You shouldn't nail things securely on outdoor structures - wood moves with moisture changes, and it will either warp, split or pull the nails out. It _will_ move, you'll never "nail it down".
Shiplap boards (any lapped boards) should be nailed with one nail at each end (maybe in the middle too), but these nails should all be in a line along the grain. Don't put lines of nails across the grain.
Expect about 10% movement (enormously rough guess) and so allow at least 15% overlap (assuming further shrinkage).