hi there,
I scratchbuilt a gunboat for some mesopotamia scenarios based on something between and insect and a fly class gunboat. A drew up a rough body plan and then cut out the sections at about 10 lengths along the ship stuck onto a base board, using thin plasti card you can bend it to the boat shape using the sections to keep the shape. Then you just need to add a deck and you've got a basic hull. You can get alot of details from the londonwarroom website they have lots of cast ship parts, hatches, WT doors vents, davits etc that can help you finish off your ship, some nice deck mounted guns up to 8" as well if your looking for some firepower.
hope this helps.
Andy