Guideboatguy, I found your first post clear, and correct. Celia had a lot of good information, but I don’t think any of it disagreed with you.
As far as answering the original question:
“If that same boat was between 15’-17’, would there be a bit more stability? Or just a longer, faster, twitchy boat?”, Guideboat’s answer is spot on:
“Yes, there are a lot of variables, but yes, given the same hull shape and width, making the boat longer will make it more stable. Adding length is just another way of putting more hull in the water, and though the effect on stability in this case is not as pronounced as it would be if simply adding width, the effect will still be quite noticeable.”
And Celia reiterates it with this:
"you are also at the top of the weight range for that boat. It is specifically designed per the manufacturer’s page for the small to medium paddler. In fact their web site shows a female paddling it. You are a medium sized paddler, and well may be sinking the boat to its optimal waterline just sitting in it without gear. A kayak that goes past that waterline gets more unstable, for example that is why you usually get the extra water out of a kayak in an assisted rescue. A waterlogged kayak is too deep in the water hence unstable and extremely likely to capsize again.
An example for myself would be me, at 190 pounds, finding a 17’ kayak as maneuverable as a 15’ kayak, but a person at 150 pounds finding the 15’ kayak more maneuverable. I need either more length or more width to have my kayak sink the same depth as the 150 pound person. And shallower draft can have an effect on maneuverability. Just one factor, but it is still a factor.
Guideboatguy and Celia both point out that you need more volume somewhere to be more stable. While adding width will make the biggest stability difference, adding length will also add stability, and could provide you with overall better benefits than simply widening for stability.
We can all be stable on an air mattress or an innertube. The world of kayaks tends to blur the difference between sea kayaking, a sea kayaker in a craft that lends itself to effective and skilled sea kayaking; and recreational kayaking, a craft that lends itself to (still very fun) floating about, and keeping unskilled paddlers (still good, wonderful, respectable people, but let’s not call everyone a sea kayaker) feeling stable and upright. There’s no right or wrong answer. It’s what you want to do with it. If you’re ready to declare the Eddyline Rio as a twitchy boat, it’s important to recognize that we’re not presently discussing sea kayaking, nor designs and attributes that any sea kayaker would care to paddle. As Peter mentioned above, he would not use Eddylines for “beginner” classes. And calling the Rio a capable sea kayak (don’t care if you can keep it upright in waves - I’m talking ability to effectively travel in textured seas and wind - whitewater kayaks and waveskis are decidedly not sea kayaks, and waves are not a problem) would be a thin stretch.
It’s hard to imagine, when first starting out, that a skilled sea kayaker has no use for the stability that you hoped for. But it is actually very true, to a surprisingly remarkable extent.
You’ve given an indication that would fly in the face of becoming a skilled sea kayaker. “I got dumped. Rio didn’t like me.”
You either focus on how you capsized the kayak, and how you could improve to make the kayak perform. Or you focus on how the kayak dumped you, and what equipment changes could prevent that from happening again. No right or wrong answer. The answer does help define you as a paddler. 5’8" 165 lbs shouldn’t be a problem in a 12’ x 24" kayak with a capacity of 270