rocker vs waterline length - Canoeing / Kayaking

Table of Contents

rocker vs waterline length - gjf12 - Jul-15-12 4:03 PM
- It's both - george4908 - Jul-15-12 9:46 PM
- Did you swap boats? - Yanoer - Jul-16-12 12:30 AM
- huh? - NewbTastic - Jul-16-12 12:35 AM
  - Well.... - mctec - Jul-16-12 7:29 AM
    - Things to - Bernie/cny - Jul-16-12 8:21 AM
      - absolutely, good point - slushpaddler - Jul-16-12 8:22 AM
    - Re: Well... - NewbTastic - Jul-17-12 3:04 PM
  - Speed difference - jaybabina - Jul-16-12 8:18 AM
    - No, its like - adbass - Jul-16-12 9:13 AM
      - Not that I've seen - alan_gage - Jul-16-12 9:19 AM
        
        Try comparing - adbass - Jul-16-12 9:57 AM
        
        this is true - slushpaddler - Jul-16-12 10:01 AM
        all I can go by - alan_gage - Jul-16-12 10:54 AM
        
        this might interest you: - slushpaddler - Jul-16-12 10:57 AM
        
        Thanks. An interesting article - adbass - Jul-16-12 11:30 AM
        
        Over report - njkayaker - Jul-16-12 6:38 PM
        
        Must be why - alan_gage - Jul-16-12 12:18 PM
        
        Exactly... - njkayaker - Jul-16-12 5:36 PM
        a more sinister reason - slushpaddler - Jul-17-12 4:36 PM
        
        The speedo (likely) is consistently LOW - njkayaker - Jul-16-12 5:30 PM
        Portable speed detectors - pikabike - Jul-17-12 1:20 PM
        
        Could it be random variation? - guideboatguy - Jul-16-12 11:15 AM
        
        Variation - njkayaker - Jul-16-12 6:36 PM
        
        It depends... - njkayaker - Jul-16-12 5:22 PM
        re: speedos are within 1mph - NewbTastic - Jul-16-12 9:17 PM
      - You have it backwards. - njkayaker - Jul-16-12 5:17 PM
        
        OTOH you have to like it - slushpaddler - Jul-17-12 4:37 PM
- Rocker and speed - CEWilson - Jul-16-12 10:22 AM
  - Rocker and speed - njkayaker - Jul-16-12 6:41 PM
    - Hmmmm - george4908 - Jul-16-12 9:04 PM
      - Agree totally - jaybabina - Jul-17-12 8:07 AM
        
        extended bow - gjf12 - Jul-17-12 9:04 AM
        
        Extended bow - njkayaker - Jul-17-12 9:56 AM
        
        Yes, shape matters too - njkayaker - Jul-17-12 10:29 AM
        
        re: hull speed - NewbTastic - Jul-17-12 3:51 PM
        
        "Fitness" paddlers - njkayaker - Jul-18-12 8:01 AM
        
        re: 'Fitness' paddlers - NewbTastic - Jul-18-12 8:09 AM
  - I don't buy that - abc - Jul-16-12 10:36 PM
    - Agree... - NewbTastic - Jul-16-12 10:58 PM
- USCA specs limit reverse rocker - Glenn_MacGrady - Jul-16-12 10:56 PM
- Whatever Boys - CEWilson - Jul-17-12 12:32 AM
  - Fact and fallacy - Glenn_MacGrady - Jul-17-12 3:12 AM
    - Usain Bolt . - Bob_d - Jul-17-12 9:03 AM
- So, okay... - NewbTastic - Jul-17-12 3:10 PM
  - Not linear - angstrom - Jul-17-12 4:50 PM
    - The goal is to - kocho - Jul-17-12 5:36 PM
  - You didn't read the thread - njkayaker - Jul-18-12 7:55 AM
    - Wrong... - NewbTastic - Jul-18-12 8:00 AM
- CEWilson has done it again - Cliffjrs - Jul-17-12 6:34 PM

Messages in this Topic

  It's both
  Posted by: george4908 on Jul-15-12 9:46 PM (EST)

But it's hard to say without a hydrodynamic study which factor is predominant in this case. Both hulls are virtually the same width, so you can largely discount that, but fuller or finer ends will affect speed, as will the shape of the bottom in cross section. The additional rocker of the Nordkapp will tend to displace water less efficiently, probably creating more wake (did you happen to notice?). But the additional length of the Cetus will help at cruising speeds as well, not just sprinting speeds, by allowing for less effort for the same speed, or a little more speed for the same effort.

