequence of the three hulls employed at the front end of the raft, provides a large buoyancy in front and a more reduced buoyancy astern, so that the raft is naturally tilted downwardly in the sea at the stern during towing. The middle hull 13 is designed with a front plate

13a extending obliquely downwards and an equivalent astern plate 13b extending obliquely downwards, which is arranged

just astern of midships.

The housing 14 for the drive means 15 of the current generator is arranged at a certain axial distance from the astern plate 13b of the middle hull 13 and is designed funnel shaped with the large funnel opening or intake opening 14a turned towards said astern plate and with the outlet opening 14b of the funnel arranged just before the rearmost portion of the raft.

Between the middle hull 13 and the housing 14 there extends an annular (bevelled) series of grill-forming connecting rods 20.

In a corresponding manner to the side hulls 11,12 the middle hull 13 is provided with a water guide duct 21 consisting of two vertical side plates 21a, 21b and a horizontal bottom plate 21c. Above the middle hull 13 is connected to the side hulls 11,12 by means of a pair of front deck plates 22.

The side hulls 11,12 and the middle hull 13 are connected to each other just behind the deck plates 22 by means of a main deck 23 which rests against the upper edge of the middle hull 13 and is supported by skirt portions 23a and 23b against the side hulls 11,12 a distance below the upper edge of the side hulls.

Sealed storage boxes 24 and 25 are shown with asso- ciated access hatches 24a, 25a on the main deck 23 and equivalent storage boxes 26 and 27 with associated access hatches 26a, 27b on one of the side hulls 11,12 respec¬ tively. At the rear portion of the main deck 23 there is fastened a collapsible mast 28 which has equivalent col- lapsible support stays 29 anchored at the stern of the raft at the end of each of their associated side hulls 11,12. In the storage boxes 24,25,26,27 and in a storage box 30 in the mast 28 diverse electrical equipment is present which is driven by the current generator in the middle hull 13.

The housing 14 for the drive means 15 of the current generator is carried on the top by a lower transversely

extending and axially extending support foil 31 which is fastened at the stern of the rε t to a lower portion of the keel-like ducts 17 of the side hulls 11,12. The hous¬ ing 14 is braced with a pair of horizontal side flaps 32, a pair of vertical side flaps 33 and a pair of obliquely extending side flaps 34 which bear against an associated construction 35 which braces and supports the foil 31 relative to the side hulls 11,12. Between the foil 31 and a pair of flaps 32,33 and 33,34 there are formed ducts 36 and 37 parallel to the housing.

By means of the front portion of the construction 35 a pair of guide surfaces 35a is formed which converge aft from the middle of the raft towards the intake opening 14a of the housing 14. It will be evident from the drawings that a water flow passage 38 which is defined between the side hulls 11,12 at the front end of the ,aft by means of the middle hull 13 is divided into two front passage portions 38a, 38b. The passage portions 38a,38b are guided together into a common passage 38c in the intermediate space between the middle hull 13 and the housing 14, the passage 38 being constricted gradually in said intermediate space. A large portion of the flow of the water in the passage 38 is forced through the housing 14, while other portions are distributed over the ducts 36,37 or are led sideways and heightwise above the ducts 36,37 and the housing 14. In this way one can ensure that during the towing operation the housing 14 is submerged in water or surrounded by the flow of water from the passage 38 during varying movements of the raft in the sea.

The illustrated placing of the drive means 15 of the current generator in combination with the tension which the raft is exposed to via the low lying line fastenings 18 (19) and the arrangement of the drive means in the middle, rear water guide duct ensures an effective and stable drive power for the drive means of the current generato