The board of supervisors of any county which shall have issued bonds under the provisions of this article shall, unless there be sufficient funds otherwise available therefor in the port bonds interest and sinking fund, annually levy a special tax upon all of the taxable property within such county at a rate which shall be sufficient to provide for the payment of the principal of and the interest on such bonds according to the terms thereof. All taxes thus collected shall be credited to a special fund in the county treasury of such county to be known and designated as the “port bonds interest and sinking fund,” and all sums credited to said fund shall be used to pay such bonds as they mature and the interest thereon as it accrues, and for no other purpose. It shall be the mandatory duty of such board of supervisors to transfer funds from said port fund to the port bonds interest and sinking fund in amounts sufficient to pay maturing principal and accruing interest on bonds issued hereunder, if balances standing to the credit of said port fund are sufficient for that purpose. To the extent that funds are thus made available for the payment of such bonds and the interest thereon, the special tax levy hereinabove provided for may be correspondingly reduced. The said bonds nevertheless shall be general obligations of the county issuing the same, and the full faith, credit and resources of such county shall be pledged to the payment thereof and the interest thereon.