I know that keyhole cockpits have become the trend on expedition sea kayaks in the past 10 years, but I definitely prefer my ocean cockpits, even (perhaps especially!) for forward paddling.
In my Nordkapp, I have both an ocean cockpit and a knee tube. That combination lets me keep my knees high and relatively centered, facilitating active leg engagement during my forward stroke, while still having instant and positive engagement with the boat for bracing. If you could look through my deck, you would see my paddle-side knee drop, driving my foot into the foot rest and hip into the back of the seat, rotating my torso fully and transmitting all forward energy into the boat.
While I could do this on a keyhole cockpit, I would have to slide my knees outboard and under the coaming knee braces if I required a solid support stroke or cockpit engagement during a capsize. With a knee tube and ocean cockpit, however, I can drive either of my knees upward and toward the center for instant boat control, but still have room to actively engage my lower body in the forward stroke.
Granted, I started paddling in the early-90s in Pintails and Nordkapps with ocean cockpits and knee tubes, so I'm very used to them. But I've also paddled extensively in Romanys, Quests, Gliders, Paradigms/Sultans, etc., with keyhole cockpits and definitely prefer the ocean cockpits in almost every situation.
Call me old school, I guess . . .
What’s a knee tube? (nm)
Knee tube
This is a knee tube in perhaps it’s least eloquent execution:
http://onkayaks.squarespace.com/journal/2007/1/7/saturday-january-7th-2007-knee-tube.html
Mine is less deep and slightly less wide, plus the front is beveled for easier entry and exit. I’ve also installed a shock-cord across the opening to prevent items from exiting the tube during surf sessions, rolling, or (hopefully the rare) wet exit.
I see
In that case, is there any reason why you can’t do the same (glue a knee tube) to a keyhole cockpit to enhance control?
yes - no room
2 reasons, it would be difficult to attach a knee tube on a keyhole cockpit and if you do, you would defeat the purpose of the keyhole, which is to be able to raise your knees in the forward portion of the keyhole.
By the way, I had not seen one of these before, but I have built a similar contraption (much narrower though) in my home-built skin on frame with an ocean cockpit. My cockpit is designed to be high-enough to allow active leg drive with kness together and has a center support b'w my kness against which I can brace. I can roll the boat without repositioning my kness. On the sides I do have knee/thigh braces for froggy-legged paddling and more bracing if desired, but for forward paddling in lumpy conditions and the occasional roll the center support is OK.
keyhole vs ocean…
“and if you do, you would defeat the purpose of the keyhole, which is to be able to raise your knees in the forward portion of the keyhole”
I never thought of the reason between keyhole vs. ocean. I thought keyholes are for easy entry (and re-entry). Naive me!
But if the reason of keyhole is to allow the raising of knees, then I assume it’s only done when condition is calm?
I learn to kayak in WW boats. So I had long ago accepted the frog legged paddling position as the ONLY way to sit in a kayak. From my prospective, I found sea kayaks’ position, even in a keyhole cockpit, a great deal more comfortable for long distance. Still, I never thought much about the frog legged position as anything but “natural” for all kayakers. 
Seems backward to me
I have an underdeck bag with my keyhole cockpit. I get in and out easily and can bring my knees to a vertical position without problem. But with an ocean cockpit a tube in the center would have to be quite narrow. The one ocean cockpit boat we owned I put foam in the center under the deck and that provided nice support for boat control. But I can’t imagine putting a tube there.
That’s what I meant
"I thought keyholes are for easy entry (and re-entry)" - that’s what I meant more or less: to raise your knees up so you can enter/exit easy. But, whether to enter or exit or to paddle with knees together your knees go in the front part of the keyhole. If you stick a tube in the middle, like Cetus or others have on a keyhole for a day hatch, then it is b/w your legs but you can’t brace against it - it is in your shin area. The one in the photo above on the ocean cockpit is much closer to you, near your knees. When paddling with knees together, generally only one knee at a time is in the center and up and the other is down, so the keyhole does not need to be too wide to allow this.
Keyhole purpose
The intent of the keyhole on sea kayaks is, indeed, two-fold. As Kocho said, it was intended to allow space for an efficient, flatwater-style lower body action. But, as ABC mentioned, it was also to allow for fast egress in surf landings. To his point, and the reason that whitewater boats have used them for decades, it’s actually easier to get a snug, rough-water fit in a kayhole with thigh braces than in a poorly padded or outfitted ocean cockpit.
But, for moderate to rough water distance paddling, a keyhole gives me two options - paddle forward efficiently with my knees in the forward section of the keyhole and sacrifice bracing, or use the thigh braces and limit my lower body contributions to forward paddling.
That’s why I’m still, after extensive paddling in both, a fan of a properly padded and outfitted ocean cockpit and knee tube for my expedition boat. I don’t mind - and in fact prefer - the keyhole on a surf, rough water play boat or a boat that I’m using for instructing, but want the other set up for long distances in mixed sea conditions.
Also as Kocho said, you could put a knee tube in a keyhole boat, but it would serve only as storage. It would be too far forward to provide any additional bracing support with you knees in an efficient paddling position.
Granted, I can’t touch my knees together like in a sprint boat, but the compromise is much more efficient than the keyhole for distance paddling on the sea . . . at least for me!
kneetube
in some of my kayaks.
http://farm3.static.flickr.com/2549/4149346431_8e8dc85bc7_m.jpg
best wishes
Roy
