Agree…
Yep, seems like thats right.
You can go over to the Tiderace site, where they list their own boats’ amount of rocker and speed (among other design characteristics).
All the high-rocker boats are slow (Xtreme, Xtra), and the faster boats are all low rocker (Pace 18, Xplore M, Xplore S).
You have to assume Tiderace knows its own boats.
Whatever Boys
– Last Updated: Jul-18-12 12:45 PM EST –
The fastest paddlecraft in the world are ICF sprint boats. The C/K4 approaching 20 mph. The solo boats~ 16.5' have about 4" rocker in the bow, ~ 3" into the stern. That is not a rule induced handicap.
The concept of differential rocker is bases upon poor paddler performance. The stern is skegged to counter the solo or stern paddlers twin problems of not getting the top hsnd outwards, across the rail and carrying the blade aft of the body. Both result in sweeping components which induce yaw, turn the hull away from the last forward stroke. Less stern rocker counters these errors. Olympic paddlers who make the team commit no such errors.
Anecdotal evidence to the contrary is just that: fairy tales based on individual observations. Comparing specific hulls we need to compute prismatic co-efficient which correlates to forward speed for given waterline length, then factor in how hard the individual is willing to work/hurt. Maximum speed always hurts.
Probably best to acquire John Winter's "the Shape of the Canoe", forward speed and forward efficiency, which aren't the same thing aren't as simple as the catalogs would have it.
Fact and fallacy
I’ll accept as fact that ICF sprint boats have bow and stern rocker. They certainly look that way from the pictures on the Nelo site.
However, one must not commit the logical fallacy called hasty generalization.
Premise: The fastest sprinter in the world, Hussein Bolt, has long arms.
Conclusion (fallacious): Longer arms make faster runners.
Premise: The fastest sprint boat in the world has rocker.
Conclusion (fallacious): Rocker makes faster boats.
The fastest sprint runner and the fastest sprint boat are what they are because of a combined package of characteristics, traits and specifications that work together in synergistic harmony. You can’t, logically, just pick one characteristic or spec out of the specialized sprint package and make a sweeping conclusion about it, unless you have further facts to support the conclusion for other types of running or paddling.
It makes sense that when you have (a) made a canoe as narrow as possible, (b) for a high kneeling human body, © maximally swede form, (d) at maximum allowed length, and (e) paddled by a super athlete who can track the hull like an arrow and drive it way over hull speed, that – under such a radical package of speed performance characteristics and specs – you can then take the liberty of reducing wetted area with some end rise (rocker) without risking energy-wasting boat yaw.
I also wonder whether the bow rocker allows the hull to “climb over” the bow wave more efficiently in a sprint boat.
However, a particularized package of sprinting specs may not be particularly good for other running or paddling usages. And this thread is about touring boats not racing boats.
Premises: Bolt can’t win a running race over 200 meters. An ICF sprint canoe can’t win the General Clinton or the Molokai (and wouldn’t be enjoyable to tour in at all).
However, it would be the fallacy of hasty generalization to conclude from those factual premises that long armed runners or swede form canoes can’t win marathons.
Agree totally
Generally in kayaks, rocker always reduces speed. Shape far outweighs wetted surface as for speed. Which is easier to push through the water, a stick or a bananna. All race boats are long with very little rocker. Al the highly rockerd sea kayaks are not fast boats, Pintail, Romany etc. Doesn’t mean they are not good kayaks and enjoyed by paddlers for a host of other reasons but they are not fast huls.
extended bow
Barton distinguishes between rocker and extended bow. In the comparison test between the Cetus and the Nordkapp LV, the Nordkapp has both more extended bow and more rocker on the portion of the hull in the water. I think it is the rocker, rather than waterline length that affects speed/efficiency, even at fast cruising, not racing speeds. Extended bow has no effect at all on either speed or efficiency. (I am not sure what, if any, benefit accrues from the extended bow.)
Extended bow
Yes. The only thing that matters in basic speed arguments is the part of the hull covered by water.
Yes, shape matters too
I didn’t quite succeed but what I was trying to say was that there’s a trade-off for reducing wetted surface area. More rocker reduces surface area (faster) but increases yawing (slower).
There are other complicated hydrodynamic issues related to shape too. These issues are complicated to convey and understand. Friction and yawing aren’t.
What people are looking for is the degree of rocker that is a balance of speed and maneuverability. The
No one wants a kayak that is just faster (it would be extremely hard to use in real conditions).
