Patent Publication Number: US-5295453-A

Title: Catamaran having a submerged airfoil

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention generally relates to a water vessel and in particular to a catamaran including a submerged connection member connecting a pair of floats or hulls. 
     2. Description of the Prior Art 
     A catamaran typically includes a pair of hulls or floats which are joined by a deck or some other connecting structure bridging between them. However, such a connecting structure normally extends in the air and not submerged under the water. In the past, the present inventor invented a working catamaran including a pair of floats joined by a submerged connecting member for use in collecting solid floating materials on the water surface as disclosed in the Japanese Utility Model Post-examination Publication No. 57-18238. The working catamaran disclosed in this publication is illustrated here as FIGS. 4a through 4c. As shown in these figures, the working catamaran generally indicated by numeral 1 includes a pair of floats or hulls 2. In the illustrated structure, a bow tip end 3 of each of the floats 2 is located toward well 4, which is a region sandwiched between the floats 2 spaced apart from each other, with respect to its longitudinal center line CL. And, each of the floats 2 has a substantially vertical inner side wall at its bow portion a, intermediate portion b and stern portion c. In particular, the inner side walls b of the opposite floats 2 extend substantially in parallel each other and with respect to the longitudinal center axis CL of each of the floats 2. 
     A connecting member 5 extends between the opposite inner side wall surfaces a of the floats 2 at those portions d that are submerged under water. The connecting member 5 is fixedly attached at each end to the floats 2 so as to provide an increased structural integrity. Because of the provision of this connecting member 5, the floats 2 may be spaced apart at a greater distance than otherwise possible. This is particularly important in such a working vessel since it allows to sweep a wider water surface. As shown in FIG. 4b, the connecting member 5 is defined as a waterfoil having a particular cross sectional shape. That is, the connecting member 5 includes a curved bottom surface 5a which extends from the bow end downwardly until it hits the bottom of the float 2 with increasing its radius of curvature, and, then, extends along the bottom of the float 2 until it reaches its downstream end 5c. The curved portion of the bottom surface 5a extends generally from the forward end of the submerged inner side wall surface portion d to the bottom e of the bow portion a, whereby a space 5d having a generally triangular-shaped cross section is defined below the bottom surface 5a. The connecting member 5 also includes a curved top surface 5b which first extends substantially horizontally from the bow end over a predetermined distance and then extends downwardly toward the downstream end 5c with its radius of curvature gradually increasing. The particular shape of the connecting member 5 as described above is advantageous in providing stability while the working catamaran is in operation. 
     As also shown in FIG. 4b, a rotating impeller 7 is disposed above the connecting member 5 and extending between the opposite floats 2. And, thus, as the working catamaran 1 cruises, any solid materials floating on the water surface is first trapped in a mouth between the opposite floats 2 and then driven into the downstream portion of the well 4 defined between the opposite floats 2 by means of the rotating impeller 7. Also provided is a deck 6 extending between the floats 2 and in the air, which provides a working area for an operator and a bridge or operating room may be provided on the deck 6. 
     Thereafter, the present inventor improved the catamaran disclosed in the above publication so as to allow to collect oil spills rather than solid floating materials and filed a Japanese Patent Application, No. 58-29378, and also a U.S. patent application, Ser. No. 582,199, which has been issued as U.S. Pat. No. 4,551,244, which is hereby incorporated by reference. This improved catamaran also includes a connecting member which is similar in many respects to the connecting member 5 illustrated in FIGS. 4a through 4c and described above. 
     As described above, the working catamaran provided with a submerged connecting member is advantageous in attaining stability in operation and such a submerged connecting member also has some merits in providing stability while cruising particularly through rough waters. However, it has been found that the bow end tends to become submerged under water when the working catamaran cruises at a relatively high speed. Since there are those cases in which the working catamaran is required to cruise at a relatively high speed so as to reach a site as soon as possible so that a collecting operation can be carried out immediately, it is also important that the working catamaran can cruise at a relatively high speed without problems. However, in the prior art structure, since the bow end tends to become submerged when the cruising speed increases, there has been an upper limit in the cruising speed. According to the experiments conducted by the present inventor, it has been found that a catamaran having a structure described above has a tendency to cause its bow end to be submerged under water when its cruising speed is increased even if the connecting member 5 has been removed. Therefore, there is an upper limit in the cruising speed for such a catamaran irrespective of whether it is provided with a connecting member or not. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is provided a catamaran comprising a pair of floats or hulls spaced apart from each other and a connecting member having, in general, an airfoil cross sectional shape, which extends between the pair of floats and which is submerged when said catamaran is afloat. In a preferred embodiment, the connecting member has a generally convex top surface and a generally convex bottom surface to define the airfoil cross sectional shape. A base line of the airfoil shaped connecting member, which is defined by drawing a straight line between its leading and trailing edges, is set at an angle in a range between 15° and 25° and most preferably approximately at 20° inclined upwardly forwardly with respect to a horizontal surface. 
