We have a series of local whitewater (mix of flatwater, Class I,II) races coming up and the lack of a snowpack here in Maine will likely mean more shallow water than usual.
My question is whether I would be better off in shallow water in the 17.5' x 19", flat-bottomed multisport kayak I usually use or in a narrower (at waterline) 14'9" round-bottomed wildwater kayak.
Which boat will be less affected by shallow water drag?
The wildwater kayak has less wetted surface area but likely has a deeper draft.
Another way of asking the question is, which is a greater factor in shallow water -- wave-making resistance (which would affect the wildwater boat more) or wetted surface (which would affect the longer multisport boat more}?
Also, can you really "pop" a kayak so that is is less affected by shallow water? If so, how?
IMHO,
would depend on how fast you plan on going, how shallow the water is and what is the draught.
In very shallow water the rounded hull would work better at low speed, provided it will not run aground - that is in VERY shallow water, say up to 10 inches as less water would be sheared along as you go. Above that a “flatter” bottom shape would probably provide more glide due to less wave generation. And in over 3 feet of water you (again, probably) won’t see any difference due to hull shape. I think you will feel more difference in boat handling because a turbulent flow in 3-foot layer of water at the same speed is considerably different from the flow at the same speed in 15-foot layer - turbulence is the key word.
It’s been a long time since I solved anything with fluid dynamics in it 
Pop a Kayak
Not sure what boat would be more suitable for you,that is a call that you will need to make regardless of all the opinions you will get.
Many moons ago, I raced in marathon events in kayak class and used a Westside boat shop RPM.
It was possible to get it to Pop going thru some shallows on a race course I was very familiar with.
You needed to be aware of where the shallows were and get into sprint speed prior to hitting them and be able to maintain it thru the shallows.
The RPM is a fast boat and mine weighted 14LBS.
Minimal issue with hull
I think shallow water and paddle blade make a difference.
If you simply can’t get the full blade in the water
you’ll loose a lot of ability to make power.
http://www.examiner.com/images/blog/EXID23194/images/Variety_Paddle_Shot.jpg
which brings up another question . . .
Do the advantages provided by a wing paddle disappear or even turn into drawbacks in shallow water?
greenland paddles
greenland paddles are a serious disadvantage when in shallow waters inasmuch as one cannot dip the blade sufficiently without the threat of sustaining impact
Hulls for shallow water
Marathon canoe racers use Swede form hulls to reduce squat in shallow water while U shaped hulls are faster in deeper water.
But…
Swede-form and U-hull are not mutually exclusive. You can have a round-bottom Swede-form hull and you can have a flat-bottom Swede-form hull. U-hull is the shape of a vertical cross-section, Swede-form is the shape of the horizontal cross-section.
whitewater and rivers
I have not seen anyone ever paddle “the stick” in whitewater. Some things are just not suitable for white water. Too little control at a short notice.
Ponderings
A long time ago someone reported that one major effect which slows the boat in shallows is the pressure wave from the bow reflecting off the bottom. I seem to remember that this effect becomes apparent gradually as the water becomes shallower, and starts to become noticeable once the water depth is less than half the length of the boat. From that point it increases rapidly as the water gets shallower. I think that the way the boat gets "trapped" within its own waves is another factor, because as the water gets really shallow, the waves of the boat wake move MUCH slower, and the boat can't go any faster than those waves (this is a lot like "hull speed" in deep water, except that the speed of the boat's wake is affected by water depth rather than by wavelength).
It seems that with a shorter boat, getting slowed by the reflected pressure wave would be less pronounced in water of moderate depth, but once the water got really shallow, perhaps it wouldn't make any difference. For getting "trapped" within your own wake, a sleek boat might exceed that "artificial hull speed" more easily, but I bet not by much. Ever notice how broad your wake becomes when slogging through the shallows - how it extends way out to each side of the boat instead of trailing behind? (I notice these things easily when rowing, but most paddlers never look behind them to assess what's slowing them down). If a sleeker boat could go a bit faster you'd see the evidence, in that it wouuld sharpen the angle of that wake more toward a "V" shape.
In short, there are probably a couple of inter-related factors, so addressing one attribute of hull shape might not provide the complete answer. Also, I suspect that you'd need a pretty long distance of shallow water for this to matter.
