the present invention will be described below in detail based on a preferred embodiment with reference to the drawings . a golf ball shown in fig1 and 2 usually has a diameter of approximately 42 . 67 mm to 43 . 00 mm . the golf ball includes an a dimple having a circular plane shape and a diameter of 4 . 5 mm , a b dimple having a circular plane shape and a diameter of 4 . 1 mm , a c dimple having a circular plane shape and a diameter of 3 . 6 mm , and a d dimple having a circular plane shape and a diameter of 2 . 8 mm . the number of the a dimples is 130 , that of the b dimples is 150 , that of the c dimples is 60 and that of the d dimples is 32 . the total number of the dimples of the golf ball is 372 . fig3 is a typical enlarged sectional view showing a part of the golf ball in fig1 . fig3 shows a section taken along the deepest portion of the dimple . in fig3 , the diameter of the dimple is shown by an arrow d . the diameter d represents the distance between both contacts with a common tangential line drawn on both ends of the dimple . the dimple volume represents the volume of a portion surrounded by the virtual sphere of the golf ball ( which is a sphere on the assumption that the dimple is not present and is shown in a two - dotted line of fig3 ) and the surface of the dimple . the area of the dimple represents the area of a region surrounded by the contour of the dimple ( that is , the area of a plane shape ) when the center of the golf ball is seen at infinity . in the case of the circular dimple , an area 9 is calculated by the following equation : in the golf ball shown in fig1 , the a dimple has an area s of 15 . 9 mm 2 , the b dimple has an area s of 13 . 2 mm 2 , the c dimple has an area s of 10 . 2 mm 2 , and the d dimple has an area s of 6 . 2 mm 2 . accordingly , the total area s of the dimple areas is 4855 . 7 mm 2 . the total area s is divided by the surface area of the virtual sphere so that a surface area occupation ratio y is calculated . in the golf ball , the surface area occupation ratio y is 0 . 848 . the surface area occupation ratio y implies an area ratio at which all the dimples occupy the spherical surface of a virtual sphere . by dividing the surface area occupation ratio y by the total number of the dimples , a mean occupation ratio y is calculated . in the golf ball , the mean occupation ratio y is 0 . 00228 . the mean occupation ratio y implies an area ratio at which dimple having a mean area occupy the spherical surface of the virtual sphere . in the case in which a designer is to design a dimple pattern having a high surface area occupation ratio y , he ( she ) can use means for increasing the number of the dimples and means for increasing the diameter d of the dimple . when the designer mainly employs the means for increasing the diameter d to achieve the surface area occupation ratio y , a golf ball having a mean occupation ratio y of 0 . 00220 or more is obtained . if the mean occupation ratio y is less than 0 . 00220 , the drag coefficient ( cd ) is increased in a region in which the flight speed is high , resulting in an insufficient flight distance of the golf ball . in this respect , the mean occupation ratio y is more preferably 0 . 00225 or higher , further preferably 0 . 00230 or higher , and particularly preferably 0 . 00250 or more . the golf ball having an extremely high mean occupation ratio y is hard to design with respect to the maintenance of the original golf ball features where the golf ball is almost a sphere . therefore , an ordinary golf ball has a mean occupation ratio y of 0 . 00300 or less . it is preferable that the surface area occupation ratio y should be 0 . 80 to 0 . 90 . if the surface area occupation ratio y is less than this range , the lift of the golf ball might become insufficient during flight . in this respect , the surface area occupation ratio y is more preferably 0 . 81 or more and more particularly 0 . 83 or more . if the surface area occupation ratio y exceeds the range , the trajectory might be too high . in this respect , it is particularly preferable that the surface area occupation ratio y should be 0 . 87 or less . in the golf ball shown in fig1 , a dimple has a contour length x of 14 . 1 mm , the b dimple has a contour length x of 12 . 9 mm , the c dimple has a contour length x of 11 . 3 mm and the d dimple has a contour length x of 8 . 8 mm . in the golf ball , the contour length x is the sum of the contour lengths x , which is 4730 . 