The City shall not sell, exchange, lease, or in any way alien or dispose of the property, easements, income or other equipment, privilege, or asset belonging to and appertaining to any utility which it may own or acquire, unless and except the proposition for such purpose shall first have been submitted to the electors of the City who are qualified to vote on questions involving the direct expenditure of money or the issuance of bonds, at an election held for the purpose in the manner provided in this Charter, and approved by them by a three-fifths majority vote of the electors voting thereon. All contracts, negotiations, licenses, grants, leases, or other forms of transfer in violation of this provision, shall be void and of no effect as against the City. The provisions of this section shall not, however, apply to the sale or exchange of any articles of equipment of any City-owned utility as are worn out or useless, or which could, with advantage to the service, be replaced by new and improved machinery or equipment.