When I loaded this model SU said it had problems. Fixing problems made the hulls unsolid. So I chose not to fix. Like you the solids did not make a solid when combined. Solidsolver could not solve it. Starting back with the three separate components: Solid inspector found one tiny tiny hole in the hull at the inside gunnel. I fixed this and made it a component and placed the hull mirrored on the opposite side. Combining the solids again failed to make a solid. Solid inspector showed numerous errors in the resulting group. Too much to work out. I scaled the model up 1000 times before using outer shell. That worked and scaled down OK to be solid.