Mark - I'm curious about what you learned and any key insights that went into your final choice. (Also, what did you choose? What were the others on the "short list"?)
In a RIB, my choice was limited to single-hulled (outer hull only, without flat floor) designs because light weight was a key criteria. But for one that stays in the water full-time; I look for a double floor model with a sump well at the drain point. That sump well feature is important so I can install a small automatic bilge pump with a hose to transom thru-hull.
Funny, a sump large and deep enough to take a bilge pump is also one of our top criteria. Our catamaran with davits make it easy to lift and carry any type of dinghy that can fit in the 12.5' space between the hulls. We consider the added weight of a double floor a good tradeoff for the dryness and level surface for the type of use we have. As for what I found:
Hull: A deep V hull forward, gradually morphing to a flatter bottom with a bit of a bustle in the aft, and having considerable chines that are designed for more than just strengthening a cheap layup on the hull. The chine design is important because the boat is going to be riding and steering on them. The bottom should sit lower than the tubes the entire run. All of this was missing in that Achilles - in particular the tubes sat below the bottom for most of the hull run and resulted in a moon-bounce ride that was actually dangerous in waves larger than 6". It was the only RIB that ever frightened me.
Surprisingly, if you look at the hull form of the West Marine 360 RIB, you will see one of the better hull form designs out there. Others that are acceptable to us were Caribe, Flexboat, Novurania, and AB (Novurania has since stop making their smaller tiller boats). Attached are bow and stern profiles of a Flexboat showing some of these design elements. There are equally valid variations on this theme. For example, Novurania's design is a deeper V at the stern with a single larger chine, while West Marine's has a pronounced flat bustle almost like a whale tail in the stern with a chine ending further forward. The overall goal is to get the boat on plane and controllable, while not reacting vertically to waves, and guiding water away from the boat instead of inside it.
Tubes: (only discussing hypalon, as PVC is inappropriate for our needs/usage) There are six grades of hypalon, but only four of them seem to be used in RIB's. Getting information out of manufacturers on what grade is used is like pulling teeth. Some that use higher grades do seem to shout it from the rooftops, but most go to lengths to keep one from finding out. All that we looked at are using Orca Hypalon. The lowest quality is Orca 215, and this is found in the West Marine boats, and only a couple other lower end manufacturers. Too bad, because their hulls are great, but one cannot expect much robustness from the tubes. This grade punctures and abrades easily, and will permeate air earlier.
Most everyone else uses two grades up Orca 820 (I don't know why the intermediate Orca 219 is skipped in RIB manufacturing). Orca 820 can take some abrasion, and is more resistant to puncturing, as well as holding air longer. This is the minimum grade for better robustness, but is still not very good. Again, pretty much everyone uses it though - AB, Achillies, Highfield, etc.
The next grade up is Orca 828, and this grade has very high robustness. It is difficult to drive a nail through it, and takes a lot of abrasion with only cosmetic damage. Novurania uses this in their smaller RIBs. Up until a few years ago when the Venezuala situation forced economic issues with Caribe, they also used Orca 828. Now they are using 820.
Above that is military and police grade Orca 866. It is almost impossible to drive a nail through this stuff, and it is made to take all beatings. Flexboat uses this exclusively, as does Novurania in their larger boats. I think AB uses it in their professional line, but I could not get this information from any means. They could just be using 828 and lots of marketing speak about "professional quality", etc.
The highest grade of Orca fabric is (confusingly) Orca 859. This does not appear to be used in any RIB manufacturing. It's properties wrt tenacity, puncture resistance, etc is double that of 866, so it must be like steel.
With a good hull design, the tubes are just to keep the boat stable when not on plane, and deflect a small amount of wave splash (spray should be handled by the chine design, and the major component of a wave handled by the bow V design). So surprisingly, tube diameter is less important than usually touted if the hull is designed correctly. Otherwise, one is asking the tubes to do things they are not fit for. Of course, there are minimum diameters for any given hull length, and 16-17" is probably minimum for a 12' RIB.
Glued on parts like rubrails, seat patches, handholds, etc: PVC is bad here, while EPDM is good. Surprisingly, even some expensive RIBs cheap out here. West Marine, of course, uses PVC parts, and these are notorious for falling off early and being unable to secure again. This is one area of quality consumers don't seem to recognize or understand the differences, so manufacturers take advantage.
Interior design: This is more personal, but we look for things like a large deep sump that can hold a small electric pump, a fiberglass pan bottom that rises a couple of inches around the tubes so that stuff doesn't become lodged between the tubes and hull, a bow locker than can take the fuel tank, with an interior chase leading aft for the fuel hose, seat(s) that are solidly held in a fiberglass pan structure instead of sitting on top of a bouncing tube, and a few other quirks we have learned to appreciate.
Armed with all of the above new knowledge, freed from our appalling Achilles and vowing to never trust a salesman again no matter how much experience they have, we set out to find a suitable RIB for us and came up with three that met our criteria almost fully - Flexboat, Caribe, Novurania, and one that would do OK - AB. I should mention that we specifically did not want an aluminum boat, so those were not considered.
Flexboat is without a doubt the best of the lot. The only downside is weight, but this is also the downside of having a good hull design and heavier tube fabric in general. All the good boats are heavier than all the cheap boats. Flexboat is made in Brazil, and there is only a single US distributor. At the time we were trying to buy one, Brazil and US were having some type of import row with shipments held up. We could not get one in the timeframe we needed it.
At the time, we were living just down the road from the Novurania factory, and I approached them about restarting their smaller RIB line again, or at least pulling out an old mold and making me a one-off. They were actually interested and designed and built a new hull form even better than their old ones. Unfortunately, they had very strong opinions on scantlings, and this hull came out grossly heavy. No amount of discussions with their engineers could convince them that this size of boat and its intended usage didn't need some of the scantling dimensions they were using, as well as some of the design features that added unusual weight. In the end, we had to continue our search elsewhere. They eventually brought the boat to market, but it failed to generate any sales because of the weight. They ended up turning it into a even much heavier console boat. I don't know how that is selling, and they don't publish weight specs for it. A tell, though, is they recommend a minimum 30hp engine for a 11.5' boat.
Next we turned to Caribe. Working directly with their factory in Venezuela, I discovered that they had just switched to cheaper Orca 820 fabric and had a new hull design in the works. The new hull design was even nicer than the older one, but the fabric downgrade was disappointing. I tried to get them to make us a boat using Orca 866, but at the time they could not source it because of the situation in Venezuela. They did still have some Orca 828 on hand, and agreed to make us a RIB using that. So that's what we ended up with. We were the first of the new hull design. It came out a bit heavier than expected (a common occurrence for the first hull of anything), but it does ride very well.
I don't know how well Caribe is doing now, or even if they are still manufacturing. The situation in Venezuela is even graver, and the Covid must be taking a toll there. Politically, I'm not sure they can even import into the US anymore.
Apologies for the length.
Mark