Along with the rope, you should also have a smaller amount of chain between the rope and the anchor. This chain will keep your rope from rubbing against the seabed and also creates the optimal angle between your rode and the seabed. The general rule of thumb is that you want approximately 1' of chain for every 1' of boat. So a 30' boat would want 30' of chain. However, often certain constraints such as weight and locker room will not allow this ideal chain amount so in these situations you should have at least 10-15' of anchor chain for the reasons mentioned above. For boaters anchoring in extreme conditions and/or for extended periods of time, you will want about 1 foot of chain for every 6 feet of rope. The reason for the different requirements is that, in theory, by having 1 foot of chain for every 6 feet of rope, an optimal angle between the rode and the seabed will be achieved.