Sketchrbob,
The anchor included by the factory with the R-25 is a clone knock-off of a Bruce anchor. I presume it is similar to that included with your R-21. It is quite adequate as a daytime lunch-hook or fishing anchor, i.e., when the skipper is awake and keeping watch. The length of chain commendably included by the factory will enhance the anchor’s effectiveness. Retrieval will not be a problem. It may meet your needs.
However, the anchor is not adequate for overnighting or when the wind kicks up. While you may not intend to overnight in your R-21, you may desire an additional margin of safety.
It is insufficient to determine the size of an anchor solely by the length of the boat. Other factors also apply, such as your intended use (lunch hook, working anchor, or storm anchor), the weight of the boat, and its windage. The prime factor is the horizontal load that the boat will exert on the anchor. Note, for example, that the R-25 is a heavy boat for its size. Based on the R-25’s working weight, most anchor manufacturers recommend a size anchor larger than that normally associated with its length (and bigger than the anchor included by the factory). A similar approach will apply to your R-21.
Quick setting and resetting are important traits for an anchor, but, as a cruiser, I am most concerned with the maximum horizontal load before the anchor drags or breaks free.
Four excellent anchors are the Fortress (a Danforth design on steroids), and the Delta, Rocna, or Quickline Ultra (all modern plow designs). The Rocna has demonstrated excellent performance with less scope. Upgrading the anchor is a sound investment in your safety. Unfortunately, you do get what you pay for.
On my R-25 I have a 22-lb Rocna. A friend has an 18-lb Ultra – very high performing, but a little pricey. Both boats have 50’ of chain. Smaller versions of these anchors may meet your needs.
Scope is scope. A 5:1 ratio is o.k. for a lunch hook. After all, you will be awake and keeping watch. Otherwise a minimum ratio of 7:1 is normal. Use more scope during a blow. You can shorten the ratio somewhat in a crowded anchorage, but the anchor’s performance will suffer. Some anchors perform notably worse with a shortened scope.
As to the desired length of chain, skippers differ in their preferences. Certainly not less than ½” of chain per foot of boat (recommended by Chapmans). 1” of chain per foot of boat is good (recommended by BoatUS). More than that is not essential, but may improve your anchor’s performance in bad weather. Boats anchoring among rocky bottoms or coral tend to use more chain. Boats on the Chesapeake Bay, with its soft bottom, can get by with less.
Good luck!
--Doug