Ah, the ole terminology problem…
yet again rears its ugly head. I like to stick to the long accepted nautical terms used by hull designers for centuries. It is a universal language understood around the world. The question here is clearly stated but those terms just get confusing, so…
Symmetry refers to the shape of a hull as seen from directly overhead (birdseye view) This is known as a “breath plan” In this view, “centerline” is an imaginary straight line drawn lengthwise from one point to the other. Now, if the hull is drawn in “breath plan” on a piece of clear tissue paper then folded at right angles to the centerline so that the points coincide and the hull lines are an exact match, then the hull is symmetrical. If the lines do not match, it is asymmetrical.
Next let’s consider the shape of the hull in “sheer plan” or what architects call “side elevation”. This is a view of the hull from the side at eyelevel. It is the view from the side, if you placed the hull on a long table at eyelevel. So, let’s draw our hull and tabletop in “side elevation” on our clear paper and fold it at a right angle to the tabletop exactly at mid-ship. If the lines exactly overlap then the rocker is identical. If one stem is lower and one higher than the other, then differential rocker is exists.
Just to avoid confusion, it might be best thet the term symmetry should not be applied when speaking of “rocker”.
Ah, the ole terminology problem…
No, sorry, still wrong terminology
I think the relevant terms for describing the symmetry of a canoe hull should be waterline, sheer line and rocker line.
The birds-eye view is not as important as the fishes-eye view of the hull's waterline, which is the footprint the hull makes in the water.
If the widest part of the hull is at the center of the waterline, this is what classically would be called a "symmetrical" canoe. If the widest part of the canoe is aft of center, this is a "swedeform" canoe. If the widest part is abow of center, this is a "fish form" canoe. (More technically, these terms refer to the location of the longitudinal center of buoyancy -- or "centroid" -- along the center line of the waterline.)
Now, it may be that the birds-eye "breadth plan" shape of a canoe is usually or almost always the same as the waterline shape, but that doesn't have to be so.
This is the curve of the gunwale as viewed from the side and as measured down to the water surface in an unladen canoe. If the sheer line rises to the same heights in the bow and stern, the sheer line would be symmetrical. Many canoes, however, have a higher bow than stern (including the Wildfire, I believe), and hence have an asymmetrical sheer line.
The important point is that the sheer line does not measure the rocker, which is at least partially under the water.
This is the curve of the bottom of the canoe hull from the center balance point of the hull on a flat surface to the points at the bow and stern where the keel line turns upward into the stems. However, there is really no universal measurement standard for determining these end points -- i.e., where the keel ends and the stems begin.
Wherever one does the end measurement, if the height of the rocker from the flat surface is different in the bow and stern, then the canoe has an asymmetrical rocker line or has "differential rocker". The differential rocker in flatwater canoes is always a lowered (or "skegged") stern rocker. In many differentially rockered whitewater canoes, however, the stern rocker is greater than the bow rocker.
The point is that the rocker line is not necessarily parallel to, or the same curve as, the sheer line. A canoe could have a symmetrical rocker line and an asymmetrical sheer line -- which I believe is the case for the Wildfire.
To repeat, I think the best use of the unspecified terms "symmetrical" and "asymmetrical" should be in reference to the waterline shape.
I’m trying to keep this simple and transpose academic hull design terminology into layman’s language but I’m 100% certain of what I state. The terms referred to in my examples are those used by naval architects universally. First there exist several “waterlines” in hull design. Which are you using in your example? There is a “Design Waterline” and many canoeists refer to the “Wetted Waterline” but it’s location is highly variable. Fact is, it doesn’t matter, the “Breadth Plan” is all one needs because all the “Buttock Lines “are typically shown in this drawing and asymmetry or symmetry is readily apparent regardless of the drafted waterlines . Secondly “Rocker” is readily apparent in the “Sheer Plan” by simply noting the gaps in stern and bow between the “BaseLine” and final “Keel Line” A “Base Line” is what I called a “tabletop” in my previous example. Rocker is made even more apparent by the presence of “Water Lines” which are a straight line grid to provide horizontal and vertical reference. The “Sheer Line” also shows up there. Please go to your reference books on naval architecture and review the various line drawing methods. I recommend “Introduction to Naval Architecture” by Gillmer and Johnson. Chapter 3 on Ship Geometry is enlightening.
Can you guys continue this
You two would have a great time.
Add Charlie and the paddling worlds problems would be solved.
You mean keep all this technical talk
about paddling just among ourselves? I see upwards of 100 posts on subjects ranging from Peregrine Falcons,bats,making coffee,the personality of some self promoter down in Georgia,and people’s pets,ad anuseum, so why not discuss paddling terminology here? I understand there is a large group here who just want mindless drivel,but submit there’s pleantly of room for scholarly discussion as well. This is a free forum, why limit ourselves?
