Dice onion and slice celery fine. Saute in dutch oven until tender. Add shredded potatoes. Cover with water and bring to a boil. Simmer partly covered over medium heat until potatoes are soft. Add sliced carrots and simmer until carrots are tender. The idea is that the potato will 'disappear' just as the carrots are cooked. Add milk to whiten the broth, clam juice, salt and pepper. It takes quite a bit of salt as the potatoes will absorb a lot. Rinse the clams well with water to remove any grit and add just as chowder comes back to a light boil. Mix flour with a cup of cold water and add slowly until desired consistency is reached. Serve with salted crackers and margarine.