I must disagree free with the missive to "avoid rope". At the very least, a boat needs a long bridal or riding line of nylon to reduce the shock loads on the anchor and on-deck gear when the chain goes bar-tight as the boat is moving aft rapidly.
We come from sailboats, with far less windage and momentum aft than a trawler, and even there the shock loads of an all chain rode have bent bolts for windlass', deformed shackles, and broken chains. If a boat is limited to protected anchorages without significant swell and wind, all chain is fine. But, in an emergency or if trapped by an engine that won't start or an anchor windlass that won't haul the gear aboard, it can be necessary to ride out a blow amongst real waves. In that case, chain will fail much more rapidly than an appropriate nylon 3-strand rope.
Obviously, one needs to keep the rope off of rock, coral, and other sharp objects. That's easily accommodated by never letting out more rope than the depth of the water. But that's not the point. The point is that the dampening of shock due to the weight of the chain stops working in strong winds and large swell. Then there is no stretch in the system and the weakest link fails. With Nylon rope, the stuff just stretches, in many case up to 15%, and the force of decelerating the boats movement aft is spread across a long period of time allowing the equipment to continuous operate on far lower loads.
Put another way, it's easy to calculate the point at which the chain is close enough to straight to shock load the system.
The other lovely benefit of rope leading down to just above the bottom is that it's quite on board and one doesn't have to listen to the grinding sounds of the chain being dragged over the bottom.