0 mm . the contour length x of the dimple represents a length measured along the contour of the dimple . for example , in the case in which the dimple has a triangular plane shape , the total length of the three sides is represented by the contour length x . since these sides are present on a spherical surface , they are not straight lines but circular arcs in a strict sense . the length of the circular arc is set to be the length of the side . in the case of a circular dimple , the contour length x is calculated by the following equation : it is preferable that the surface area occupation ratio y and the total contour length x should satisfy the relationship in the following expression ( i ). the golf ball has a smaller total contour length x as compared with the surface area occupation ratio y . the golf ball has a small drag coefficient ( cd ) during a flight and an excellent flight performance . in the knowledge of the inventor , there has not been a golf ball satisfying the expression ( i ). in respect of a reduction in the drag coefficient ( cd ), it is more preferable that the total contour length x and the surface area occupation ratio y should satisfy the following expression ( ii ), further preferably the following expression ( iii ), and particularly preferably the following expression ( iv ). in order to maintain the original feature that the golf ball is an almost sphere , the total contour length x and the surface area occupation ratio y are to satisfy the relationship in the following expression ( v ). the total contour length x is properly determined based oh the relationship with the surface area occupation ratio y within the range to satisfy the expression ( i ), and is usually 2800 mm to 5000 mm , particularly , 3100 mm to 4700 mm . with respect to a reduction in the drag coefficient ( cd ), the number of the dimples having a contour length x of 10 . 5 mm or more is preferably 91 % of the total number of the dimples or more , and particularly preferably 95 % or more . the ratio is ideally 100 %. while the size of each dimple is not particularly restricted , the circular dimple usually has a diameter d of 2 . 0 mm to 8 . 0 mm , and particularly 3 . 0 mm to 7 . 0 mm . it is possible to form a dimple of a simple kind or plural kinds . a non - circular dimple ( a dimple having no circular plane shape ) may be formed in place of the circular dimple or together with the circular dimple . the total volume of dimples is preferably 300 mm 3 to 700 mm 3 . if the total volume is less than this range , the trajectory might be too high . from this viewpoint , it is particularly preferable that the total volume should be 350 mm 3 or more . if the total volume exceeds the range , the trajectory might be lower . from this viewpoint , it is particularly preferable that the total volume should be 600 mm 3 or less . the total number of the dimples is preferably 200 to 500 . if the total number is less than this range , it might be hard to cause the golf ball to take on the shape of an almost sphere while maintaining a predetermined surface area occupation ratio y ( in other words , the smoothness of the surface of the golf ball might be damaged ). from this viewpoint , it is particularly preferable if the total number should be 250 or more . if the total number exceeds this range , the mean occupation ratio y might be reduced . from this viewpoint , it is particularly preferable that the total number should be 400 or less . the structure of the golf ball is not particularly restricted and a so - called wound golf ball or a solid golf ball ( a one - piece golf ball , a two - pieces golf ball , a three - piece golf ball or the like ) may be used . moreover , the material is not particularly restricted and thus well - known materials can be used . a core formed of a solid rubber was put in a mold and an ionomer resin composition was subjected to injection molding to form a cover around the core . the surface of the cover was coated so that a golf ball according to an example 1 which has a dimple pattern shown in a plan view of fig1 and a front view of fig2 was obtained . the golf ball had an outside diameter of approximately 42 . 70 mm , a weight of approximately 45 . 4 g , a compression of approximately 93 ( by an atti compression tester produced by atti engineering co ., ltd .) and a total dimple volume of approximately 500 mm 3 . the golf ball includes 130 a dimples having a circular plane shape and a diameter of 4 . 5 mm , 150 b dimples having a circular plane shape and a diameter of 4 . 1 mm , 60 c dimples having a circular plane shape and a diameter of 3 . 6 mm , and 32 d dimples having a circular plane shape and a diameter of 2 . 8 mm . in the golf ball , the total number of the dimples is 372 , atotal contour length x is 4730 . 0 mm , a surface area occupation ratio y is 0 . 848 and a mean occupation ratio y is 0 . 00228 . a golf ball according to an example 2 which has a dimple pattern shown in a plan view of fig4 and a front view of fig5 was obtained in the same manner as in the example 1 except that the mold was changed . the golf ball includes 170 a dimples having a circular plane shape and a diameter of 4 . 4 mm , 120 b dimples having a circular plane shape and a diameter of 4 . 1 mm , 60 c dimples having a circular plane shape and a diameter of 3 . 5 mm , and 12 d dimples having a circular plane shape and a diameter of 2 . 4 mm . in the golf ball , the total number of the dimples is 362 , a total contour length x is 4645 . 8 mm , a surface area occupation ratio y is 0 . 838 , and a mean occupation ratio y is 0 . 00232 . a golf ball according to an example 3 which has a dimple pattern shown in a plan view of fig6 and a front view of fig7 was obtained in the same manner as in the example 1 except that the mold was changed . the golf ball includes 72 a dimples having a circular plane shape and a diameter of 5 . 