(Corran Addison; Oct 30th, 2019)
“Edges” on a kayak is really a misnomer. People refer to sharp edges as doing this or that and in fact, it's so much more complex, with the actual sharpness of the edge in question (usually people are referring to X in the image above) having the least overall effect on both the positive and negative performance characteristics.
Reality is that this edge, while important, probably has the least role to play. Comparing boat A and D, they have similar shapes and sidewalls, with X being rounded off in D. The reality is that D is not going to be any more forgiving than A (in water-rock impact is another story), but it will be less loose and slower on a wave. Thus, the “predictability gain” in going from A to D is nominal, but the performance loss is notable.
AS you move from A to C, keeping X the same “sharpness”, the boat progressively gets more predictable. The angled sidewall, more than anything, helps in forgiveness and predictability. The water has less chance of slamming into the sidewall and then like a switch, engaging that edge and trying to flip you.
Conversely, the water running under the boat gets to the initial release edge, then curls up to the upper edge X. Some release happens between the two. When the water leaves X heading away from the boat, it has the hardest time curling up the sidewall of A. This makes A the loosest and fastest of the three. When you get to C, the water, at slower speeds, can make that curve onto the sidewall, and this starts to “see” the kayak as a displacement boat.
In all three - A to C, the “edge” is just as sharp. Its the side angle to the edge that has changed.
Planing hulled creek boas tend to look like C. Not as loose (especially on smaller waves), but far more forgiving in funky water and when glancing off rocks.
ICF style freestyle boats tend to be like B, being somewhat forgiving (because flushing off a wave is no good when you have 45 seconds to win). Still loose but nothing WOW.
My wave surfing river playboats are more like A. You’re not in super hard water so the odd brace or roll is worth it to gain speed and looseness on even the smallest waves.
The faster and bigger the wave, the more A to C feel alike. The water is going so fast it can't make the turn onto the sidewall of C. So in all 4 of the places in the world with super-fast waves, A or C is pretty much the same. Not as “exciting feeling” with C, not quite as fast or loose, but totally fine. Put these (all other things being the same - rocker, outline, etc) onto some small SE waves, and, with A, you’re spinning and, with C, you’re locked in a front surf.
A is more like the 303. D is more like the Funky Monkey. The Funky Monkey has D because it is also taken into rocky environments, and the rounded edge is not for forgiveness in water and currents, but glancing off rocks. It’ll get less damaged, hook up less on rocks and so on. It’s not quite as loose as A on a wave, or as “exciting” to surf, but it's not bad. It’s a small difference and worth the trade-off for the rocky environment, it can go in.
As for the name, in surfing X called the rail. In canoes the chine. Most kayakers just refer to all of it as edges (even ones on a deck, or at the gunnel/seam line) like they’re all one and the same