The members of the Navigation Commission for the Delaware River and its navigable tributaries, hereinafter referred to as the Commissioners, shall hold office for a term of four years, and until their successors are appointed and qualified, and may be eligible for reappointment to office. They shall serve without compensation, but shall be reimbursed for necessary expenses. A majority of the Commissioners, appointed by the Governor and the Mayor of the City of Philadelphia, shall constitute a quorum for the transaction of business.