Pagayeur, I will believe what you say about what can be read from a naval architect’s paper designs. But I have never looked at a paper design for a canoe, and no one else does either. (Well, there’s you, Charlie Wilson, John Winters … but I mean Jill Sixpack.)
I was describing what you see when looking at an actual canoe. I also think my terminology is commonly used. If you look at an actual canoe from a birds-eye view, you will not see any of the waterlines.
I’m talking about the design waterline. If the LCB is at the middle of the centerline of the design waterline, that’s a “symmetrical” canoe, even if it doesn’t pass the folded paper test. I agree with you that using the term to refer to rocker is confusing.
I don’t agree with this sentence of yours:
“If one stem is lower and one higher than the other, then differential rocker is exists.”
My post explains why. Perhaps you misspoke.
Let me start again:
I don’t suggest anyone draw anything on paper. I am trying to explain that hull designers have been using a certain terminology for centuries and perhaps we can all be on the same page if we adopt these terms which are universal. For instance, the terms “symmetrical” and “asymmetrical” are used to describe the hull shape as seen in “Breadth Plan” ( rather than “Sheer Line Plan”). The confusion comes when folks apply these terms to rocker which can be seen only in “Sheer Line Plan”. So have someone tilt the hull on its side then sit on the ground and look at it’s shape in “Breadth Plan” Is it symmetrical on both sides of the beam or not? Now turn the hull so that it rests on its bottom on a flat surface (base line) Is the rocker the same on both sides of the beam? If so, let’s not apply the the same terms, “synmmetrical or asymmetrical” to rocker or confusion results. Let’s call it identical rocker or differentical rocker. I speak of paper only to illustrate what the various classic “views” and which terms apply to them. Look, I got in a long discussion with a guy who swore the hull under discussion was asymmetrical. I had been told by D. Y. that this was a symmetrical hull. I told him that and the reply was that he had put the hull on a flat surface and the rocker was different on either side of the beam. He had applied a term to the “Breadth Plan” shape and thus confusion.So if folks can understand the various views and which terms apply to them then, voila, we can have cogent discussions even on line.
BTW, I think your decription of rocker and mine are pretty much the same. Hope this makes sense.
Holy Cow Guys!
I've specified length, width, tumblehome and rocker for 10 hulls to a very qualified designer and have been invited to help with another six. It's always a long discussion.
Of course, my 10 plus pale into insignificance compared to Bill Swift's total of over 50 or Mike Chick's number, probably well over a hundred hulls speced.
Symmetry can be had in top view; waterline or max or rail dimensions and in side view; shear or rocker. If you are unhappy with the design terminolgy we use, I'm sorry, but that won't change the terms used between builders and designers.
Come on! You are both friends, I've shared hotel rooms and a couple board tables with with John and various discussions with Glenn and value both your opinions, but you're not going to dictate new terminology to the paddlesport industry. You are parsing a perfectly polished and symmetrical stainless steel sphere!
[I love alliteration.]
this is just a discussion not a knife fight. I feel no contentiousness. Glenn, sorry if you felt that any angst from my direction. It was never intended that way.
I’m not nieve enough to think that I can change the world overnight. My goal is merely to bring educated suggestion into the mix. Any change must start with new ideas and that always yields differing opinions. And so we think, then learn. Thanks for your willingness to play peacemaker but for my part it’s not necessary.
Perhaps you mean "woe"
Pagayeur, I had no angst and enjoyed our exchange. In many of my posts I am attempting to give my knowledge and experience, FWIW, on some paddling subject. I see you doing that, too.
I think we were both trying to clarify the subject of “assymetrical” or “differential” rocker, specifically, and also pointing out the more general terminological confusion that is endemic in the paddling world.
(BTW, freestyle canoeing is an example of a discipline that has adopted a terminology that confuses many.)
The only point of substantive disagreement on my side was as to the sentence of yours that I quoted, which I think was not really a disagreement on substance but simply word ambiguity. I thought your sentence could be read to measure rocker as the distance from the TOP of the stems (the ends of the sheer line) down to the flat surface. I was pointing out that rocker is measured by the distance from the BOTTOM of the stems (the end point of the “rocker line”) down to the flat surface.
Come to think of it, I think I made up the term “rocker line”. It’s a term I use in my head and writings, but maybe no one else does. I think the more normal name for the shape of the bottom of the canoe would be “keel line”.
An Einsteinian thought experiment easily demonstrates that the sheer line of a canoe cannot be parallel to the rocker line … uh, that is … to the keel line. If these two lines were parallel, the canoe would have uniform depth at all places in the hull.
Moreover, the hull would be infinitely long. This, of course, would be greatly beneficial. An infinitely long hull, based on some screwy formula that only John Winters understands, would have an infinite top hull speed. Now, that’s a fast boat … maybe as fast as a British boat.
However, the average Pnet paddler could not cruise at an infinite hull speed … unless they were sitting on the bottom of the canoe and using a Greenland paddle.