How did the Hurricanes of 1775 and 1780 affect American History?
It is hard to argue that these hurricanes had any huge impact on US history. The first hurricane was a terrible disaster in Newfoundland, but not in what is now the US. The second pounded the Caribbean, but did not affect the American mainland that much. Both hurricanes had minor impacts on Revolutionary War activities.
The 1775 hurricane drove one British ship, H.M.S. Liberty aground. It was captured and burned by colonists in Virginia. This was certainly an act of rebellion, but it was not a huge one compared to things like the fighting that had already occurred in Massachusetts.
The 1780 hurricane destroyed British and French warships in the Caribbean. This, of course, reduced the two navies' abilities to operate in that area. This could be said to have helped the cause of American independence by reducing Britain's naval advantage. This was important because Britain was involved in a world-wide war with France and Spain in addition to the conflict in the American colonies. Any serious reduction in British power was helpful to the American cause.