All conveyances which are used or intended for use in the willful discharge of harmful or destructive substances shall be subject to forfeiture to the State, except that:
a. No conveyance used by any person as a common carrier in the transaction of business as a common carrier shall be forfeited under this act unless it appears that the owner or person in charge of such conveyance was a consenting party or privy to a violation of this act. The term conveyance shall mean aircraft, vessels, vehicles, other equipment or containers;
b. No conveyance shall be forfeited by reason of any act or omission, established by the owner thereof, to have been committed or omitted without his knowledge by any person other than such owner while such conveyance was unlawfully in the possession of a person other than the owner in violation of the criminal law of the United States or of any state;
c. The forfeiture of any conveyance encumbered by a bona fide security interest shall remain subject to the interest of the secured party if he had no knowledge of the act or omission.