It can be confusing to cook a piece of meat in liquids only to find it dried out. The problem usually lies with cooking temperature. A large piece of chuck or rump roast has a good deal of connective tissue. These need to be cooked very slowly so that they get tender and break down. If they get hot too fast, like when your simmering liquid crosses into a boil, those fibers shorten and contract. This will squeeze the natural moisture right out of your roast.  It can’t be said enough, low and slow is the best way to ensure tender pot roast.