To turn a wandering hammer brother (the ones that walk about on the world map) into a white ship full of coins, collect a number of coins that is a multiple of 11. Make the tens digit in your score match the multiple of 11. Stop the timer at the end of the stage that you are playing in on an even number.

One combination would be: 11 coins, 9,310 as your score, and the timer at 104. This only works in worlds 1, 3, 5, and 6.