Bob chose the San Juan and it's difficult to argue with that choice given the boat's reputation. I think it's a bit more easily found that the Cal 21' as well, unless you're on the west coast.
The big advantage, and disadvantage, of the swing keel trailer sailers is the swing keel. It does allow an amazing flexibility of use but not without trade-offs. Some of those are design specific such as the ability to lock the keel down, the hoisting method, and the ingress of water. Glancing at the Ensenada's web-site I see that some have modified the boat to be able to lock it down. That's a project, and a capability, that you're going to want to investigate thoroughly before purchase.
The Cal 21's keel is lowered via portable winch from the cockpit and then a keel keeper board is inserted and a deck footing installed over that. This produces a very dry ride as far as water ingress from the keel trunk goes. Some leave their cable attached to the keel after lowering and this would also allow you to raise it when running. I find it more practical to detach it and leave the keel down until maybe landing. In lake sailing, left attached it's just something to snag weeds in the skinny stuff. The keel can be locked down from within the cabin with a pin near the pivot. I'd not want to be in a trailer-sailer with a swing keel that didn't have this capability unless only doing small lake sailing. A severe knock down or broach could cause an unlocked keel, which are in excess of 400 lbs, to slam up into the keel trunk causing untold damage far offshore.
Otherwise the Cal 21' is similar to the San Juan in most aspects. Weighing in at only 1100lbs it's quick and very responsive. It's simple as can be to sail and a great learning boat. A big advantage to it and the SJ is that you'll be sailing, and sailing fast, in the lightest of airs. Large sail area and light displacement will do that for you. The Cal and the SJ will both handle better than most all of the other boat's mentioned they'll also sail faster up to hull speed. You won't be lagging far behind anything in it's class. Build quality on the Cal is excellent with the keel pivot being any area to examine for wear or damage.
The Potter 19 is a completely different boat. It's more cramped in the cockpit and more commodious below. It's probably a more seaworthy boat when things get rough due to it's hard chine and freeboard but it is a real stone. If you're looking for a camper sailer, it and the Compac are good choices. I'd probably give the nod to the Potter on build quality but I like the Compac's looks much better. Com Pac has a nice tabernackle set-up for raising the mast as well, no small issue if you're going to be trailering it each time out. But neither one of them will keep up with the Cal or the San Juan.
"Nodrog" a Cal 21' that has been in the same family since built was fore sale recently up in Rhode Island I believe. No trailer, though. Never thought that family would sell her. Might still be available. A common comment of former Cal 21 owners seems to be that they wish they still had the boat. Even though they've gone on to other bigger boats they miss the quick and fun to sail Cal. $2000-3000 should find you a decent one.
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