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Measuring and changing rocker

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    #16
    Originally posted by memaquay View Post
    Interesting, I was always under the impression that widening the gunwales would decrease rocker/increase hogging.
    If you open an envelope (widening gunwales) the bottom edge curves up (more rocker). Close the envelope and the bottom goes flat again.

    Alan

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      #17
      Originally posted by memaquay View Post
      Interesting, I was always under the impression that widening the gunwales would decrease rocker/increase hogging. I had a friend with a 16 foot kevlar prospector who tucked the gunwales in from 36 to 33, certainly didn't seem to decrease rocker.
      Like Alan said it will increase the rocker on an envelope, in real life it is a bit hard to notice and the changes are subtle.... The longer the boat and the less changes you'll notice, and if you go behind a certain point you will end up with a reverse rocker... Like if you go way wide!! It is all a fine line!!

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        #18
        Originally posted by MagicPaddler View Post
        I had a different canoe that the only way to stop a turn was to drag a paddle or slow down. When I notice that with this canoe it required more effort to control the canoe the faster we were going I got suspicious . . . .
        Sorry, but I'm going to repeat myself.

        What you should be suspicious of is paddling technique and weight trim, not rocker, because those are the causes of the symptoms you describe, such as continuous carving until you can't stop the turn. Paddled with proper technique, a canoe with any type of rocker -- including none or slightly negative (hogged) -- can be paddled under control and effortlessly, and even if the trim is somewhat out of balance.

        Forget whether the child in front is paddling or not. Can you paddle the canoe in a straight line effortlessly from the stern? If not, the issue is likely your technique, possibly worsened by bad trim, and not the shape of the gunwale line, water line or rocker line -- unless you're in some sort of unfamiliar radical hull such as a whitewater rodeo hull.


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          #19
          Glenn
          Thank you for the response.
          I like the way a canoe paddles with a little rocker and was looking for experience other may have had in changing the rocker on a canoe.

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            #20
            Originally posted by Alan Gage View Post
            If you open an envelope (widening gunwales) the bottom edge curves up (more rocker). Close the envelope and the bottom goes flat again.
            That is a familiar changing-rocker analogy that makes sense in theory, and in practice with a paper envelope. If you further open an envelope the ends to do rise (more rocker)

            But it is an imperfect analogy at best, at least for changing gunwale width by any normal amount.

            An envelope is 9.5 inches long and a 16’ canoe is 192 inches long. Opening an envelope by another inch does visibly raise the “stems”, but the equivalent on a canoe would be widening the gunwales by something like an additional 17 or 18 inches (somebody check that math).

            I have never heard of anyone changing the gunwale distance by anything close to that. I doubt it is even possible. And canoes are not made of paper.

            Originally posted by MagicPaddler View Post
            I like the way a canoe paddles with a little rocker and was looking for experience other may have had in changing the rocker on a canoe.
            My preferred tripping canoes are “soloized” composite or Royalex tandems with moderate rocker, and I have narrowed the gunwales on some by one or at most two inches for easier reach with no apparent impact on rocker. I should note that in the process of “soloizing” those canoes I first removed all of the seats and thwarts before narrowing the hull and reinstalling a single seat and thwarts at my preferred locations, so any change along the length of sheerline was incremental from stem to center.

            I had one too wide thin skinned kevlar hull solo with flared sides on which I narrowed the gunwales by 4 inches at center, which was all the hull would allow even when stripped of seats and thwarts. All that did was induce some tumblehome, and sharpen the chine curves so much that they eventually cracked from the added stress.

            A change in gunwale width could induce hog in the bottom, especially on a flat bottomed hull. Less likely with the bottom structure of a shallow arch or shallow vee.

            The Polaris specs say that it has 2 ½” of bow rocker and 1 ½” of stern rocker. While there is no industry standard for measuring rocker the difference in rocker should be visually apparent with the hull resting on a flat floor, and negative rocker does sound like a manufacturing issue.

            This negative rocker thread may be of interest:

            http://www.canoetripping.net/forums/...d-canoe-handle

            It could have been a factory issue, layup problem or came off the mold poorly or etc.

            I have seen some whacky factory mistakes simply in the trim department; miss drilled hardware, a seat canted in the wrong direction, yokes installed backwards, even canoes factory badged with the wrong name (that’s not a Legend, it is a Horizon). I can’t imagine Northstar making this mistake, but have you check the sheer to make sure the bow (20 ½” high) is actually the bow and not the stern (17 ½”)?

            Having a skegged stern up front would be problematic.

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              #21
              I have KeelEazy keel guards which are about 1/16 thick and if they were removed the negative rocker would be gone so the boat is not badly hog back. It also does not have what I expected when I used the manufactures specks to choose a boat. My guess is it is a misprint on the manufactures web site.
              It is the correct length and the bow is taller than the stern.
              It is plain to see this boat does not have any rocker over the center 14 foot of canoe.
              I have measured the rocker with 7 different gunnel widths and there is very little difference in rocker over the range I feel safe in spreading the gunnels. (like Mike said)

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                #22
                Originally posted by MagicPaddler View Post
                Glenn
                Thank you for the response.
                I like the way a canoe paddles with a little rocker and was looking for experience other may have had in changing the rocker on a canoe.
                Well, I refocused on paddling technique when you stated that you had trouble controlling two different canoes.

                If you just want to talk about rocker for some reason, I've seen many paddlers, mostly whitewater paddlers, try to increase rocker by lengthening the thwarts. It always seemed to me that this worked better on Royalex canoes than composite canoes, but that it didn't work significantly. In addition, it undoubtedly changes other paddling characteristics and also puts permanent stresses on the hull -- especially a composite hull -- because hulls want to remain the shape of the mold they were formed in.

                The way you measured rocker in your OP suggests to me that your canoe has basically a flat bottom for most of its length, but is slightly hogged due either to manufacturing defect or pressure injury. This photo of the Northstar Polaris seems to visually confirm that there's no or insignificant rocker (in the banana sense) along most of the keel line:



                Some manufactures call slight uplift only at the ends "longitudinal deadrise" rather than rocker. I'm not sure the Polaris even has much of that. I'd question the veracity of the manufacturer's claim of 2.5" of rocker in the bow and 1.5" in the stern, and I'd also question whether, regardless of some end lift deadrise or "rocker", whether Polaris's come out of the mold slightly hogged in the middle. Too bad you didn't take your measurements before or upon delivery.

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