Depending on how you feel and if you want to change things slightly you could use the Frog God Games book Fire as She Bears.
The ships are easiely re-built form scratch in here and the have detailed rules on ship building (although you can still keep it simple if you wanted). If you use this system the players ships will not overpower the others unless they spend heaps of coin on modifications. It should help with the balance a bit.
Everything is based on 20ft cubes and each suare on the battle mat is 20ft not 30 ft.
It's not that difficult to convert just roound the ships up or down as you feel. (Man's Promise I re-did as a 120ftx 40ft wide ship.
For me and a fair few others this books is an excellent resoure for this AP and greatly enhances the ship combat experience.
I have used a combination of this book and the Ship combat rules in the players guide and I must say the ship combats are so much better and more satisfying.