Six Pirates and 100 Coins

Six pirates ranked 1 (most senior) to 6 must divide 100 gold coins. Starting with the most senior, the proposer suggests an allocation; all living pirates (including the proposer) vote. If at least half vote yes the plan passes; otherwise the proposer is thrown overboard and the next most senior proposes. Pirates are perfectly rational and value, in order: survival, then maximizing their own coins, then (as a tiebreaker) seeing others thrown overboard. How many coins does the most senior pirate keep?