What is the measurement of "rocker"

We always talk about how much rocker a boat has, or doesn't have, What is the measurement? And how does one know that one boat has more than the other. I would assume that the builders have "borrowed" one another's boats to see and compare, I have seen diagrams of Kayaks and surfski's with cross sections, But it really looks like a bunch of lines to me since I have nothing to relate them to. I have also seen boats mounted on walls and pictures taken but again without reference the pictures don't mean much.

This is a good link on design and building of surfski's and where I got the diagram http://www.surfski.info/content/view/218/156/

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Submitted by OceanOhana on Tue, 11/25/2008 - 2:13pm



Think of a 'rocking chair'.

Different rocking chairs can have different rockers. So do boats.
It relates to the curve of the underwater hull as seen in the side view.

The rocker can have all imaginable shapes/curves, that's why there is no standard description about it.
Stated differently: unless the bottom of a hull is absolutely straight in the side view, it has a rocker.

It makes as much a difference in a rocking chair as in a boat.

For the function of a rocker think of an inverted wing profile:

The rocker determines how the water flows under the boat.

Example: a lot of rocker/curve = longer travel of the water under the hull, that's why higher speed of the water traveling, that's why lower pressure under the hull, that's why boat sucked down and a little slower.
But: less side area front and/or back, that's why less resistance when boat wants to turn, that's why better maneuvers.
Example: no rocker - just the opposite of the above.

Further: the entire shape of your underwater hull counts as much as your 'rocker'.
Every section can be, and likely is, rockered. If not, you have a box.

Thus an actually better equivalent to understand would be a rocking chair mounted on the inside of a bowl.


#1 Tue, 11/25/2008 - 3:44pm


The pictures of the plug being carved is real, the drawings of the plans are of one of the many prototypes. Epic are not silly enough to publish their plans, though their skis have been "copied" direct from the hull.

Pretty good likeness huh!!

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Rambo


#2 Tue, 11/25/2008 - 3:41pm


It is my contention.the amount of rocker determines how straight the canoe will go.

  • average rocker would be 3 inches
  • 4 of rocker would make it hard to go straight
  • and 2' rocker would make it hard to turn

#3 Tue, 11/25/2008 - 4:37pm


here's one way to get rocker measurements

1) turn the boat upside down.

2) situate it so that the center of the boat will balance a level at level.

3) pull a super tight and level string above the boat.

4) measure the rocker as distance from the string at whatever interval you like.

5) the rocker will be the difference in measurement at center to measurement at nose, and likewise for the tail.

yay.


#4 Tue, 11/25/2008 - 4:41pm


I understand what rocker is, but how is it measured. How can someone say a Fusion has more rocker than a pueo, or a pegasus, With the boats so close in design I find it very hard to believe that someone can simply look at a boats and say one has more/less, without taking precise measurements. It may appear that it has more/less but that may be an optical illusion.


#5 Tue, 11/25/2008 - 4:46pm


The best way to measure rocker is to place the canoe in a swimming pool or other still water with a paddler on it and mark the water line, from there you have a perfect reference to measure wetted surface area, displacement and the rocker either with the string line method or a simple pointing laser.

The rocker is more than just the measurement along the keel line, it's the entire underwater area (wetted surface) of the canoe.

Ramboscie


#6 Tue, 11/25/2008 - 5:10pm


Go to http://www.epickayaks.com/news/news/want-rocker-think-again

May make sense of Rocker measurement etc


#7 Tue, 11/25/2008 - 7:29pm


The rocker is, if you will, simply the line from bow to stern.

This curvature can have thousands of variations.

Just to say 'this is a 3 inch rocker' does not tell you very much; where is the deepest point, where and how is it sloped, is there any flat area, is it continuous etc..
One boat has a 3 inch rocker front and no rocker aft, the next boat has a 3 inch rocker front and a 3 inch rocker aft.

If an experienced guy says that there is 'more' rocker, he says 'the line is more curved'.
If all OC1 have comparable rockers, lets say a continuous rocker, deepest point under the seat, more front than aft, then that statement makes some sense.

If you compare boats with different rockers such as a non continuous with a flat section versus continuous, then the statement does not say much, imo.


#8 Tue, 11/25/2008 - 7:39pm


The point i was trying to make, is you can have a dead straight line along the keel from bow to stern, but the shape of the wetted surface area either side of this line (it could be a convex at the bow like the V6Proto below which allows use a straighter rocker line) determines how it handles.

Notice the almost straight rocker line.