Consider this. A buddy mine tells me that mountain-bike races are usually won by the riders with the best hill-climbing ability rather than the ones with crazy downhill skills. On a bike course, the uphill distances are equal to the downhill distances but much more TIME is spent going uphill, so any advantage in downhill speed is usually trumped by an advantage in climbing speed. By the same token, for a boat race, if the shallow sections are not a big part of the course, the boat's deep-water performance would probably be more important, but shallow-water performance would be what you want if most of your time is spent paddling the shallows. That's what makes sense to me, but I can hardly imagine trying to quanitify all the variables even if I knew how.
Nantahala (II - III)
I was standing on the bridge at NOC enjoying an amber when I noticed a guy getting out of his Wavesport EZ. The interesting thing was that he had a Greenland stick. I went and talked to him. I don’t remember exactly what he said but I think he had just tried it to see how it would go. I think his opinion was that the lack of bite in the aerated water was a bigger issue than the depth of the stroke but I could be mis-remembering.
Seriously?
Ok, I know that the fluid dynamics will, in theory, show that there is a difference.
But seriously, hitting one rock, or scraping bottom on a really shallow spot will in all likelihood make far far more difference in time than your choice of hull.
Take the boat that you are most comfortable in, and focus on making a clean run. If the water depth may be measured in inches over a significant distance of the course then go for the shallowest draft.
A valuable insight…
...and a fitting comparison.
One thing I'll add - a lot depends on the nature of the water and properties of the bottom. Moving water and flat water are two entirely different things. In a turbulent flow the issues of wave generated by the boat reflecting off the bottom will become impossible to estimate.
Theory…
is a wonderful thing. It gives people ammo for loooong and protracted agreements (and some real fights!).
I don’t blame him at all for wondering
It wasn't until I first encountered the apparent paradox of the bike-racing problem I referred to in my earlier post that I came to realize how the extremely tiny speed differences during agonizingly slow portions of a race are more important than big speed differences at higher speed, IF the time spent in those slow situations is significant. A shallow river might be a classic illustration of this principle. Wave-limited speed in extremely shallow water - almost to the point of rubbing the bottom - is slower than a person walking slowly, so in those locations, increasing your speed by just 0.1 mph would have about the same proportional effect per time spent paddling as adding another 0.5 mph when traveling at hull speed in deep water. Going 0.5 mph faster than hull speed isn't something anyone can do for any length of time with a non-racing hull, but I bet the right hull would make a speed increase of 0.1 mph in shallow water possible. Throw in the complication that the time spent in the shallows may be great enough to offset any amount of speed difference between racers while in deep water, and the effect of this difference becomes huge. Hitting a rock might cause you to lose 50 feet while your competitors keep moving, but a couple hours spent paddling 0.1 mph faster than them puts you an additional 1,000 feet ahead of them, so which is more important? Regardless of what the actual differences turn out to be, even if they are very small, there's certainly reason to believe that the right hull could be the key factor.
As another illustration, if you can lose a race by 50 feet because you hit a rock, consider that you can win a two-hour-long race by the same 50-foot distance simply by going 0.005 mph faster than the second-place boat. In easier-to-visualize terms, that's a speed difference of about one-half of a foot per minute, and that isn't much (it would take you half an hour just to gain a boat length on the other guy)! Think the right hull might give you such a tiny advantage?
Moving water versus flatwater
Yeah, I can’t imagine actually calculating any of this stuff even under ideal conditions, much less in current, where the water is swirling every which way. It’s a topic for someone a lot smarter than me.
Me neither…
…it’s plain impossible to calculate. One would need a supercomputer to model it and even then it will be iffy. Just remembering fluid dynamics from Uni makes me cringe with pain 
Paddle Blade - River Know How
I’m thinking the guy who best understands “flow”
of water in shallow river situations will get the
most “bang for his buck”.
Getting maximum paddle blade bite and maneuvering
the boat in and out of turns properly will pay off.
http://vcsar1.org/images/swiftwater/river_bend.jpg
Yep
Tiny advantages in boat speed can be negated by choosing the wrong path every so often, and tiny disadvantages can be negated by making the right choice IF the other guy makes the wrong one. Still, the only way to compare any given factor is to assume all other things are equal, and that’s what’s going on here. Knowing that if if you do everything the same in either of two boats, the faster boat WILL put you farther ahead, would you choose the slower one instead? Heck no.
Agreed!
Thanks for all the responses. Lots of food for thought. I am fully aware that hull profile is only one of many factors to consider. Just trying to get a handle on how to figure that one factor.