Abstract:
Disclosed is an oar, which has a supplementary blade pivotally connected to one end of the blade adjacent to the shaft thereof to increase the area of the water expelling surface, enhancing the rowing efficiency.

Description:
BACKGROUND OF THE INVENTION 
   (a) Technical Field of the Invention 
   The present invention relates to an oar for rowboat and more particularly, to such an oar, which is equipped with a pivoted supplementary blade to enhance the rowing. 
   (b) Description of the Prior Art 
   When rowing a rowboat, an oar or two oars shall be used. An oar for this purpose comprises a shaft, a grip fixedly provided at one end of the shaft, and a flat blade fixedly provided at the other end of the shaft. For comfortable gripping of the hand, the grip of an oar is made in a cylindrical shape. When rowing a rowboat, the user who sits in the rowboat must insert the blade of the oar into the water and then move the oar to force the blade against the water, and then lift the oar from the water, and then repeat the rowing procedure again and again. When forcing the blade against the water, a resisting force is produced to move the rowboat on the water. 
   A person who learned fluid mechanics or mechanics knows that the wider the area of the water expelling surface of an oar is the greater the water resisting force will be, i.e., an oar having a relative bigger blade surface area can move the rowboat on the water more quickly. However, it requires much effort to row an oar having a relatively greater blade surface area. 
   SUMMARY OF THE INVENTION 
   The primary purpose of the present invention is to provide an oar, which enhances the rowing efficiency by: increasing the pressure bearing surface of the blade when rowing the oar against the water and reducing the pressure bearing surface of the blade when lifting the oar from the water. According to one aspect of the present invention, the oar comprises a shaft, a fixed blade axially extending from one end of the shaft, and a supplementary blade pivotally connected to the connection area between the blade and the shaft and turns relative to the fixed blade between a received position and an extended position. The supplementary blade is received to the fixed blade without increasing the water resisting force when the user lifts the oar from the water during rowing. The supplementary blade is turned outwards from the fixed blade to the extended position to increase the pressure bearing surface (water expelling surface) of the oar, when the user rowing the oar against the water, thereby enhancing the rowing efficiency. According to another aspect of the present invention, a constraint member is provided to limit the turning angle of the supplementary blade relative to the fixed blade to a predetermined range. Preferably, the turning angle is within 90°˜180°. The constraint member can be a soft cord member connected between the fixed blade and the supplementary blade. Alternatively, the constraint member can be a stop rod fixedly provided at the connection area between the fixed blade and the shaft. 
   The foregoing object and summary provide only a brief introduction to the present invention. To fully appreciate these and other objects of the present invention as well as the invention itself, all of which will become apparent to those skilled in the art, the following detailed description of the invention and the claims should be read in conjunction with the accompanying drawings. Throughout the specification and drawings identical reference numerals refer to identical or similar parts. 
   Many other advantages and features of the present invention will become manifest to those versed in the art upon making reference to the detailed description and the accompanying sheets of drawings in which a preferred structural embodiment incorporating the principles of the present invention is shown by way of illustrative example. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an elevated view of an oar according to the present invention. 
       FIG. 2  is a schematic drawing showing the supplementary blade turned outwards from the received position to the extended position during rowing of the oar according to the present invention. 
       FIG. 3  is a schematic drawing showing the supplementary blade moved from the extended position toward the received position during rowing of the oar according to the present invention. 
       FIG. 4  is a schematic drawing showing an alternate form of the oar according to the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The following descriptions are of exemplary embodiments only, and are not intended to limit the scope, applicability or configuration of the invention in any way. Rather, the following description provides a convenient illustration for implementing exemplary embodiments of the invention. Various changes to the described embodiments may be made in the function and arrangement of the elements described without departing from the scope of the invention as set forth in the appended claims. 
   Referring to  FIG. 1 , an oar  1  in accordance with the present invention is shown comprising a shaft  12 , a fixed main blade  11  extended from one end, namely, the bottom end of the shaft  12 , and a movable supplementary blade  13  pivotally connected to the main blade  11  adjacent to the shaft  12 . The supplementary blade  13  can be turned between a received position where the contained angle between the main blade  11  and the supplementary blade  13  is zero, and an extended position where the contained angle between the main blade  11  and the supplementary blade  13  is within 180° but greater than 90°. 
   The main blade  11  has two lugs  111  arranged in parallel at one side thereof adjacent to the shaft  12 . The supplementary blade  13  has two lugs  131  disposed at two opposite lateral sides near one end thereof and pivotally connected to the lugs  111  of the main blade  11  with a pivot. 
   Referring to  FIG. 2 , a constraint member, for example, a cord member  14  is connected between the main blade  11  and the supplementary blade  13  to limit the maximum turning angle of the supplementary blade  13  relative to the main blade  11  to the aforesaid extended position. 
     FIG. 4  shows an alternate form of the present invention. According to this embodiment, the constraint member is a stop rod  15  fixedly provided at the connection area between the main blade  11  and the shaft  12  and adapted to stop the supplementary blade  13  when the supplementary blade  13  is turned outwards from the main blade  11  to the aforesaid extended position. 
   Referring to  FIG. 2  again, when rowing the oar  1  to force the main blade  11  against the water and to further move the rowboat on the water, the supplementary blade  13  is forced outwards from the main blade  11  by the resisting force of the water to the extent that the cord member  14  or stop rod  15  stop the supplementary blade  13  in the extended position. At this time, the main blade  11  and the supplementary blade  13  provide a big area of water expelling surface, producing a relatively greater force to move the rowboat forwards. 
   Referring to  FIG. 3 , when the user lifting the main blade  11  out of the water during rowing, the supplementary blade  13  is forced toward the main blade  11  by the water, thereby reducing the water resisting force, and therefore the user can lift the main blade  11  and the supplementary blade  13  from the water with less effort. 
   It will be understood that each of the elements described above, or two or more together may also find a useful application in other types of methods differing from the type described above. 
   While certain novel features of this invention have been shown and described and are pointed out in the annexed claim, it is not intended to be limited to the details above, since it will be understood that various omissions, modifications, substitutions and changes in the forms and details of the device illustrated and in its operation can be made by those skilled in the art without departing in any way from the spirit of the present invention.