CREATE TABLE table_17421 ("Number" FLOAT, "Builder" CLOB, "Built" CLOB, "Rebuilt" CLOB, "Name as rebuilt" CLOB, "Scrapped/Sold" CLOB)

What was the built data for Knocklayd rebuild?
SELECT "Built" FROM table_17421 WHERE "Name as rebuilt" = 'Knocklayd'
SELECT "Built" FROM "table_17421" WHERE "Name as rebuilt" = 'Knocklayd'