However, to prove that it actually was done, one needs to have the evidence you spoke of.
Thus, his voyages proved it could be done, which negates the impossibility argument. When people, including scientists, believe something is impossible, they will tend to go to great lengths interpret away evidence which tends to prove that it happened.
That is why you shouldn't dismiss his work as irrelevant.
If he had failed and lost his life at sea, would that have proved it impossible?
No.
The "impossibility" arguement had no sound scientific basis.
It was merely a matter of conjecture on the laws of probablity that govern the struggle of Man vs. the forces of Nature.
Thor Heyerdahl's accomplishments were indeed remarkable and admirable.
But they are not "sience".
Similar junk science is pursued today, most vividly involving speculation regarding the method of constructing the Pyramids. Like many, I find the various clever approaches tested by archeological enthusiasts and broadcast on the Discovery or History channels to be fascinating and educational entertainment. But I don't regard it as "science". Proof will only come from hard archeological evidence, if such proof still exists.