     In a preferred embodiment, the connecting member has a two-part structure, i.e., airfoil-shaped part and a tail part which extends continuously from the trailing end portion of the airfoil-shaped part in the downstream direction. The tail part is preferably comprised of a parallel plate which extends in the downstream direction from the downstream end portion of the airfoil-shaped part. The tail part may extend generally horizontally or at an angle, if desired. 
     It is therefore a primary object of the present invention to provide an improved catamaran which can cruise at a relatively high speed without problem. 
     Another object of the present invention is to provide an improved working catamaran which is stable both at high speeds and low speeds. 
     A further object of the present invention is to provide an improved catamaran which can be advantageously used for collecting floating materials, solid or liquid, on the water surface efficiently. 
     Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1a is a schematic illustration showing in transverse cross section a catamaran constructed in accordance with an embodiment of the present invention; 
     FIG. 1b is a schematic illustration showing in side elevation with a part of the catamaran of FIG. 1a removed; 
     FIG. 1c is a schematic illustration showing in plan view the catamaran of FIG. 1a; 
     FIG. 2 is a schematic illustration showing on an enlarged scale the airfoil shaped connecting member provided in the catamaran shown in FIGS. 1a through 1c; 
     FIG. 3 is a graph showing the cruising performance of the catamaran shown in FIGS. 1a through 1c; and 
     FIGS. 4a through 4c are schematic illustrations showing a prior art working catamaran suitable for use in collecting floating materials on the water surface. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIGS. 1a through 1c, a catamaran 1 constructed in accordance with one embodiment of the present invention will be described in detail. As shown, the present catamaran 1 includes a pair of floats or hulls 2 which are elongated in shape and spaced apart from each other over a predetermined distance to thereby define a well 4 therebetween. In the illustrated embodiment, each of the floats 2 has a bow end 3 which is shifted in position toward the well 4 with respect to its center line CL. Each of the floats 2 also has a substantially vertical inner side wall surface at its bow portion a, intermediate portion b, and stern portion c. The side walls of the intermediate portions b are substantially parallel to each other and with respect to the center lines CL of the floats 2. It is to be noted, however, that the present invention should not be limited only to floats having such a specific structure, and the present invention may be applied to floats having various other structures. 
     The catamaran 1 further includes a connecting member 5 extending between the pair of floats 2 as fixedly attached thereto at its opposite ends. In this embodiment, the connecting member 5 is disposed to extend between submerged portions d of the inner side wall surfaces at the bow portions a as extending from the bow end to a bottom e of the float 2. Thus, the connecting member 5 remains submerged in the water when the catamaran 1 is afloat. As described before, the provision of such a connecting member 5 allows to set a distance between the opposite floats 2 as wide as possible because of an increased structural integrity given by the connecting member 5. As will be described in detail later, the connecting member 5 of the present invention has a generally airfoil-shaped cross section. In the illustrated embodiment, the connecting member 5 has a two-part structure, including a forward airfoil-shaped section 5A and a tail section 5B which is connected to the trailing end of the forward section 5A. As far as the prevention of bow dipping is concerned, the forward section 5A plays a major role since this is the section which provides a lift to the catamaran 1 during operation. The tail section 5B mainly serves to guide the flow of water toward its stern. As will be described in detail later, the forward airfoil-shaped section 5A is preferably inclined forwardly at a predetermined angle, which ranges between 15° and 25° , most preferably at 20°. 
     The catamaran 1 is also provided with those elements which are ordinarily provided in a water vessel, such as an engine, a transmission, a screw and a rudder, so that the catamaran 1 can cruise by itself at any desired speed and in any desired direction. The catamaran 1 is also provided with a deck 6 extending in the air between the pair of floats 2. The deck 6 provides a space for mounting thereon a bridge or operating room. In the present embodiment, since the catamaran 1 is mainly designed to serve as a working catamaran for collecting floating materials on the water surface, a rotating impeller 7 is also provided above the airfoil-shaped connecting member 5 extending between the pair of opposite floats 2. The rotating impeller 7 is disposed such that it is partly submerged in the water at its bottom portion. Thus, when the catamaran 1 cruises with its impeller 7 in rotation in a predetermined direction, any floating material on the water surface which has been trapped in a mouth defined between the pair of opposite floats 2 is positively guided into a storing section defined in the downstream side of the well 4. In this case, since the tail section 5B is provided at the trailing end of the airfoil-shaped forward section 5A, the floating material positively guided by the rotating impeller 7 is properly guided into the storing section of the well 4 and prevented from being pushed deeper into the water to escape from the well 4. It should be noted, however, that although the present invention is advantageously applicable to such a working catamaran for collecting solid and liquid floating materials on the water surface, the present invention should not be limited only to such applications, and the present invention is also applicable to any other types of catamarans, such as those intended to be used for pleasures, and to a working catamaran for collecting oil spills as disclosed in the above-identified United States Patent. 
