Success rates were very high based on the evidence we have; computer modeling and oral traditions.

Geoffrey Irwin's computer modeling showed that the survival rate of voyaging was >99% when traveling between two known points and nearly always >90% when traveling between a known point and an unknown one. Now survival rate is not success; success rates were typically high, however when navigators were sailing between two known points, they often knew of other islands they could land on if trouble arose. Even traveling between a known and unknown point, there is a high likelihood, that even if a navigator does not reach the unknown point, they will be able to arrive at a known point-- or another yet discovered island.

There are two different kinds of voyaging that I think you are really asking about; voyages that result in the discovery of new islands and voyages of settlement to known islands. Going between two known islands can be fairly straight forward, especially if we are only talking about a few hundred kilometers (or even less for inter-archipelago travel).

Voyages of discovery could take place for many different reasons, just based on oral traditions people might be sailing out over the horizon for religious reasons-- contacting a divine entity, or after a message from one, to prepare for exile, to relieve population pressure, to go fishing, and perhaps just out of curiosity. A voyage into the unknown East would involve packing enough food supplies to last the better part of a month or longer and a relatively small crew, perhaps a dozen or so men. One of the key strategies on these voyages was to sail into the wind, to head out at oblique angels in a generally Eastern direction. This is a great safety feature when you are sailing out into the unknown; though it may take you a while to make a lot of progress toward the east, ultimately if anything goes wrong-- or you need to get back to known waters, all you need to do is turn around and with the wind in your sails you can return home much faster than you left.

There are a lot of different clues that navigators could use in order to find land (many of these are true across voyaging traditions even outside of the Pacific). Many of the islands can appear to be 'smallish' but their effect on the ocean can make them visible from very far away. Key clues that a navigator might look for would include flotsam (floating debris), changes in wave patterns/water temperature, build up of clouds over islands (even flat coral atolls trap clouds), looking for land in the shadows of the clouds (as they are reflected), looking for marine life that congregates near islands, or looking for bird life-- typically birds fly away from islands in the morning to get food and then back in the evening. All of these different signs would be something you would spend decades learning and developing a sense to see before becoming the sort of master navigator leading an expedition across vast distances or into the unknown. This is of course to say nothing of supernatural signs that navigators might have access to as a part of the mysteries revealed to them during their training/initiation.

Voyages of settlement were a bit of a different beast. In this case you knew where you were going. Depending on the sort of island you were planning to settle, there were different steps that might need to be undertaken. Remember, the further east you move in the Pacific, the less likely the islands are to have edible plants or familiar large animals. Indeed, the fertility and the foods we associate with the Pacific are virtually all there because of human intervention (most of the fruits the result of 19th century colonialism). For instance, when navigators first arrived in the Hawaiian archipelago, there were all of two edible ferns they could have eaten, a few large birds, and then more familiar marine resources. When the first settlers arrived on the islands, they needed to bring with them their entire agricultural toolkit. They needed saplings, seeds, and animals. Settlement of new islands could take generations-- high fertile islands such as the Hawaiian Islands could be settled by a founding population without too much difficulty after clearing land (likely through fire/burnings). Yet, smaller islands, such as the Marquesas, show greater gaps from when human first arrived and began transforming the landscape and when they finally begin settling in larger numbers. There is a transformation stage that needs to take place to prepare the island for larger settlement. Coral atolls could take generations to transform into a places where humans could live permanently; stages of transformation would take place from discovery, to temporary habitation (use for hunting/fishing), and eventually more permanent settlement as important resources like coconuts were planted in greater densities.

A few readings for anyone with the time and resources :

• Irwin, Geoffrey. The Prehistorical Exploration and Colonisation of the Pacific. Cambridge: Cambridge University Press, 1994.

• Kirch, Patrick Vinton. A Shark Going Inland is My Chief: The Island Civilization of Ancient Hawaiʻi. Berkeley: University of California Press, 2012.

• Kirch, Patrick Vinton. On the Road of the Winds. Berkeley: University of California Press, 2002.

• Finney, Ben. Voyage of Rediscovery: A Cultural Odyssey through Polynesia. Berkeley: University of California Press, 1994.

• Lewis, David. We, the Navigators: The Ancient Art of Landfinding in the Pacific. Honolulu: University of Hawaii Press, 1972.