I have heard recently about a new way of anchoring where you set your anchor on a 4:1 scope, regardless of weather or conditions. This concept is being propagated by those who claim that modern anchors work so well that they don't need much chain for them to function properly.
This concept seems logical at first. Modern anchors are far superior when compared to old style anchors.
When you look at historical ships, you will note that the anchors they carry are rather "small" for the size of vessel they are. While the anchors were small, the chain they employed was massive! The concept was simple: set enough chain so that the resistance of pulling the chain along the bottom would resist the movement of the yacht. At the end of this chain that was holding the yacht was a small anchor that would serve as a terminal point for th rode.
The shift to modern anchors is quite dramatic. Instead of using massive chains with small anchors, the current trend is to use tiny chains with massive anchors. Since the anchor is doing so much work, why set all that chain?
Having a long rode means that there is more rode to set and recover when arriving and leaving. You also will swing further, causing you to bump into neighboring boats when anchoring close to other yachts. Lastly, that long rode can wrap around various obstacles on the bottom and cause problems when it is time to retrieve the anchor.
So, by setting 4:1 scope, you are eliminating many of these issues. The short scope means that less ground tackle is needed to be handled. You will swing less meaning that you can anchor very close to other yachts and not swing into them. And lastly, since your rode is short, only the anchor will contact the bottom reducing the risk of having your rode wrap around obstacles on the sea floor.
I feel that the reason this push is occurring is because people view boats as another vehicle that they are familiar with: cars. Cars are our most commonly used vehicle for transportation, and features of cars have made their way into yachts over the years. Headlights on cars have spawned the search light on the bow of boats. The maneuverability of a car has pushed the use of bow and stern thrusters on almost all new boats (I once saw a 15 foot center console boat with bow and stern thrusters). Lastly, people park their car in parking lots, crammed in ontop of each other, so why not do the same with their boats?
The issue is, boats are not cars. Boats are maneuverable in a different way. For a long time, they did not have thrusters, and you simply did what the single screw was capable of. Prop walk was used to your advantage, and situations where it would hurt you were avoided.
Anchoring was another technique where you set your yacht far from other boats, with enough swing room to spin around the anchor and not touch anyone else. If you let out a lot of scope, you will also anchor far from everyone else.
So, if modern anchors can hold so much better, why use all that rode? It just makes more work for you in the end? Well, anchors holding power is scope dependent. At 4:1, you will have around 30% of the anchors holding power. At 7:1, you will have around 70% of the holding power, and at 10:1, you finally have 100% holding power.
If you are anchoring for a few hours in calm and settled conditions, a short scope is acceptable. When you are anchoring over night or in unsettled conditions, short scope is a liability and should be avoided.
The push for 4:1 scope will lead to more vessels dragging anchor, and winding up in trouble. While anchorages get filled more and more tightly, we choose to have plenty of scope out and plenty of swing room by staying far from other boats. This allows us the peace of mind that we won't drag anchor, and plenty of time to react when someone else starts to drag anchor with their short scope.
I feel that this whole "short scope" push is being propagated by marina dwellers who never anchor and armchair sailors who dream of sailing but never leave the comfort of their couch. They have never anchored and do not realize the forces involved. The problem is that they are a very loud group who is constantly telling everybody how to do it, and as more people hear about short scope, more people will begin to try it.
Short scope works find in calm conditions, but when conditions deteriorate, these same people will face a lot of damage as they will begin to drag anchor and not know why.
If you are told to anchor with 4:1 scope, or even presented with articles about anchoring with short scope, ignore it and do 7:1 scope (with all chain) as a minimum and as your standard. This will ensure that your anchoring is uneventful and safe. Boats are not cars, so you shouldn't attempt to handle your yacht like you do your compact commuter car.