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#9 Tue, 11/25/2008 - 9:23pm


3 feet of rocker

alt text


#10 Tue, 11/25/2008 - 10:15pm


Not much "wetted "surface area on him jc ..... hahaha

Rambo


#11 Tue, 11/25/2008 - 10:24pm


lol


#12 Tue, 11/25/2008 - 10:32pm


Good topic , as eckhart said , rocker / amount thereof / location on the hull / flat areas / etc , make comparing boats trickey for sure.

One of the best boat designers I` ve ever seen is Ted Van Deusen of :

http://www.vandusenracingboats.com/

Ted has been able to design boats for peak gliding performance at various paddler, power outputs.

For example , his "Mohawk" one person sprint kayak was a " break through" design .

And if I have this right and I think I do , the boat doesn`t have spectacular glide at lower speeds but once powered up to reach its peak part of the , speed curve , the boat kind of " lets go " of drag forces a little better than other designs.

This whole subject of boat performance gets really complicated in a hurry , many factors go into designing fast boats especially when the difference between "fast " and "slow " is only a fraction of 1 mph .

Ordinarily I would go off on a "chine" discussion at this point but I think you all have been well educated on the virtues or lack thereoff of chines.

Fast boats displace water more efficiently than slow boats . The very fastest boats climb out of the water and plane , or at least lift a little more than the others.

In a fast ocean , the paddler who can plane longest and thereby save energy and connect more waves will probably win most of the time.

Have a very happy Thanksgiving ... :)

" All Truth All The Time"


#13 Wed, 11/26/2008 - 5:39am


all the various combinations of measurements like width, depth, length, height, rocker, and especially the shape, make the design of a canoe and then there's the construction with moor variations.how they make it what they make it with. a side note I worked at a paddle company for a while and we made thousands of paddles and we always did it the same way with the same stuff and they would come out ever so slightly different. And also application, we all have different places to paddle.

sorry blabbering on. What is the measurement? was the question.

the only way to tell is to measure it as accurately as you can or have the resource's to measure it.


#14 Wed, 11/26/2008 - 6:41am


I just about passed out when Fuze passed on another "chine" discussion.
Are you alright man?
Must be the Obama effect.


#15 Wed, 11/26/2008 - 9:07am


That's a rocker


#16 Wed, 11/26/2008 - 9:23am


OO ,
if you want compare the bottom curve's of different hulls, have to use some standardised measurement system, like on the pic.
With the measured data you can not only compare hulls with each-other but also you can read from the curves the characteristic of the hull.
Hope this helps a little.

Ecky, maybe add a new column??

Laz


#17 Wed, 11/26/2008 - 9:47am


Heavywater, I added a column here:

http://oc1design.blogspot.com/2008/11/oc-1-designing-and-building-oc-1-r...


#18 Wed, 11/26/2008 - 11:56am


The oc-1 comparison chart is a good example of why I posted this question, what is the measurement of the rocker on each boat, On the Huki it says less, Less than what? Minimal or Moderate is a better way of generalizing the amount, But did anyone really measure these boats to get an actual number that says for certain that the Hurricane is less than the Fuze.
Can a number or percentage be assigned to a boats rocker? If we are going to say that all OC boats have moderate rocker, does the fuze have more or less moderate rocker than the Zephyr?


#19 Wed, 11/26/2008 - 1:15pm


I’m hoping a few builders can jump in here. After reading the answers provided here and taking at some of the attachments and links provided in this thread, I began to feel some of the frustration I imagine Ocean Ohana is having right now. In order to approach any measurement scientifically, a system must be introduced that provides adequate estimates of precision; meaning that the measurement has to be taken in the same way every time and must use the same unit of measurement every time.

So, per previous posts in this thread, some of the things that comprise the elusive OC1 “rocker” are the amount of “curve”, the point on the hull where that curve begins and ends, the pitch or angle of the curve compared to the surface area, the “wetted surface”, and probably a few others I missed or misunderstood.

This makes for a pretty complex measurement. Is it a measurement of angle? Is it a measurement of displacement? Surface area? A combination of all the above?

Again, it’d be nice to hear from some of the visitors and posters who have some formal education in this arena. But it seems to me that a standard for measurement involving rocker would need to be described by several separate measurements.

I’m guessing what Ocean Ohana is ultimately seeking is a standard, non-biased measurement that can be applied to each canoe for comparison. Just like we compare canoe length and canoe weight which are easily quantifiable. It’d be nice to have all these measurements plus some others like displacement on all canoes for side-by-side comparison. Rambo & Eckhart, this seems to be your department.


#20 Wed, 11/26/2008 - 1:51pm


Before I started building my boat I really wanted to know what rocker might be the best to choose.

