As stated by hannsvillejohn, the Fortress is a good anchor given the right bottom, especially sand or mud. We used to carry one as a spare on a previous boat. Liked it because it was very lightweight and folded into a very small package. Having said that, I would not choose a Fortress or other danforth style anchor for our R-31CB. They are notoriously hard to set in weeds or grass and unreliable on a rocky bottom. That may be more of an issue here in the PNW than in your area on the east coast.
We have a Rocna 15 as our primary anchor and carry the OEM claw (“Bruce”) as our spare. If I were going to spring for a better spare, I would get the Mantus 15kg, which I believe is equal to or better than the Rocna, but (unlike the Rocna) disassembles to store in a more compact package. The best place I have found to store the spare anchor is under the forward portion of the V-berth.
A spare anchor should generally have a similar length of rode as that needed for the primary anchor, though you may want to reduce the portion that is chain. One rule of thumb says that you should have at least one boat length of chain.
I’m pretty sure other people will weigh in here since the subject of anchors is usually a hot topic and people have strong opinions.
John
