One needs to really SET an anchor at way more than idle speed. See below from a post Maine Sail made on our C34 Forum.
In order to answer the OP's original question, have a look at the ANCHOR SYSTEM sizing tables here:
Ground Tackle & Anchor System Sizing TABLES
Then size appropriately for YOUR use.
Last summer on a friends boat he left me at the helm while he went to drop his CQR. I backed down, like I always do, gradually increasing to 80% throttle and the anchor dragged!
Here's how the conversation went "Geez that's never happened before","Really? Lets try it again",.
On the second attempt it had an initial bite (starting to burry) but when I applied power it broke free. "Your giving it to much throttle and ripping it out of the bottom", "it's an anchor!", "let me try", "ok".
So I now go up to let the anchor down & he puts the boat in reverse gets it moving and then puts it in neutral and we get an initial bite. "There see it's set", "No it's just starting to dig in it now needs to be set", "It's always held me before", "Have you ever experienced a 30 knot blow on the hook?", "No" "Well a 30 knot blow on your boat is the equivalent of roughly 900 pounds of pull on the anchor did you know that?", "No", "Did you know that the motor on this boat can barely re-produce 350 pounds of pull wide open?" "No", "Well let's let it set your way and in a couple of hours we'll simulate 20 knots of wind with the motor and see and happens", "You're on". You can probably guess what happened. Because we never properly set the anchor it dragged! We did get it to set that day using a 10:1 scope then shortening to 5:1. My friend could not beleive that the CQR could hold his boat using 80% throttle and was totally surprised by it! Scary I know.... From my experience I find a CQR likes a minimum of a 7:1 to set but it sometimes prefers more..
He now understands that an anchor should hold your boat at wide open in reverse without moving. This is a guy who has been sailing for 25 years and admittedly dragged "perhaps 20 times but never with my CQR"! Once is to much! It's imperative the anchor gets "set" properly. Yes the CQR sets better in soft bottoms than in sand but not all boaters are lucky enough to always drop the hook in a soft bottom. So if you're in a hard bottom make sure to get it set. The CQR will set well but it may take more than one attempt. Don't ever be fooled by the "initial bite". With a CQR this is a situation where the anchor is laying on it's side with the tip just starting to dig in. Like the picture at the beginning of Sail Magazines article. If you stop there on any sort of wind or current shift the anchor will twist out. A CQR needs to be vertical and buried to the shank or it's not properly set. If it's properly buried it can sometimes survive a 180 shift without "breaking free". I suggest some of you begin diving on your anchors in a shallow spot to see what's going on down there I think you'd be surprised...