  Did you swap boats?
  Posted by: Yanoer on Jul-16-12 12:30 AM (EST)

Could've been that the Cetus fit it's paddler better than the 'Kapp fit it's paddler and thus gave him some mechanical advantage, which resulted in more speed?

  huh?
  Posted by: NewbTastic on Jul-16-12 12:35 AM (EST)

Is 0.25 knots really 'dramatically' faster?

  Well....
  Posted by: mctec on Jul-16-12 7:29 AM (EST)

...All factors considered...If you are paddling in a group for an hour, and trying to keep up, at the end of the hour you would be a quarter mile behind.

No doubt you could paddle harder and keep up. But after a while on a full day trip, that could be a significant disadvantage.

I paddle a WS Tsunami 145, and I know what I'm talking about here. :)

  Things to
  Posted by: Bernie/cny on Jul-16-12 8:21 AM (EST)

consider also include paddlers skill.Boat,paddler and gear weight and hull width/design.A paddler with better technique alone will easily outpace you all else being equal.

  absolutely, good point
  Posted by: slushpaddler on Jul-16-12 8:22 AM (EST)

  Re: Well...
  Posted by: NewbTastic on Jul-17-12 3:04 PM (EST)

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. =(

  Speed difference
  Posted by: jaybabina on Jul-16-12 8:18 AM (EST)

If the speed is 4 kts, a loss of .25 is like you getting into your car and and you can't reach 60 mph on the highway but have to stay�at 50. You would be at the repair shop right away.

  No, its like
  Posted by: adbass on Jul-16-12 9:13 AM (EST)

expecting 60 mph and getting 56.25 mph, actually.

And unless you're measuring car speed by GPS*, that is probably what you are getting anyway!

*Speedometers usually read lower than the true value.

  Not that I've seen
  Posted by: alan_gage on Jul-16-12 9:19 AM (EST)

Speedos are generally within 1mph.

Alan

  Try comparing
  Posted by: adbass on Jul-16-12 9:57 AM (EST)

your speedometer with a GPS reading, Invariably, the GPS will give a lower reading. For example, the speedo in the rental car I drove last week read 75 or 76 mph when the GPS read 70 mph (note I was in the UK) and I have seen similar in other cars. I believe (but can not prove) that this a problem with the speedometers rather than the GPS system. Note that in many jurisdictions, speedometers are required to be accurate between -0% to +10% of the true speed. Many factors can effect the operation of a speedometer (e.g., tire pressure and wear) and it seems reasonable that a manufacturer would adjust their speedometers to slightly over-estimate car-speed.

  this is true
  Posted by: slushpaddler on Jul-16-12 10:01 AM (EST)

A few manufacturers got nailed for this not too long ago: inflated performance data and "optimistic" speedos. From what I've heard the typical car speedo can be +/- up to 4 MPH, and that's with the proper tires with the proper tread.

The "+/- 10 MPH" thing is actually the range of accuracy for radar detectors at highway speeds. This is why you have a good chance of beating a highway speeding ticket if you were 10 over or less.

  all I can go by
  Posted by: alan_gage on Jul-16-12 10:54 AM (EST)

is what I see in cars I drive everyday (I'm a mechanic). The speedometer is always within 1-2mph of the actual VSS/WSS (Vehicle and Wheel Speed Sensors) readings for the engine controller and ABS. Of course it's possible the sensors themselves are reading inaccurately but I find it hard to believe since they always agree with each other on the same car and so many things rely heavily on those readings, from transmission shifting to ABS and airbag. I see no good reason for the manufacturer to intentionally make the speed sensors read low, seems it would only make for a lot more work and complications.

Also, in the past few months they've been putting up those little portable radar detectors with the digital readout in different places around town. One was on our normal test drive route and I don't remember driving any car past that detector that was off by more than 1-2mph.

Alan

  this might interest you:
  Posted by: slushpaddler on Jul-16-12 10:57 AM (EST)

http://www.caranddriver.com/features/speedometer-scandal

I think we're splitting hairs, I just thought you might find the article interesting.

  Thanks. An interesting article
  Posted by: adbass on Jul-16-12 11:30 AM (EST)

indeed. So cars manufactured in-, or exported to- Europe are most likely to over-read speed.

I'm a bit sorry for starting this speedo business. :(

  Over report
  Posted by: njkayaker on Jul-16-12 6:38 PM (EST)

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.

