What shortcut eliminates some 8,100 miles of the otherwise 13,000 mile trip via sea routes from New York around the southern tip of South America to Los Angeles?

Going across the Isthmus of Panama is the efficient approach that significantly reduces the travel distance between New York and Los Angeles. The Panama Canal, which cuts through the isthmus, allows vessels to pass directly from the Atlantic to the Pacific Ocean, eliminating the need for a long and treacherous voyage around the southern tip of South America, which is not only lengthy but also subject to hazardous conditions.

The canal’s construction provided a strategic shortcut that transformed maritime trade routes by minimizing travel time and costs. The distance by using the canal is much shorter, saving approximately 8,100 miles in the journey.

Alternative routes such as traveling via the Intercontinental Waterway System or going east around South Africa do not offer the same level of efficiency for this specific trip. Direct flights may suggest air travel, which, while quicker, wouldn't apply to a sea route context as asked in the question. Thus, utilizing the Isthmus of Panama presents the best solution for shortening the voyage considerably.