I'm reading James Michener's Hawaii, a historical fiction about Hawaii. It references the voyage of the first Protestant missionaries from Boston to Hawaii in 1819. In Michener's book (assumed to be well-researched), they sail south on the Atlantic to round Cape Horn at the tip of Chile before crossing the Pacific to reach Hawaii. Why did they not first travel across land to California and set sail from there? Seems way safer and less unpleasant than spending five straight months on the open ocean. I haven't been able to find an answer online.
Traveling across the continent was not as easy and straightforward as it appears.
Remember, 1819 is not even two decades after the Louisiana Purchase (1803) which dramatically extended the US westward and just over a decade since Louis and Clark (1803-6) mapped a route to the Pacific Coast. Traveling westward was arduous and potentially perilous (as anyone who played Oregon Trail can probably attest).
It is also thirty years before California came under US control (1847) and right about 50 years before the transcontinental railroad (1869) will finally link the Eastern and Western Coasts of the US. So travel to California was neither easy nor quick and, as it was not part of the US, there would have been little reason for missionaries to take that route.
On the other hand, whaling ships from New England regularly made the journey to the Pacific to hunt whales with stopovers at Hawaii, and ships engaged in trade with China often left from the east coast to collect furs in the Northwest or cut sandalwood in Hawaii or across several Pacific Islands (for any of these activities they might start in Hawaii to load supplies and take on a crew). Additionally, Hawaiians had already visited New England serving on whaling ships, with merchants, or searching for other opportunities. For the missionaries, one of their chief inspirations was Henry 'Ōpūkaha'ia (known as Obookiah to the missionaries) who attended the Foreign Mission School after its founding in 1816 (indeed his presence in New Haven was part of the drive to create it). Ultimately he died before he could return to Hawaii to proselytize, but he created tools such as grammar guides that the missionaries then used to help learn Hawaiian.
New England continued to be the primary point of connection until after the annexation of California. There was no regular shipping from California until the Gold Rush created a demand for agricultural goods from Hawaii as well as education for upper-class elite, often anglophone or Protestant, Californians at Oahu College. Thereafter connections grew and the east coast route for passengers slowly declined by the end of the 19th century, especially after the transcontinental railroad cut down travel time between the coasts. Until that point, even lots of shipping between the East and West Coasts usually involved an intermediary stop in Hawaii as ships went around South America and made straight for the Kingdom before then going to the West Coast.
Igler, David. The Great Ocean; Pacific Worlds from Captain Cook to the Gold Rush. New York: Oxford University Press, 2013.
Chappell, David. Double Ghosts: Oceanic Voyages on Euroamerican Ships. Armonk, New York: M.E. Sharpe, 1997.
Grimshaw, Patricia. Paths of Duty: American Missionary Wives in Nineteenth-Century Hawai’i. Honolulu: University of Hawaii Press, 1989.
Kashay, Jennifer Fisher, "Agents of Imperialism: Missionaries and Merchants in Early-Nineteenth-Century Hawaii" New England Quarterly vol. 80 no. 2 (2007): 280-298.