  Must be why
  Posted by: alan_gage on Jul-16-12 12:18 PM (EST)

the ones I drive are so accurate. 90% domestic vehicles and 50% of those are GM. :)

I guess I can see why they'd tend to run them just slightly high. If they ran them just slightly low, even .5mph, someone would make a big stink about it and they'd be fending off lawsuits from people wanting them to pay for their speeding tickets.

Alan

  Exactly...
  Posted by: njkayaker on Jul-16-12 5:36 PM (EST)

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.

  a more sinister reason
  Posted by: slushpaddler on Jul-17-12 4:36 PM (EST)

In Mazda's case it coincided with inflated horsepower ratings. I believe Mazda actually had to offer a partial rebate for the false info.

  The speedo (likely) is consistently LOW
  Posted by: njkayaker on Jul-16-12 5:30 PM (EST)

"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.

  Portable speed detectors
  Posted by: pikabike on Jul-17-12 1:20 PM (EST)

-- 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.

  Could it be random variation?
  Posted by: guideboatguy on Jul-16-12 11:15 AM (EST)

-- Last Updated: Jul-16-12 11:16 AM EST --

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. Years ago I used to read "Cycle World", and in those days, every bike they tested had what seemed to me to be a very large error in the speedometer reading, much larger than what you usually would see in cars at the time. Remember that a mechanical speedometer is a very crude device. It would be asking a lot for it to be accurate over the whole range. Not sure if the new electronic ones are much better or not.

  Variation
  Posted by: njkayaker on Jul-16-12 6:36 PM (EST)

"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".

1) actual/true speeds.
2) speed reported by the computer.
3) 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).

  It depends...
  Posted by: njkayaker on Jul-16-12 5:22 PM (EST)

VW speedometers (as an example) read lower than what the car computer reports.

That is, the speedometer consistently reports a speed that is consistently lower than the ODBII data reports.

The speed measurement is dependent on tire diameter (which is dependent on the tire air pressure).

If there's a built-in reporting error, it would be higher than actual to reduce the likelihood that a driver using the speedometer reading to avoid speeding would actually be speeding.

  re: speedos are within 1mph
  Posted by: NewbTastic on Jul-16-12 9:17 PM (EST)

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.

  You have it backwards.
  Posted by: njkayaker on Jul-16-12 5:17 PM (EST)

"*Speedometers usually read lower than the true value."

Speedometers would read -higher- than true value. That would avoid actual speeding if you use don't exceed the speedometer reading.

  OTOH you have to like it
  Posted by: slushpaddler on Jul-17-12 4:37 PM (EST)

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...)

  Rocker and speed
  Posted by: CEWilson on Jul-16-12 10:22 AM (EST)

-- Last Updated: Jul-18-12 6:38 AM EST --

Rocker doesn't decrease forward speed and usually increases it. 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.

A peek at race boats tells us a lot. ICF boats usually carry 3-4" rocker into the bow, while USCA boats have straight keel lines? The difference is in water conditions, ICF courses have a required depth. USCA races often cross riffles and deal with shallow water dynamics.

John Winter's "the Shape of the Canoe" remains the best synopsis of hull shape.

  Rocker and speed
  Posted by: njkayaker on Jul-16-12 6:41 PM (EST)

-- 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
  Posted by: george4908 on Jul-16-12 9:04 PM (EST)

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 ...

  Agree totally
  Posted by: jaybabina on Jul-17-12 8:07 AM (EST)

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
  Posted by: gjf12 on Jul-17-12 9:04 AM (EST)

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
  Posted by: njkayaker on Jul-17-12 9:56 AM (EST)

Yes. The only thing that matters in basic speed arguments is the part of the hull covered by water.

  Yes, shape matters too
  Posted by: njkayaker on Jul-17-12 10:29 AM (EST)

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.

  re: hull speed
  Posted by: NewbTastic on Jul-17-12 3:51 PM (EST)

"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.

  "Fitness" paddlers
  Posted by: njkayaker on Jul-18-12 8:01 AM (EST)

It's like "fitness" bikes. Those users are not typically going that fast/hard.

  re: 'Fitness' paddlers
  Posted by: NewbTastic on Jul-18-12 8:09 AM (EST)

"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.

  I don't buy that
  Posted by: abc on Jul-16-12 10:36 PM (EST)

"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!

  Agree...
  Posted by: NewbTastic on Jul-16-12 10:58 PM (EST)

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.

  USCA specs limit reverse rocker
  Posted by: Glenn_MacGrady on Jul-16-12 10:56 PM (EST)

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.

  Whatever Boys
  Posted by: CEWilson on Jul-17-12 12:32 AM (EST)

-- 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
  Posted by: Glenn_MacGrady on Jul-17-12 3:12 AM (EST)

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, (c) 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.

