How to determine the length of the chain when setting a buoy in the ocean?
Multiply the depth of the water, (highest annual tide) by 1.5 to determine the length of the chain to be used. So if the depth is 50 feet the chain length, rule of thumb, would be 75 feet. If the current is fast, or tidal change is rapid some extra chain is suggested.
What size of anchor to choose when mooring a buoy?
As a rule of thumb:
If the buoy is 1 ton then the anchor is to be 500kg.
If a buoy is 2000 pounds then the anchor is to be 1000 pounds.
Note this is for serrated cast anchors/sinkers as cast iron or steel is almost the same weight in air as in sea and will also grip the ocean floor if dragged. Concrete anchors are much lighter in the sea as in the air and slide often on the ocean floor.
If the current is high, or the tidal chain is rapid somewhat heavier anchors are suggested.