An SEC filing revealed his Cayman Islands investment company Tiger Container Shipping had 20,000 preferred shares converted into 2.32m common shares this year.
This slice would be worth $51.53m based on the last price of $22.45 in New York.
Tiger has 6.99m shares and Porter another 5,500 himself.
The investment vehicle has added 37,639 shares in the open market since July.
Seaspan itself is considering acquiring Greater China Intermodal Investments (GCI), its boxship investment partnership with affiliates of Porter, largest shareholder Dennis Washington and private equity giant The Carlyle Group .