     Referring now to FIG. 2, the structure of the airfoil-shaped connecting member 5 provided in the catamaran 1 shown in FIGS. 1a through 1c will be described in detail. As shown in FIG. 2, the connecting member 5 constructed in accordance with one embodiment of the present invention generally has a two-part structure, i.e., a forward section 5A and a rearward or tail section 5B. The forward section 5A is so designed to have an airfoil-shaped cross section and the tail section 5B may be any shape, such as a plate having generally parallel top and bottom surfaces as shown in FIG. 2. 
     The airfoil-shaped forward section 5A in the illustrated embodiment is defined by a pair of top and bottom curved surfaces, both of which are smooth curved surfaces and which are both generally convex with respect to its base line defined by a straight line connecting the leading and trailing edges of the airfoil-shaped forward section 5A. In the illustrated specific embodiment, the base line has a length of 1,522 mm. Also of importance, the airfoil-shaped forward section 5A is so arranged with its base line inclined forwardly to define a predetermined attack angle, which may range between 15° and 25°, most preferably approximately at 20°. Although the airfoil-shaped connecting member 5 of this embodiment has a smooth and convex curved surface for both of its top and bottom surfaces, the connecting member 5 may have any other shapes without departing from the spirit and scope of the present invention as long as such other shapes can provide a proper lift to the catamaran 1 so as to prevent the bow from being submerged when cruising at a relatively high speed. For example, the bottom surface of the airfoil-shaped forward section 5A may be convex with respect to its base line at least partly. 
     As described before, the forward section 5A has a main function of providing a lift to the bow of the catamaran 1 when cruising at a relatively high speed, and the tail section 5B has a main function of guiding the flow of water after passing the airfoil-shaped forward section 5A. Thus, without departing from the spirit and scope of the present invention, the tail section 5B may be omitted if there is no need to provide such a guide for the flow of water in the downstream of the airfoil-shaped forward section 5A. For example, when the present invention were applied to a working catamaran for collecting floating materials, such as debris or oil, on the water surface, then the provision of such a tail section 5B would be required because of its function. However, if the present invention were to be applied to a catamaran for other purposes, such as sightseeing, transportation or the like, then the provision of such a tail section 5B would not be necessary. 
     FIG. 3 shows a lift characteristic of the airfoil-shaped connecting member 5. In FIG. 3, the ordinate T is an indication of a height of the bow, where T=(da-df)/L, and the abscissa FN is the Froude Number which is an indication of the cruising speed, where FN=V/SQRT(L×g). Here, da=forward draft, df=aft draft, L=effective length of the catamaran, V=cruising speed, g=9.8 m/sec 2 , and KT=knot. In the specific embodiment shown in FIG. 3, L=11.5 meters. As shown in FIG. 3, when the cruising speed was 7.9 KT, T=L D  (=-10 cm) where L D  is a non-dimensional depth for the water surface, so that the bow was pulled into the water by 10 cm as compared with the case when the catamaran 1 is not cruising or cruising at a very slow speed. On the other hand, as shown in FIG. 3, when the cruising speed of catamaran 1 increased beyond 7.9 KT, the bow started to received a lift so that the bow started to pushed upward relative to the water surface. And, at the cruising speed of FN=0.49 (10.1 KT), T=L U  (25 cm) where L U  is non-dimensional height from the water surface, so that the bow was raised in height relative to the water surface as compared with the case when the catamaran 1 was not cruising or cruising at a relatively low speed. 
     As indicated in FIG. 3 and described above, the catamaran 1 starts to receive enough lift to keep its bow properly above the water surface once it exceeds a critical speed so that the present catamaran 1 can cruise stably at a relatively high speed. In addition, the bow is not pulled downward excessively at low and intermediate speeds, at which a collection operation is carried out most effectively, so that the present catamaran 1 is stable at any cruising speed. 
     While the above provides a full and complete disclosure of the preferred embodiments of the present invention, various modifications, alternate constructions and equivalents may be employed without departing from the true spirit and scope of the invention. Therefore, the above description and illustration should not be construed as limiting the scope of the invention, which is defined by the appended claims.