I don't know about Nelson's patent bridge but in context it sounds something like what the Greeks used. They would stow a long, heavy board with large spike on the end up alongsside the mast. When they engaged the enemy vessel at close enough quarters, they would drop the board onto the enemy deck and Greek troops would race across the plank to board the enemy vessel with the intent of capturing it.

It would be interesting to know if Nelson adapted an idea he learned of through his classical studies and applied it to "modern" warfare 22 centuries later.

Sourdough