So I read about everything that is published on the net, asked every builder etc until I reached the following understanding - and stopped thinking about 'rocker':

"rocker' seems to be a term that describes what you see/"feel" when you look at the bottom of a boat. You see more or less of a curve. You can measure this curve, but that will give you a two dimensional picture, a lengthwise slice cut out at the mid-line of the hull only.

How much does that tell you ? Not that much - it is only a slice.

It does tell you quite a bit for comparison only if you leave all other parameters constant or close to each other:

If two boats have the same/similar displacement volume, length of waterline, beam at waterline, sharpness bow and stern - then I'd say it makes sense to say " one has more rocker and will be easier to control, ... " ; simply because the boats are alike otherwise.
OC1 are quite alike; so it makes sense to compare based on the rocker.

Other than that: a box = no rocker, behaves different from a ball= max rocker - common sense.

If you want to use a precise description, you need to take 'all the rockers' of a hull, that is, put a 3 D reflection of the hull into a program and let it calculate what pressures/frictions etc. you have at any given point of the hull, as you suggest.

Regarding rocker - it is a slice only. That's all you measure. There is no standard because this slice can have any shape/slope/apex imaginable. There is not even an agreement re where to take any measurements. It is frustrating, indeed.

Don't expect " 3 dimensional " information from a 2 dimensional measurement.

While you are building, the rocker is of utmost importance because it 'guides' the general shape of the hull. How much slope aft to get best take off surfing ? Hm - where are you surfing - Makapu'u, Hawaii Kai ?
The likely way to do it, is to look at other comparable boats and come up with a general idea. Then you build it and hope that it works.
Other things such as V shape, roundness etc are also very important.

The only easy scenario: a flat water boat that does not need to turn = minimal or no rocker.

I gave up thinking about it. Surfskis have little rocker and do well. Wave conditions are different every day and everywhere. And many other variables. You end up starting to build with a moderate rocker, change later if you are not satisfied with the performance.

Nice picture, Heavywater, I will put it into my post if you don't mind.


#21 Wed, 11/26/2008 - 4:07pm


Six as one, the answer Oceanohana is seeking is not that simple, rocker on it's own cannot be defined as a useful measurement.

As Ecky said, it's the 3 dimensional aspect that is the more complete guide, hull designers talk in Prismatic and Block Coefficients that encompass more than just a single measurement.

For us Gumby's, as a guide, bananas surf better than celery sticks and a demo ride will tell you more about the boat than just looking at it.

Cheers Rambo


#22 Wed, 11/26/2008 - 5:29pm


I liked your pointing laser suggestion Rambo Ive seen them for 30 $ but don't have one. and a demo ride would be the best way to find out.

If i had to find out Exactly what the bend is. I would pull a straight line with a string. lay the canoe on its side. and measure across. I would leave the string a few inches away from the canoe and minus that later. but try to get it equal on front and back. and go every two feet that would be very close to the exact measurements.

maybe we should all try this with our different canoes .I don't have access to to many different top canoes. I could do my Millennium S.S. and Hurricane.I still think it will be close to 3 inches maybe 3 1/4 and compair data.

O.C. paddler data bank.


#23 Thu, 11/27/2008 - 7:23am


When I was in high school a rocker was defined as someone who wore spandex, used a lot of hairspray to get their hair just right, and loved to listen to their music loud. Ahhhh...those were the days...
Moanalua High would never be the same!


#24 Thu, 11/27/2008 - 7:40am


mulus, if you give a single number as rocker wouldn't that be the same as the draft ?


#25 Thu, 11/27/2008 - 11:12am


A draft? is a drawing, sketch, or design. and the single # like 3 is from the highest point to the lowest point of the rocker and that I would call the rockers measurement.

The original question was. We always talk about how much rocker a boat has, or dose not have, What is the measurement?

I was telling my brother who also builds canoes, about this question and he said there's a difference in me and him of 60 lbs or so and that reacts different in water ( wetted surface area ) and as you could Imagine new paddlers to experienced paddlers there's differences of how the canoe reacts.

I think it would be interesting to see the differences and compare them. Canoe builders could use this info to make better canoes.


#26 Thu, 11/27/2008 - 2:37pm


See here; the word has more than one meaning. The frustrating part about the rocker as a single number is that it does not tell where that deepest point is.

http://www.oneoceankayaks.com/smhydro/hydro.htm#draft


#27 Thu, 11/27/2008 - 3:25pm


If I can remember something from my boatbuilding apprenticeship days, I think the most accurate / efficient way to quantify rocker would be to relate it back to the block co-efficient.
Imagine the form of a house brick has a value of 100 and every time you shave into it and reduce the "blockness" thereby possibly increasing the surface area you are reducing the co-efficient value, so yes Ramboscie a banana would have a lower vale than a stick of celery, and hence possibly more rocker.
Im sure someone else can explain it more clearly but I think thats the path to follow.
over to you, boffins.


