The board shall, at least thirty days before the meeting of the board of supervisors at which the general tax levy is fixed, notify the board of supervisors in writing of the amount of money necessary to be raised by taxation to meet the payment of principal and interest on outstanding bonds which will become payable before the proceeds of another tax levy made at the time of the next general tax levy for county purposes can be made available for payment of said principal and interest. In fixing the amount of money to be raised by taxation the Board of Harbor Commissioners may take into account all moneys on hand and set aside in a fund for the purpose of paying said principal and interest and the amount of moneys to be raised by taxation shall be the amount required in addition to any moneys on hand and so set aside for the purpose of payment of said principal and interest.