  Usain Bolt .
  Posted by: Bob_d on Jul-17-12 9:03 AM (EST)

Is quite fast too.

Didn't know he had a cousin in Iraq.

  So, okay...
  Posted by: NewbTastic on Jul-17-12 3:10 PM (EST)

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

  Not linear
  Posted by: angstrom on Jul-17-12 4:50 PM (EST)

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
  Posted by: kocho on Jul-17-12 5:36 PM (EST)

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...

  You didn't read the thread
  Posted by: njkayaker on Jul-18-12 7:55 AM (EST)

Rocker is not the only thing that determines speed.

Rocker relates to wetted area and that relates to friction.

  Wrong...
  Posted by: NewbTastic on Jul-18-12 8:00 AM (EST)

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...)

  CEWilson has done it again
  Posted by: Cliffjrs on Jul-17-12 6:34 PM (EST)

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 !

	It's both Posted by: george4908 on Jul-15-12 9:46 PM (EST)
	But it's hard to say without a hydrodynamic study which factor is predominant in this case. Both hulls are virtually the same width, so you can largely discount that, but fuller or finer ends will affect speed, as will the shape of the bottom in cross section. The additional rocker of the Nordkapp will tend to displace water less efficiently, probably creating more wake (did you happen to notice?). But the additional length of the Cetus will help at cruising speeds as well, not just sprinting speeds, by allowing for less effort for the same speed, or a little more speed for the same effort.

	Did you swap boats? Posted by: Yanoer on Jul-16-12 12:30 AM (EST)
	Could've been that the Cetus fit it's paddler better than the 'Kapp fit it's paddler and thus gave him some mechanical advantage, which resulted in more speed?

	huh? Posted by: NewbTastic on Jul-16-12 12:35 AM (EST)
	Is 0.25 knots really 'dramatically' faster?

	Well.... Posted by: mctec on Jul-16-12 7:29 AM (EST)
	...All factors considered...If you are paddling in a group for an hour, and trying to keep up, at the end of the hour you would be a quarter mile behind. No doubt you could paddle harder and keep up. But after a while on a full day trip, that could be a significant disadvantage. I paddle a WS Tsunami 145, and I know what I'm talking about here. :)

	Things to Posted by: Bernie/cny on Jul-16-12 8:21 AM (EST)
	consider also include paddlers skill.Boat,paddler and gear weight and hull width/design.A paddler with better technique alone will easily outpace you all else being equal.

	absolutely, good point Posted by: slushpaddler on Jul-16-12 8:22 AM (EST)

	Re: Well... Posted by: NewbTastic on Jul-17-12 3:04 PM (EST)
	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. =(

	Speed difference Posted by: jaybabina on Jul-16-12 8:18 AM (EST)
	If the speed is 4 kts, a loss of .25 is like you getting into your car and and you can't reach 60 mph on the highway but have to stay�at 50. You would be at the repair shop right away.

	No, its like Posted by: adbass on Jul-16-12 9:13 AM (EST)
	expecting 60 mph and getting 56.25 mph, actually. And unless you're measuring car speed by GPS, that is probably what you are getting anyway! Speedometers usually read lower than the true value.

	Not that I've seen Posted by: alan_gage on Jul-16-12 9:19 AM (EST)
	Speedos are generally within 1mph. Alan

	Try comparing Posted by: adbass on Jul-16-12 9:57 AM (EST)
	your speedometer with a GPS reading, Invariably, the GPS will give a lower reading. For example, the speedo in the rental car I drove last week read 75 or 76 mph when the GPS read 70 mph (note I was in the UK) and I have seen similar in other cars. I believe (but can not prove) that this a problem with the speedometers rather than the GPS system. Note that in many jurisdictions, speedometers are required to be accurate between -0% to +10% of the true speed. Many factors can effect the operation of a speedometer (e.g., tire pressure and wear) and it seems reasonable that a manufacturer would adjust their speedometers to slightly over-estimate car-speed.

	this is true Posted by: slushpaddler on Jul-16-12 10:01 AM (EST)
	A few manufacturers got nailed for this not too long ago: inflated performance data and "optimistic" speedos. From what I've heard the typical car speedo can be +/- up to 4 MPH, and that's with the proper tires with the proper tread. The "+/- 10 MPH" thing is actually the range of accuracy for radar detectors at highway speeds. This is why you have a good chance of beating a highway speeding ticket if you were 10 over or less.

