T5 (the other T5) to the rescue! Change it to a 5-level class with the following as its Special column:
Level 1: Bonus feat, dirty strike +1d4, sailor lore, seamanship +1
Level 2: Back-to-back +1, dirty strike +2d4, seamanship +2
Level 3: Bonus feat, dirty strike +3d4, seamanship +3
Level 4: Back-to-back +2, dirty strike +4d4, seamanship +4
Level 5: Bonus feat, dirty strike +5d4, seamanship +5
Compared to the whole 20-level class, it's missing 2 bonus feats and +3 to its Back-to-back. I'm not sure if it would even hurt to give it the full Back-to-back, but I erred very slightly on the side of caution.
*This is what I converted it to last December/January when I did the T5 document (think E6, but 5, and buffing low-tier classes significantly and eliminating dead levels), which I need to update and reupload.