rocker vs waterline length

A longtime paddling friend and I rented a Cetus MV and a Nordkapp LV from CCK and paddled flat water in the Oakland Estuary. We generally cruise compatibly between 4.0 and 4.5 kts. On this occasion the Cetus was dramatically (about 0.25 kts) faster than the Nordkapp, which could not keep up to the cruising Cetus.

The Cetus has at least a 1 ft longer waterline and much less rocker. I attribute the great difference in efficiency at these moderate speeds more to the lower rocker of the Cetus, than to its longer waterline. It seems to me that the waterline difference will only predominate at speeds of about 5 kts and above.

What do you think?

It’s both
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?
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?

Is 0.25 knots really ‘dramatically’ faster?

…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. :slight_smile:

Speed difference
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.

Things to
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

No, its like
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
Speedos are generally within 1mph.


Try comparing
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
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.

Rocker and speed

– 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.

all I can go by
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.


this might interest you:

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

Could it be random variation?

– 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.

Thanks. An interesting article
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. :frowning:

Must be why
the ones I drive are so accurate. 90% domestic vehicles and 50% of those are GM. :slight_smile:

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.


You have it backwards.
"*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.

It depends…
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.