No need to panic about zinc in the alloy. If the boat is home made, and more than 15 years old, there would not likely be a lot of zinc available at that time. If you can file off a flat clean surface and apply a drop of muratic acid to the clean lead surface, it will bubble or fizz and indicate the presence of zinc. If it just turns dark, you will be good to go. This should answer the question of the presence of zinc.