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These anchors are used to control boat movement in some form. Usually they are the shape of a parachute or a tapered cone with the end cut off.
Some common ways in which drogues are used are:
Offshore anchor
With this style usually a large cone or parachute style anchor is deployed off the bow of the boat. Usually you are doing this because the weather has got real bad. It is used to slow the boats drift down and keep the boat pointing into the wind. Some people like to attach another line to the drogue and run it to an aft winch . When you crank on the winch it will angle the boat a little to the weather and make the ride more comfortable.
This is a genaral sizing chart for offshore anchors . The sizes can vary quite a lot in relation to anchor design.
(Boat Loa /ft) (weight/tonnes) (anchor diameter/ft)
20 2 6
25 4 9
33 5 1/2 12
40 11 1/2 15
45 18 18
90 43 24
120 90 32
150 135 40
Drift Anchors
Another popular use for these style anchors is a drift anchor. They are much smaller than the offshore sea anchors and are commonly used to stabilise sideways boat movement in rivers. This helps a lot when you are trying to fish. There are quite a few manufactures of these anchors with slightly different designs so I will not do a sizing chart for these.
Other uses for drouges
Another use for a drogue is to drag one behind your boat if you are being towed. This will help reduce the whiplashing effect and make for a more comfortable ride.
When towed off the rear windward 1/4 a drogue will help stabilize the boat when negotiating dangerous harbor entrances.
A drogue can also be used as an emergency steering system when used in conjunction with a bridal.
They can also be used to slow a boat down while trolling. This is sometimes necessary if you don't have a small trolling motor and the main motor trolls too fast.
If you intend to get into serious offshore cruising you should look into a good offshore sea anchor. You should practice putting it out in rough weather and get confident with it. It could save your life.
Hope I have helped you gain some insight into anchoring. It really is worth getting a good system and be good at using it. Have fun.