Preheat the oven to 350°.

In the large bowl of an electric mixer, beat the oil with the sugar until it is well mixed. Use the paddle attachment if your mixer has one. Add the egg yolks one at a time, beating well after each. Slowly pour in the hot water and continue to beat until the mixture is light.

Sift 1/4 cup of the flour over the sweet potato in a large bowl and toss so that it is lightly coated. This will keep the potato from sinking to the bottom of the cake.

Sift the remaining flour with the baking powder, soda, salt, ginger, and nutmeg. Add the dry to the wet ingredients and mix well, then mix in the vanilla and the sweet potatoes.

Beat the egg whites until they hold stiff peaks, then fold them gently but thoroughly into the batter. Pour the batter into a 10-inch tube pan and bake for 1 hour, or until the cake is lightly browned and a straw poked into the cake comes out clean. Remove from the oven, invert the pan, and allow to cool completely.