It depends on the boat and is indicated on the plans or at the study plans page. Some general guidelines: . For a small boat that does not stay in the water, inexpensive Lauan or other exterior plywood is just fine. . For a utilitarian style boat like our OD16-18 or our garveys, good exterior with no voids is a good choice. . For offshore going sailboats, fast planing power boats and for all boats that will stay in the water for more than 2 weeks at a time, we specify marine plywood. This is required for structural reasons mostly. Those are the minimum specifications. Compared to the total cost of the boat, the difference in cost between quality marine plywood and cheap plywood is small: less than 10%. Marine plywood like Okume or Meranti are the best choices: they have superior mechanical characteristics, bend easily and are easy to work with. Build with marine plywood to save many hours of work and obtain a much better looking boat with a higher resale value. A good compromise if plywood cost is an issue is to use marine plywood for all hull panels and exterior for the framing and inside.