In a medium saucepan cook potato cubes in water for 12 minutes or until tender. Do not drain.
Cool mixture to 110 degrees F (43 degrees C). Set aside 1/2 cup of potato water.
Mash potato and remaining liquid; if necessary add warm water to make 2 cups potato mixture.
In a large mixing bowl, sprinkle yeast on reserved 1/2 cup potato water.
Add mashed potato mixture, 2 cups of the flour, sugar, shortening, and salt.
Beat with an electric mixer on low speed for 1/2 minute, scraping sides of bowl constantly. Beat 3 minutes at high speed.
Stir in as much of the remaining flour as you can mix in by hand. Turn dough out onto a lightly floured surface.
Knead in enough of remaining flour to make a moderately stiff dough. Knead for 6 to 8 minutes or until dough is smooth and elastic.
Place in a lightly greased bowl; turn once to grease surface. Cover. Let rise in warm place until doubled in bulk (about 1 hour).
Punch down and turn out onto a lightly floured surface. Divide dough in half.
Cover and let rest for 10 minutes.
Shape each half into a loaf. Place in greased 8 x 4 x 2 loaf pans. Cover.
Let rise until nearly double (about 35 minutes). Before baking, brush tops with a little water and dust with additional flour.
Bake at 375 degree F (190 degrees C) for 40 to 45 minutes. Cover with foil for the last 15 minutes of baking to prevent over-browning. Remove from pan; cool on wire rack.