Hollow golf club head

1. A hollow golf club head comprising:

a face; and

a sole,

wherein:

the face has a face center;

the sole includes a groove extending from a toe side to a heel side;

the whole groove is located below the face center;

a bottom part of the groove is curved so that it projects to a lower side along a shape of the sole; and

a maximum value of a groove depth of the groove is 5% or greater but 15% or less of a height of a center of gravity of the head.

2. The golf club head according to claim 1 , wherein the maximum value of the groove depth is equal to or less than 10% of the height of the center of gravity of the head.

3. The golf club head according to claim 1 , wherein the golf club head satisfies the following item (x), (y), or (z):

(x) the groove depth is gradually decreased as the groove depth approaches a toe end in a toe side end part of the groove, and the groove depth becomes zero in the toe end;

(y) the groove depth is gradually decreased as the groove depth approaches a heel end in a heel side end part of the groove, and the groove depth becomes zero in the heel end;

(z) the groove depth is gradually decreased as the groove depth approaches the toe end in the toe side end part of the groove, and the groove depth becomes zero in the toe end; and the groove depth is gradually decreased as the groove depth approaches the heel end in the heel side end part of the groove, and the groove depth becomes zero in the heel end.

4. The golf club head according to claim 1 , wherein the maximum value of the groove depth is 0.8 mm or greater but 3.2 mm or less.

5. The golf club head according to claim 4 , wherein the maximum value of the groove depth is equal to or less than 2.7 mm.

6. The golf club head according to claim 4 , wherein the maximum value of the groove depth is equal to or less than 2.2 mm.

7. The golf club head according to claim 1 ,

wherein:

the groove includes a center groove part, a toe groove part located on the toe side with respect to the center groove part, and a heel groove part located on the heel side with respect to the center groove part; and

the golf club head satisfies the following item (a), (b), or (c):

(a) a groove depth of the center groove part is smaller than a groove depth of the toe groove part;

(b) the groove depth of the center groove part is smaller than a groove depth of the heel groove part;

(c) the groove depth of the center groove part is smaller than the groove depth of the toe groove part, and the groove depth of the center groove part is smaller than the groove depth of the heel groove part.

8. The golf club head according to claim 1 , wherein if a toe-heel direction length of the groove is defined as Lm, and a toe-heel direction width of the head is defined as Lh, Lm/Lh is 0.7 or greater but 0.92 or less.

9. The golf club head according to claim 1 , wherein the golf club head satisfies the following item (z):

(z) the groove depth is gradually decreased as the groove depth approaches a toe end in a toe side end part of the groove, and the groove depth becomes zero in the toe end; and the groove depth is gradually decreased as the groove depth approaches a heel end in a heel side end part of the groove, and the groove depth becomes zero in the heel end.

10. The golf club head according to claim 1 , wherein the groove depth is equal to or less than 5 mm at all positions in a toe-heel direction.

11. The golf club head according to claim 1 , wherein a sole thickness in the groove is 0.5 mm or greater but 1.4 mm or less.

12. The golf club head according to claim 1 , wherein if a height of the face center from a horizontal plane is defined as H1 (mm), the height of the center of gravity of the head is (H1−5) mm or greater but (H1+1) mm or less.