If it is deemed advisable or found necessary to redeem any or all of such warrants, the department is authorized to issue its revenue refunding bonds for that purpose. The bonds shall constitute obligations only of the department, and shall not be obligations of the state of Washington. The refunding bonds shall be payable solely out of part or all of the revenues derived from the operation of the Puget Sound ferry system as shall be provided in the resolution authorizing their issuance.