I hatched some Rouen duck eggs I ordered along with chicken eggs in my little still air HovaBator and all eggs hatched. Because I had both in at same time, I made a sort of nest with a wet wash cloth and set the duck eggs in middle. I would re-wet it with warm water each night and turn the eggs and put them back on it. All chicks and all ducks hatched just fine and those same ducks are now laying fertile eggs at 4 months old and I have them in bator now.
Now that I see how the Rouens and Mallards are laying eggs in their duck yard I understand why they need the extra dampness. They are digging pretty deep nests and laying several layers of eggs - the oldest being completed buried in the moist dirt. Then when they leave the nest they bury it. They just started laying about two weeks ago and so far none have gone broody so once a week I gather up the eggs on top and put them in bator.