Grease a 9x13 inch pan and preheat oven to 350 degrees F (175 degrees C).
Drain crushed pineapple and reserve juice. Combine margarine, brown sugar, pineapple and 2 tablespoons pineapple juice. Spread onto the bottom of a 9x13 inch pan. Add enough water to remaining juice to make 1 1/3 cup of liquid. Set aside.
Combine flour and baking powder. Set aside.
Cream shortening and white sugar until light and fluffy. Add eggs and vanilla, and beat for 1 minute. Add flour mixture alternately with juice, beginning and ending with the dry ingredients.
Pour over pineapple mixture in pan. Bake at 350 degrees F (175 degrees C) for 40 minutes or until toothpick inserted in center comes out clean. Cool in pan 5 minutes and then turn out.