#28 Thu, 11/27/2008 - 4:54pm


I asked the question on boat design forum and one of the senior members (jehardiman) gave this answer:

Aqua;

Rocker is one of those things like the proper sweep of a sheerline, you know it when you see it. There is no definite measurement of it.

Rocker is put into a basic hullform for 2 main reasons. First is to improve turning ability by reducing and concentrating the submerged profile towards the center of mass. The second reason is to manage the placement of the center of submerged volume. This is important in making an burdensome but easily driven hull. Either of these two factors can be managed by other means, so a boat with more rocker does not always mean better handling or lower resistance. The other thing is that rocker measured at the bow and at the stern may be different, however this is not usually the case for canoes and kayaks (not sure about dugouts though).

It is possible to define rocker in terms of rise/keel length or rise/maximum hull depth but these type of measurements really have little information in them without a visual of the hullform.


#29 Fri, 11/28/2008 - 10:59am


The amount of rocker depends on lots of factors like lenght,width,displacment and target speed .
Ted Vandusen is one of all time greats .But you are only mixing things up with the K1 boats as they are designed for much higher speeds and for very short distances ,those boats are real energy vampires and can barely sustain half their design speed on long run .But can if enough propulsive power is provided go much faster than Skis and Ouriggers. Skis are as easier to sustain at speed than k1 but cant be pushed near their top speeds and the Oc-1 dont have propulsive power to go any faster so need to be optimised for lowest drag for sustained speed.
Here is where the rocker starts to be interesting ,at K1 racing speeds wave drag is quite a dominant factor so there in not much rocker in the bow in adition to much rocker would make the boats move to much sideways on the start,once you slow down to lets say 5-6knots wave drag becomes much less important and weted suface starts to play a mayor role.so you start taking away unecesary surface,bow now only barely needs to touch water and the tial the same that creates a hull with some rocker ,then if you narrow the hull(Oc compared to Ski)you start loosing volume which you again regain by making the boat deeper ,which means even more rocker ,after that you start pushing the CG forward to make the hull more efficient which creates flater forward part of the hull botom creating nice long sweeping rocker line in the back and a bit short but still rockerd at the front. And then you play to get that just right feeling (while comparing all the evolutionary steps for preformance)
One reason why the rocker is usualy smaller that it could/should be is because the builders don't build boats for only for target paddlers weight ,they usualy include hefty margins ,which is bad for preformance but good for bussines.

Before enyone starts protesting to much note that boxy hulls like hurricane play by a bit diferent set of rules.
And rocker a as fact is not designed in for manuvrability (rather the manuvrability(maybe not on oc-4) is a byproduct and is definetly overrated ) in any flat water boat and Oc-1 and Skis are design wise purely flatwater boats.Boat with no rocker with same displacment and dimansions would have cca1/6 - 1/4 more weted surface and 5-10% more drag and and a quite a bit less speed .

For ilustration in K1 1% diference in speed is almost unbeatable in top class event ,which was the case of Vandusen's boats for the 88' Seul games.Which were developed at a great expense with unprecedented R&D using money that was left from Los Angeles games to develop sports hardware for US athlets for thech heavy sports at which US wasnt very succesfoul then. An that is a fact


#30 Fri, 11/28/2008 - 1:55pm


Then you throw in wave riding and things change again.

Ride as many canoes as you can, you will soon instinctively know what works in what conditions and what doesn't.

Maybe we shouldn't be pigeon holing canoes.

Rambo


#31 Fri, 11/28/2008 - 3:22pm


Wave riding doesn't change things as much as you might think.
In next couple of years the new ski designs will be much more optimised for flat water

And have to say that the paddlers feed back is very dependent on the paddler and i have worked with some of very best in WW paddling and very few (realy tiny number)of them really knew how and why things work the way they do.With some 1L=1kg was hard to explain as people soon start dreaming to much about hulls planing and creating more lift etc.

The most important factor in any design is designers feel for that just the right look,which also improves with experience.
A British friend of mine, one of the best canoe designers ever ,himself never sat in and canoe or kayak ,but he has a sharp mind thinks logicaly and has a feel for speed.Plus he methodicaly builds a boat for a certain paddler for which he feels that has the potential winning races and for developing a boat and that is able to give him good feedback ,to continue to evolve his designs .He designs 2-3 designs per year as Slalom boats tend to evolve very rapidly and in 2 years they are antiques.