	all I can go by Posted by: alan_gage on Jul-16-12 10:54 AM (EST)
	is what I see in cars I drive everyday (I'm a mechanic). The speedometer is always within 1-2mph of the actual VSS/WSS (Vehicle and Wheel Speed Sensors) readings for the engine controller and ABS. Of course it's possible the sensors themselves are reading inaccurately but I find it hard to believe since they always agree with each other on the same car and so many things rely heavily on those readings, from transmission shifting to ABS and airbag. I see no good reason for the manufacturer to intentionally make the speed sensors read low, seems it would only make for a lot more work and complications. Also, in the past few months they've been putting up those little portable radar detectors with the digital readout in different places around town. One was on our normal test drive route and I don't remember driving any car past that detector that was off by more than 1-2mph. Alan

	this might interest you: Posted by: slushpaddler on Jul-16-12 10:57 AM (EST)
	http://www.caranddriver.com/features/speedometer-scandal I think we're splitting hairs, I just thought you might find the article interesting.

	Thanks. An interesting article Posted by: adbass on Jul-16-12 11:30 AM (EST)
	indeed. So cars manufactured in-, or exported to- Europe are most likely to over-read speed. I'm a bit sorry for starting this speedo business. :(

	Over report Posted by: njkayaker on Jul-16-12 6:38 PM (EST)
	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.

	Must be why Posted by: alan_gage on Jul-16-12 12:18 PM (EST)
	the ones I drive are so accurate. 90% domestic vehicles and 50% of those are GM. :) I guess I can see why they'd tend to run them just slightly high. If they ran them just slightly low, even .5mph, someone would make a big stink about it and they'd be fending off lawsuits from people wanting them to pay for their speeding tickets. Alan

	Exactly... Posted by: njkayaker on Jul-16-12 5:36 PM (EST)
	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.

	a more sinister reason Posted by: slushpaddler on Jul-17-12 4:36 PM (EST)
	In Mazda's case it coincided with inflated horsepower ratings. I believe Mazda actually had to offer a partial rebate for the false info.

	The speedo (likely) is consistently LOW Posted by: njkayaker on Jul-16-12 5:30 PM (EST)
	"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.

	Portable speed detectors Posted by: pikabike on Jul-17-12 1:20 PM (EST)
	-- 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.

	Could it be random variation? Posted by: guideboatguy on Jul-16-12 11:15 AM (EST)
	-- Last Updated: Jul-16-12 11:16 AM EST -- 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. Years ago I used to read "Cycle World", and in those days, every bike they tested had what seemed to me to be a very large error in the speedometer reading, much larger than what you usually would see in cars at the time. Remember that a mechanical speedometer is a very crude device. It would be asking a lot for it to be accurate over the whole range. Not sure if the new electronic ones are much better or not.

	Variation Posted by: njkayaker on Jul-16-12 6:36 PM (EST)
	"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". 1) actual/true speeds. 2) speed reported by the computer. 3) 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).

	It depends... Posted by: njkayaker on Jul-16-12 5:22 PM (EST)
	VW speedometers (as an example) read lower than what the car computer reports. That is, the speedometer consistently reports a speed that is consistently lower than the ODBII data reports. The speed measurement is dependent on tire diameter (which is dependent on the tire air pressure). If there's a built-in reporting error, it would be higher than actual to reduce the likelihood that a driver using the speedometer reading to avoid speeding would actually be speeding.

	re: speedos are within 1mph Posted by: NewbTastic on Jul-16-12 9:17 PM (EST)
	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.

	You have it backwards. Posted by: njkayaker on Jul-16-12 5:17 PM (EST)
	"*Speedometers usually read lower than the true value." Speedometers would read -higher- than true value. That would avoid actual speeding if you use don't exceed the speedometer reading.

	OTOH you have to like it Posted by: slushpaddler on Jul-17-12 4:37 PM (EST)
	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...)

	Rocker and speed Posted by: CEWilson on Jul-16-12 10:22 AM (EST)
	-- Last Updated: Jul-18-12 6:38 AM EST -- Rocker doesn't decrease forward speed and usually increases it. 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. A peek at race boats tells us a lot. ICF boats usually carry 3-4" rocker into the bow, while USCA boats have straight keel lines? The difference is in water conditions, ICF courses have a required depth. USCA races often cross riffles and deal with shallow water dynamics. John Winter's "the Shape of the Canoe" remains the best synopsis of hull shape.

	Rocker and speed Posted by: njkayaker on Jul-16-12 6:41 PM (EST)
	-- 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 Posted by: george4908 on Jul-16-12 9:04 PM (EST)
	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 ...