NET
10 If an iron axhead is blunt and a workman does not sharpen its edge, he must exert a great deal of effort; so wisdom has the advantage of giving success.
Darby
10 If the iron be blunt, and one do not whet the edge, then must he apply more strength; but wisdom is profitable to give success.
KJV
10 If the iron be blunt, and he do not whet the edge, then must he put to more strength: but wisdom [is] profitable to direct.