9 mm , 24 b dimples having a circular plane shape and a diameter of 4 . 5 mm , 88 c dimples having a circular plane shape and a diameter of 3 . 9 mm , 112 d dimples having a circular plane shape and a diameter of 3 . 6 mm , and 24 e dimples having a circular plane shape and a diameter of 2 . 8 mm . in the golf ball , the total number of the dimples is 320 , a total contour length x is 4229 . 8 mm , a surface area occupation ratio y is 0 . 819 , and a mean occupation ratio y is 0 . 00256 . a golf ball according to a comparative example 1 which has a dimple pattern shown in a plan view of fig8 and a front view of fig9 was obtained in the same manner as in the example 1 except that the mold was changed . the golf ball includes 30 a dimples having a circular plane shape and a diameter of 4 . 3 mm , 130 b dimples having a circular plane shape and a diameter of 4 . 0 mm , 180 c dimples having a circular plane shape and a diameter of 3 . 7 mm , 60 d dimples having a circular plane shape and a diameter of 3 . 4 mm , and 32 e dimples having a circular plane shape and a diameter of 2 . 8 mm . in the golf ball , the total number of the dimples is 432 , a total contour length x is 5053 . 6 mm , a surface area occupation ratio y is 0 . 829 , and a mean occupation ratio y is 0 . 00192 . the dimple pattern according to the comparative example 1 has been disclosed as an example 1 in the japanese laid - open patent publication no . hei 4 - 347177 ( u . s . pat . no . 5 , 292 , 132 ) described above . a golf ball according to a comparative example 2 which has a dimple pattern shown in a plan view of fig1 and a front view of fig1 was obtained in the same manner as in the example 1 except that the mold was changed . the golf ball includes 30 a dimples having a circular plane shape and a diameter of 4 . 3 mm , 130 b dimples having a circular plane shape and a diameter of 4 . 0 mm , 180 c dimples having a circular plane shape and a diameter of 3 . 7 mm , 60 d dimples having a circular plane shape and a diameter of 3 . 4 mm , and 20 e dimples having a circular plane shape and a diameter of 2 . 8 mm . in the golf ball , the total number of the dimples is 420 , a total contour length x is 4948 . 0 mm , a surface area occupation ratio y is 0 . 816 , and a mean occupation ratio y is 0 . 00194 . the dimple pattern according to the comparative example 2 has been disclosed as an example 2 in the japanese laid - open patent publication no . hei 4 - 347177 ( u . s . pat . no . 5 , 292 , 132 ) described above . a golf ball according to a comparative example 3 which has a dimple pattern shown in a plan view of fig1 and a front view of fig1 was obtained in the same manner as in the example 1 except that the mold was changed . the golf ball includes 132 a dimples having a circular plane shape and a diameter of 4 . 4 mm , 60 b dimples having a circular plane shape and a diameter of 4 . 2 mm , 60 c dimples having a circular plane shape and a diameter of 3 . 5 mm , and 60 d dimples having a circular plane shape and a diameter of 3 . 3 mm . in the golf ball , the total number of the dimples is 312 , a total contour length x is 3898 . 1 mm , a surface area occupation ratio y is 0 . 686 , and a mean occupation ratio y is 0 . 00220 . the dimple pattern according to the comparative example 3 has been disclosed as an example 1 in the japanese laid - open patent publication no . sho 62 - 192181 ( u . s . pat . no . 4 , 813 , 677 ) described above ., 20 golf balls according to each of the examples and the comparative examples were prepared and were maintained at 23 ° c . on the other hand , a driver comprising a metal head ( trade name of “ xxiow # 1 ” produced by sumitomo rubber industries , ltd ., loft : 8 degrees , shaft hardness : x ) was attached to a swing machine ( produced by golf lab co ., ltd .). machine conditions were set to have a head speed of 50 m / sec , a back spin amount of approximately 2000 rpm obtained immediately after hitting and a launch angle of approximately 10 degrees , and the golf ball was hit and a flight distance ( a distance between a launch point and a stationary point ) was measured . the following tables 1 and 2 show the mean value of the results of measurement for the 20 golf balls . as is apparent from the tables 1 and 2 , the golf balls according to the examples 1 to 3 have greater flight distances than those of the golf balls according to the comparative examples 1 to 3 . from the results of evaluation , the advantage of the present invention is apparent . the above description is only illustrative and can be variously changed without departing from the scope of the present invention .
a golf ball having a large number of dimples on the surface wherein the surface area occupation ratio y of the dimples is 0 . 80 to 0 . 90 ; the mean occupation ratio y which is a value obtained by dividing the surface area occupation ratio y by the total number of the dimples , is 0 . 00220 or more ; the total contour length x of the dimples and the surface area occupation ratio y satisfy the relationship indicated by an expression : x ≦ 3882 * y + 1495 ; and number of dimples having a contour length x of 10 . 5 mm or more to the total number of dimples is 91 % or more .