The “hull speed” arguments are generally irrelevant to most users because most users don’t paddle fast enough to be anywhere near it.
Portable speed detectors
– Last Updated: Jul-17-12 3:43 PM EST –
I, too, like to compare what those say vs. what's on my speedometer. When I am driving on a deserted highway with mile markers, I also like to see how closely a mile passes on the speedo if I drive at 60 mph (does the mile's passage match 1 minute?).
Conclusion: the truck's speedo is close to the other devices, with actual speed just slightly under the speedo's stated speed. Whether it was more accurate with the OEM tires that had less beefy tread, I don't know. These are upgrade tires and might be a tiny bit larger in circumference.
Re: Well…
Well, I wanted to reply and say that even so, 0.25 knots still isn’t ‘dramatically’ faster.
But then you went and told me you have a Tsunami 145, and I felt bad about arguing with you.
I’ve been in that boat, it’s a barge. =(
So, okay…
To the ‘more rocker = more speed’ crowd… okay, if that’s so, howcum all the slow playboats have a ton of rocker, and all the fast expedition boats have a lot less?
Do all these famous kayak designers know they’ve been doing it completely wrong all these years? =o
I bet Aled Williams is banging his head against a wall right now, yelling, “D’oh!!”. =D
re: hull speed
"The “hull speed” arguments are generally irrelevant to most users because most users don’t paddle fast enough to be anywhere near it."
Are we going by the traditional hull speed formula here?
Because using it, a typical 17 ft sea kayak with a 15 ft waterline works out to hull speed a hair over 5 knots.
Seems like a strong paddler with good form would come very close to that (or even hit it) while doing something like a 10K time-trial, or just plain ol’ paddling at a good exercise pace.
If you’re talking about just cruising along, then yes, you aren’t very near hull speed. But not everyone just cruises along. There’s plenty of ‘fitness’ paddlers out there.
a more sinister reason
In Mazda’s case it coincided with inflated horsepower ratings. I believe Mazda actually had to offer a partial rebate for the false info.
OTOH you have to like it
Margin of error in my speedo plus margin of error on a radar gun is what allows me to travel at 10 mph over on the freeway (OR at least CONVINCE me that I’m going that fast…)
Not linear
I think a more accurate statement – and what Charlie probably meant – would be “some rocker can increase speed”, not “more rocker always increases speed.”
Hull design is always a tradeoff. It’s easy to say “all else being equal…”, but in reality, it’s very difficult to change just one parameter/dimension and keep everything else equal.
The goal is to
Minimize wave making and drag due to friction at speeds that are high enough to matter (e.g., where the paddler has to really paddle to achieve). I have a very simple and unproven theory about the bow rocker: just like we want a full plumb bow that widens as slowly as possible when seen from above, I think we also want that same shape to be present when seen from the side. Basically, a very sharp half-cone as the bow, with the tip at the water surface would probably have the least resistance. And that means rocker has to be present since in the middle of the boat (or thereabouts) there would be several inches of draft, to provide the design displacement. So, the boat should have about 3" of bow rocker… Of course, such a shape will not have very good tracking, and unless one sits on the bottom of the rounded hull, will have little stability too…
CEWilson has done it again
I wish he’d stop confusing these issues with fact. I am so much more comfortable with those who begin with “I believe” (I always need to look up when reading this), “I think” (Nice to know you’re awake.), so here I am with Charley, and actually learning something once again.
Thank you !
You didn’t read the thread
Rocker is not the only thing that determines speed.
Rocker relates to wetted area and that relates to friction.
Wrong…
I did read the thread, actually.
It’s just that some of things said in it don’t seem to make much sense. Or maybe need further exploration in order to do so.
And yes, I know about wetted surface area and its impact on speed, particularly at lower cruising speeds.
(Of course, not everyone paddles at lower cruising speeds…)
“Fitness” paddlers
It’s like “fitness” bikes. Those users are not typically going that fast/hard.
re: ‘Fitness’ paddlers
"It’s like “fitness” bikes. Those users are not typically going that fast/hard."
That seems an odd (over-)generalization to make.
I myself am part of a bike club, and plenty of ppl in it go like hell, all the time. It’s not terribly uncommon behavior.
Perhaps you’re confusing the guy who rides solo on the MUT on the Wal-Mart special he picked up to “get in shape”, and all the testosterone-addled youngsters on their $3000+ carbon-fiber racing steeds.
There’s riding, and then there’s RIDING. And there’s plenty of the latter.