For now most Oc's ,Va'a ,skis have been built without any real design ,trough evolutionary steps by trial and error ,computer work is only now entering this arena at a bigger scale but still computers can only analize what designers feed them and if the concept and design is not right no amount of computer work will make it right.


#32 Fri, 11/28/2008 - 4:08pm


And when designing an OC would the ama's volume displaced and rocker enter the calculations.

Also two celery sticks traveling side by side would track of each other.

I'm a Gecko holding canoes


#33 Fri, 11/28/2008 - 4:10pm


"Boat with no rocker with same displacment and dimansions would have cca1/6 - 1/4 more weted surface and 5-10% more drag and and a quite a bit less speed ." - Unless I misunderstand what you are trying to say I'd argue that this is just not the case.

If you keep the displacement, but change the rocker, the dimensions must change - you must distribute the displacement volume somewhere.

I googled for Rambo's 'Rocker' recently; time to retire from this thread.


#34 Fri, 11/28/2008 - 7:28pm


I still recon this is the fastest shape on the planet

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Cheers Rambo


#35 Fri, 11/28/2008 - 7:48pm


Look at my previus posts ,but here it is again ,bow and stern of a no rocker boat provide very little buoyancy for the amount of weted surface they present ,when you take that out by reducing the draft of the bov and stern you ad it on to the middle of the hull by deepening it but due to the mid section beinng relatively full the increase of volume caries much smaler penalty in terms of weted surface.


#36 Fri, 11/28/2008 - 11:07pm


but it could help with tracking. which would add to speed.do you think?


#37 Fri, 11/28/2008 - 11:24pm


Hey canoemaker, you explain the facts very well.

I think that 'rocker', as a simple term for bottom curvature of a hull, does not explain much besides the reduction in wetted skin compared to a rocker-less form of same volume.

Measurement of rocker profile from a chosen waterline explains little about a hulls volumetric distrubution or functional characteristics


#38 Sat, 11/29/2008 - 12:42pm


I stepped away from this one for a few day to eat and paddle, eat and paddle, etc. during the holiday.
After reading through some of the really informative posts, I'm still left with the feeling that quantifying "rocker" is still very elusive for those of us without a boat building background and yet one of the great topics of conversation on every beach I've paddled from is "this canoe has more rocker than that canoe" or "that canoe has better rocker than this canoe" or "this canoe has the best rocker for here" and yet even the pros can't easily explain how we arrive at that "more" or "better".
When I suggested a common measurement, I didn't expect a single number; we are after all dealing with three dimension. What would be nice for the rest of us is a common understanding and a standardized way to discuss "rocker".
After the majority of the conversations I hear on this topic and from what I've read in this thread I'm reminded of the old joke about art, "I don't know what it is, but I know it when I see it." I was just hoping for something less subjective.
Thanks everyone that has contributed so far for helping to clarify it for the rest of us.


#39 Sun, 11/30/2008 - 5:22pm


" A description of the amount of curvature in the bottom of the hull. Often measured in inches of height to bow or stern from the lowest point in the hull's bottom. Unfortunately, this can cause a lot of confusion because the bow and the stern are always even with the water surface and the lowest point of the boat is the draft which really does not change all that much between various boats, really all you are talking about is the draft. Rocker really tells you how quickly the keel of the boat approaches its full depth.

A boat with a lot of rocker will slope gradually down to its deepest point, where a boat with little rocker will slope down quickly and then it will have a relatively straight keel. "

source 'kayakforum' - good general description, imo.


#40 Tue, 12/02/2008 - 8:51am


" Rocker really tells you how quickly the keel of the boat approaches it's full depth............. A boat with a lot of rocker will slope gradually down to its deepest point, where a boat with a little rocker will slope down quickly and then it will have a relatvely straight keel"

So...if the slope towards the full depth of keel (draft) is near the nose end, after which the bottom curvature slopes gradually up towards the tail, then the rocker is all in the nose??.....is greater because there is more curvature over shorter distance??.....or else the hulls draft is merely greatest forward of midships and it does not actually have much rocer at all........or else..............or else


#41 Mon, 12/01/2008 - 9:34am


if you talk to surfboard shapers they would normally consive rocker as the bottom curve towards the nose.................cause the tail one is called Kick.

to me the bottom curve of a canoes is the rocker, from nose to tail.......and its hard subjet to mesure, cause as a lot of prior posts said, the variables in this ecuations are sensitive to each design....

i agree with the ones that resume by saying, more rocker good for surf.....less rocker good for flats.....happy middle?

happy middle.


#42 Tue, 12/02/2008 - 7:29am


nicely said.


#43 Tue, 12/02/2008 - 6:34am


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