Yes, scope is the term, probably just a typo above... the "formula" is generally considered to be a 5:1 ratio. The distance from the surface of the water to the bow (say, 4-5' on a small boat...) must be included. The more horizontal the pull on the anchor line/chain, the more digging in power.
Therefore, if anchoring in 30ft. of water with the bow heighth above the surface included, your scope shoud be 150' minimum. In crowded anchorages 3:1 is common but not ideal.( I sometimes use 4:1 but don't like to leave the boat (37' Motorsailer) to go to town with less than 5:1. If other anchored boats give you crap, comment on their lack of seamanship IMHO.
Commercial fishermen often use 2:1 but that's just asking for trouble. I set my anchors using 2:1, back down hard to dig in making sure the boat springs forward after setting, then let out to 5:1. We've been in some hellacious blows and never dragged. The only time we have dragged has been on an initial set when backing down. Try to back down with your stern pointed towards shore, rocks etc. so that your boat is between the anchor and a lee shore. (If there's an offhore breeze obviously that won't work.)
Using an anchor rated for a larger boat than yours is not a bad idea. Definitely have at least 3 fathoms of chain between your anchor and rope to avoid chafing/cutting of your anchor line.
Scope is the correct term for how much chain/rope is out and the angle of dangle.
If you use rope mostly (usual on small boats due to weight saving measures, "messengers"-additional weights with an eye for the line to run through can be let down your anchor rope putting a catenary (bow) in your anchor line which helps to keep the anchor in the bottom should wakes or surge be present.
Be sure to use a Bruce or CQR plow for sand or mud. Danforth style anchors are in my personal experience a russian roulette anchor. All it takes is for one tang to impale a beer can or seaweed on the bottom and the holding power will be halved.
After 25 years of boating in the Pacific N.W. my wife and I have seen countless Danforth equipped (or similar) boats drag anchor. Some swear by them but I sure don't. For newer small boat owners, do NOT use a mushroom style anchor like one would use for bass fishing in a lake. The don't dig into anything and it's just weight holding you in place. Suicide! Catherine, please don't do this unless you know you can reach shore.
The boat in any appreciable breeze will drift fairly quickly away-even with no current.
Even swimming around the anchor line may not allow you to swim fast enough to catch up.
IMHO diving from a boat without a crew member on board who knows how to drive the boat and do minor engine troubleshooting is also potential suicide. Even solo diving, with redundancy, you can be ok. Without a boat for the return trip home-there isn't any redundancy plus that gets kind of expensive!
Hope that helps someone, especially newbie boat owners/divers.
