The speedo (likely) is consistently LOW
"The speedometer is always within 1-2mph of the actual VSS/WSS (Vehicle and Wheel Speed Sensors) readings for the engine controller and ABS. "
The speedo is (likely) going to be consistently lower (assuming wacky tire sizes/pressure aren’t being used). That would reduce the likelihood that a conscientious driver would actually exceed the speed limit.
There’s no good reason the speedometer would unintentionally be off by 1-2 mph from what the engine computer (ODBII) reports.
Exactly…
Even ignoring the “stink”, it’s actually helpful to drivers who want to avoid speeding.
Either the radar is more accurate than the car could ever be (given the car’s measurement of speed is dependent on tire pressure and tire size) OR the radar is going to read high. (The radar reading low is of no concern.)
Reporting high gives a bit of space for real-world errors.
Variation
"I’ve driven cars in which the speedometer was “right on”, and other in which it read a little higher than actual. I haven’t notice an actual trend for the speedometer reading to err on the high side."
There are three “speeds”.
- actual/true speeds.
- speed reported by the computer.
- speed reported by the speedometer.
If there’s a difference between 2 and 3, it almost certainly is deliberate. (The error/variation would be very small.)
The speed reported by 2 depends on things like tire pressure and tire size. To deal with the variation resulting from these things, some manufacturers (such as VW) make the speedo (3) report a higher speed than what the engine computer (2) is reporting. That way, the speed being reported is “conservative” (errs on the high side).
Over report
The speedos “over report” speed (displays a speed higher than the speed the engine computer is reading from the wheel rotation).
The speed measured by the engine computer could be a bit higher or lower than the actual/true speed due to things like tire diameter and tire air pressure.
Rocker and speed
– Last Updated: Jul-17-12 10:17 AM EST –
Rocker reduces wetted surface area, decreasing friction, and increasing speed (as you said).
Rocker tends to reduce tracking, which means more energy goes into the boat yawing (and yawing is exaggerated with stronger paddle strokes).
Thus, the "right" rocker is a balance of these two opposite effects. (And the appropriate rocker depends on conditions and the use as well).
Hmmmm
Doesn’t seem to add up. If you take a relatively straight hull and add more rocker, the ends lift out of the water. But the weight of the kayak is the same, and having reducing bouyancy in the ends, the center now sits lower in the water, displacing exactly the same amount of water.
Assuming constant displacement, take the two kayak forms to the extreme – a pencil vs. a sphere. Which is faster through the water? The sphere may have slightly lower wetted surface, but underway it plows.
Maybe we need a naval architect in here …
re: speedos are within 1mph
Mines haven’t been.
My last two cars have had speedos that read 2mph fast compared to the GPS speed. This is also confirmed by those helpful “You’re going this fast” side-of-the-road displays the police and CHP set up sometimes.
FWIW, I’m sort of thankful for the speedo error. One time I set my cruise control to 10 mph over the limit according to the speedo, and got tailed by a CHP who was pretty obviously radar’ing me.
He finally left me alone, but I have the distint feeling that if I’d been a true 10mph over the limit, he would’ve ticketed me. Close one.
I don’t buy that
"First, rocker decreases wetted area, so lessens skin friction, hence drag. Secondly, to reduce wave making resistance, we want to move water away from and back into the hole created by maximum bean and draft. Since much of that water moves downwards, rocker reduces wave making resistance"
If rockers are so great, how come most boat don’t have a ton? Faster, AND more menuverable. What’s not to like?
Just look at all the boat descriptions. More rocker for playboating, “yet reasonably fast”. Hmmm… it isn’t fast, it just managed not to be a dog! Reading between the lines, rocker and speed don’t go together except as compromise of the opposite end of boat characteristic!
USCA specs limit reverse rocker
The USCA, which governs most of the flatwater canoe racing in the US, has specs that limit the amount of reverse (or negative) rocker along a canoe keel to 1/8". The spec reads:
“Reverse Rocker Specification: The limit of negative rocker (hog) at the 4 inch water line jig point shall be limited to 1/8 inch at the center of a six (6) foot straight line along the longitudinal center of the canoe.”
Why is this? Is it because negative rocker, as opposed to positive rocker, is what can increase the speed of a given waterline length hull?
I find this suggestion interesting because I have heard that a national champion whitewater racer believes negative rocker may increase speed in downriver races.
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.