The US coast guard, and many countries have what is declared a "Manifest unsafe voyage" docterine. They might stop a 12 foot outboard from trying to voyage to Hawaii. A friends son sailed a Montgomary 15 to Hawaii with no problems. Teak Ladies (18 foot) have crossing the Pacific several times. But the boat's condition, the crew's abilities etc are all taken into account. Also many countries license a boat for certain types of voyages. For example you will not be allowed to take a boat licensed for near coastal across an ocean, nor can a skipper licensed for near coastal captain a boat licensed for ocean voyaging across an ocean. Fortunately we don't have quite so severe restrictions yet.... Bob Austin