wemfish says:

a flat bottom ( for the roof bars)

If you carry your boat upside-down, it has two advantages:

1. The sheerline (upper edges of hull) is necessarily at the same level each
side, reducing the need for a flat bottom.
2. The boat doesn't fill up with water/road crud while you drive. The
rainwater can be a major source of irritation when you try to unload, or
corner, or.....

Steve "there's a reason why most car=topped boats are upside-down..."