ELI5: Why do car engines tend to have an even number of cylinders but ships often seem to have odd numbers like 3, 7 or 9 cylinders? How do the engineers who design engines decide that 7 cylinders might be better than 6 or 8 for a ship’s engine?

ELI5: Why do car engines tend to have an even number of cylinders but ships often seem to have odd numbers like 3, 7 or 9 cylinders? How do the engineers who design engines decide that 7 cylinders might be better than 6 or 8 for a ship’s engine?