he doesn't plan to use the ship as a gunship and he is going to raid the duke's lands so why not just make a deal with a willow trader to transport the cannon and the supplies to the destination? it would be a hell of a lot cheaper than building a new ship and staffing it with people. you can do all that after you've looted and gained new wealth
Later version gets steam engine so it doesn't require Wendy to help at all, and even become a new industry for his town when other merchants learn of these 'boats that don't rely on wind'