CREATE TABLE table_49309 ("Year" FLOAT, "Entrant" TEXT, "Chassis" TEXT, "Engine" TEXT, "Tyres" TEXT, "Points" TEXT)

Which engine of Rob Walker Racing Team has 47 points and F tyres?
SELECT "Engine" FROM table_49309 WHERE "Points" = '47' AND "Tyres" = 'f' AND "Entrant" = 'rob walker racing team'
SELECT "Engine" FROM "table_49309" WHERE "Entrant" = 'rob walker racing team' AND "Points" = '47' AND "Tyres" = 'f'