What is the measurement of "Rocker"

[aquafiend65]

I frequent an outrigger paddling forum that has recently asked the question "What is the measurement of rocker".
Just to Clarify, not what is rocker, or how do you measure it.
It is often stated "this canoe has less/more rocker so it will be better in these conditions"
Here is a link to the original post for clarity.

[url="http://www.ocpaddler.com/node/4497"]

We're struggling with it so I thought I'd bring it to the experts.

[Tad]

aqua....

There is no measurement, you are free to make one up if you like

It's just like the measurement of sheer, there's a lot, or there's not much, or it's straight. But keel/fairbody profiles are as individual as the boat, if it's a dead straight line we all agree there is no rocker. But beyond that there may be a little or a lot, perception and comparison are all we have.

[jehardiman]

Rocker, similiar to deadrise, is usualy measured in inches (or cm) from the lowest part of the main hull shell (i.e. exclusive of keel structure) to the "turn" of the profile or the waterline.

So if the canoe is said to have 2" of rocker, that means that the line of the keel springs 2" between the forefoot/transom and the lowest part of the hull along the centerline.

Also note, as Tad pointed out, that comparison of rocker is relative. A 10' hull and a 20' hull could both be said to have 2" of rocker, but the 10' hull would be said to have more rocker because it has more spring for it's length.

[aquafiend65]

I think what the original poster was referring to is a "known" measurement like weight. If you measure the canoe and it weighs 25 lbs, you know that there will be a displacement of 25 lbs, correct? Terms like "rocker" get thrown around like dung in the monkey cage:-) There could be 2 canoes from diferent builders that have exactly the same rocker but the shape of the hull, its entry, displacement, reserve buoyancy, etc could all be different. In using jehardiman's definition of rocker, isn't it possible for a canoe with a lower jehardiman rocker to actually have more rocker based on the other variables? And if thats the case, what then is rocker?

[jehardiman]

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.

[mick_allen]

Rocker is measured at some proportion or percentage of the length of the longest waterline. Typically listed in inches, rocker would be more descriptive to be given as a proportion of this already proportioned LWL, as jehardiman infers.

In a perfect world, rocker would be given as the simplified curvature of the keel at the chosen proportion location (inverse of an idealized simple radius of the proportioned keel). But unfortunately, rocker is typically given in absolute dimensions such as "1 1/2 inches at the bow and ½ inch at the stern”, etc.

And while I would prefer the LWL proportion to be smaller, 90% LWL is used by at least one canoe/kayak designer as the location for the rocker to be measured from.

So in the absence of a more defined location (‘turns’ of curvature, etc are too ambiguous), I use the 90% LWL proportion as the driver for defining or taking definitive measurements and then give qualifying statements for idiosyncracies after that.

-mick

[Village_Idiot]

Originally Posted by jehardiman

Aqua;

Rocker is put into a basic hullform for 2 main reasons.

You forgot a third reason... rocker may be added to help manhandle a 20-foot flat-bottom boat off of a sandbar when you run aground in the middle of the river/bay.

[Chris Ostlind]

Man o man, this is one of those really fun discussions that usually end up being.... undefined.

I liked Tad's description right off the top, "There is no measurement, you are free to make one up if you like." While this is certainly true in an empirical sense.. and you'll get similar answers from Naval Architects all over the world, it does leave things rather cloudy.

I like the turn of the curve profile as stated by John H. I happen to use that type of reference for my own design work... but there's a caveat and it should be understood before dashing away.

These chosen reference points have to relate to something and that something is about two leg lengths of typed data. Mostly, though, it has to relate to how the boat works and to really discover that, the boat needs to be built and then used in over a wide latitude of conditions. Over time, the relationship will mean something to the designer as it speaks to a "feel" that is desired in the final design.

When I first started doing this, I was banging out kayak and canoe ideas as I thought they might work. Truly, I was guessing, but I did have a bit of an historical reference in all the boats that were out there already that I could paddle and experience. Once I got that on-the-water info in my head, I started to measure the boats so that some kind of norm could be applied.

After making many boats of my own, paddling hundreds of others, talking to a collection of well-respected designers and accumulating a pile of information, I managed to bring a cohesive understanding of the subject into focus. OK, so I'm kinda slow on the uptake, but it did make for a nice, comfortable relationship for me and I can apply it with a degree of confidence.

The designers I asked about the topic offered-up, literally, a dozen different methods for coming to the rocker description. In some ways they were no help at all. It may just be a quiet conspiracy among working boat designers to inhibit the intrusion of the lesser weevils, but eventually, I saw the method in the madness.

Most of all, though... enjoy the journey.

[alan white]

Rocker is one of those descriptors that works well if you have a skiff or a canoe, and less well if you have a tug boat. But enough has been said (and said well) about the subject---- I wanted to comment on something aquafiend said about displacement.
If a canoe weighs 25 lbs, is the 25 lbs the displacement? I don't think so.
Since there's so much confusion out there regarding weight vs displacement, maybe that term should be cleared up too.
As for my definition of weight, it is clearly the best way to measure the performance potential and the ease of launching/carrying/retrieving of a light boat like a kayak or a canoe, since it should describe the boat alone.
The weight, however, doesn't tell you the carrying capacity, which you want to know especially in a small boat where the occupant(s) weighs far more than the boat itself, so displacement minus boat weight should equal carrying capacity, and displacement can also be broken down into several catagories based on kinds of carrying capacity to produce specifications like light, design waterline, heavy, and maximum (for legal purposes) displacement.
How often have I seen an advertisement for a boat or a boat design, and a figure such as "625 lbs" is shown in the specs, and I can't tell whether it means displacement or trailer weight. Obviously, in describing a canoe, you know which, but especially in boats between 200 lbs and 2000 lbs or more, there can be confusion.

Alan