When There's No Room For Scope
When it comes to scope, there's no two ways about it: more is always better. More scope means less vertical strain on the anchor thereby decreasing the chances of unsetting the anchor. Unfortunately, in small or crowded anchorages there's often not enough room to allow for a lot of scope. What can you do?
When facing this delimna, the aim is to increase horizontal rode tension. This is a basically a fancy way of saying you want the rode to sit as horizontal as possible in the water.
There are a couple of ways to do this. The most common way is to add 20 feet or so of chain to nylon rope. Using his excellent tension calculator (see link below), John Holtrop showed adding just 20 feet of chain to 180 feet of nylon rode increased horizontal strain nearly 250%. This is the result of the extra weight pulling the rode to the bottom. By using 200 feet of chain you can increase horizontal tension nearly 800% from all nylon rope. However, it's debatable whether the tension gains achieved in an all chain setup offset its disadvantages, most importantly the strains on one's arms! Without a doubt though, at least 20 feet of chain should be added to all rode setups.
The other popular option in increasing horizontal strains is to add a kellet. A kellet is a weight added somewhere along the rode. Adding the kellet as close to the chain-rope connection as possible results in the largest tension gains but many prefer to place it about half way along the rode to ease the shock loads. Holtrop showed that by adding a 20 lbs kellet half way up (or down) a 200 foot rode setup one would achieve 300% horizontal tension gains over an all nylon setup.
Credit
Holtrop, John. 2006. http://www.johnsboatstuff.com/Articles/anchor.htm
