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Measuring and changing rocker

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    Measuring and changing rocker

    I have a smaller tandem canoe about 3 years old. The only person in the front seat has been a 60 pound youngster. He is developing his paddling skill and last summer he worked hard at doing his share of keeping the canoe moving. I noticed when he was paddling hard the canoe was harder to control. Reminded me of a canoe I owned that had no rocker and was hard to control.
    Last week I set the canoe on a couple of carpenter horses and setup a laser level over it and measured the rocker. Starting in the center and using a felt tip I marked off one foot points to take measurements from. The Canoe is 16’ 9” and there are 15 points covering 14’ of the canoe. My measurements show a flat to slight negative rocker. Manufactures speck is 2.5”bw. and 1.5” st. The canoe has not been over any beaver dams.
    I have tried to change the rocker by pushing the gunnels out and have had some success by removing all 3 thwarts and pushing the center out by 1&1/2 inches. It now has a positive rocker of about 1/3 inch. Yet to try is pushing the gunnels out at the other 2 thwarts.
    Anyone have any experience at doing this pros cons. Will I notice it in the handling of the canoe?

    #2
    The Canoe is 16’ 9” and there are 15 points covering 14’ of the canoe.
    I take that to mean there is 2 1/2' of the canoe that isn't being measured? Rocker measurements have always been so ambiguous. It could be that the manufacturer was measuring at some point along the stem as it was curving up.

    What's the model of the canoe and what material is it made of. Perhaps others here have experience with it.

    What did you mean by hard to control? Hard to turn or it turned like it had a mind of its own?

    Alan

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      #3
      By hard to control I meant one a turn has started the canoe wants to continue turning that direction. The faster it is going the more the effect is noticed.
      That is correct 2&1/2 ‘ of the canoe is outside the area measured. Any further out toward the ends and a little error in location makes a large error in measurement.
      I have had very good canoes from this manufacture so I do not want to bad mouth them. In order to see if others have had experience with this model canoe I will name it. It is a Polaris by NSC. The canoe is Kevlar with aluminum gunnels.

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        #4
        Since it's a composite boat, I assume with a foam core, it's doubtful that it's changed shape since it left the mold and I also doubt you'll be able to make a large difference in the amount of rocker. I'm assuming they measured hull rocker beyond where you're measuring.

        I've paddled a handful of canoes with no rocker and I never found them difficult to control in the way you describe. They were certainly different to control, in that you couldn't turn as fast and had to anticipate your moves ahead of time, but otherwise they responded as expected.

        It sounds to me like the canoe is bow heavy, which doesn't really make sense with a 60 pound bow paddler. Is there any extra weight up there? Or are you paddling from the center? Or paddling it backwards to better distribute the paddlers weights?

        Alan
        Last edited by Alan Gage; 12-26-2019, 12:28 PM.

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          #5
          I mentioned 60 lb bow paddler because I have seen problems with heavy bow paddlers. We are bow high in the water although I put the heavier pack right behind him.

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            #6
            It sure sounds like the bow is digging in. Negative rocker...wow, that sounds pretty bad. You have to measure right at the tip of the bow and stern...maybe sit it on a flat surface upright and stand back to eyeball it. Did it sit out in the hot hot sun somewhere?

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              #7
              Originally posted by MagicPaddler View Post
              By hard to control I meant one a turn has started the canoe wants to continue turning that direction. The faster it is going the more the effect is noticed.
              Doesn't sound like a rocker problem to me, but rather a paddling technique problem.

              If a turning force stroke is applied to a canoe while the canoe is heeled (leaned), the canoe will start to carve away from the side of the paddle. With a low rocker canoe this usually happens more so with the canoe heeled to the outside of the turn, although some canoes (usually highly rockered ones) will carve more with an inside heel. Carving is cause by the build up of differential water pressures on the bow planes. The canoe will carve to the side with the lesser bow plane water pressure.

              You don't say whether you paddle tandem hit & switch or single-sided with a stern correction stroke. In either case, the strokes should be simultaneous, so that one paddler doesn't overpower the other in terms of turning forces. Because of the different lever arms, the stern should always be able to overpower the bow paddler's turning force, or reduce the stern force, so as to keep the canoe on an arrow-straight line. In addition, the canoe shouldn't be heeled or bobbled from side-to-side while paddling, or else it may begin to carve away from the stronger paddler's turning force.

              So, try to: (a) stroke in unison; (b) equalize the turning vectors of each forward stroke, which is mainly the stern's job, by keeping as vertical paddle as possible and stroking parallel the keel line, which are both the paddlers' jobs; and (c) don't heel the boat side-to-side when going straight -- heeling is a technique to enhance turning.

              Comment


                #8
                With this boat and at the speed we are paddling and stern heavy I can still control it. 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 and finally got around to checking. We do not stroke together he is a small boy and I do most of the work of getting us where we are going. In a couple of years we will probably be able to do that.
                I have eyeballed down the bottom and it is flat from end to end.

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                  #9
                  If the boat is supposed to have rocker and you have laid it on a flat surface and it doesn't seem to have rocker at all, then it is likely time to bring the manufacturer in on the conversation. that is likely the only way this will get resolved or explained.


                  Brian

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                    #10
                    I'm not quite sure I'm following. Is the canoe hogged? Is that what you mean by negative rocker?

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                      #11
                      Originally posted by MagicPaddler View Post
                      I mentioned 60 lb bow paddler because I have seen problems with heavy bow paddlers. We are bow high in the water although I put the heavier pack right behind him.
                      I hesitate to write because there are more experienced canoeists here, but....

                      I believe that when you run bow high, you've effectively put more rocker in the bow (think popping a wheelie) and the canoe is going to naturally turn more easily away from the side being paddled by the airborne bow paddler (assuming they can reach the water with their paddle).

                      With a 60 pound bow paddler and an adult in the stern you are always going to run bow high unless (1) you are an adult elf who also weighs 60 lbs. or (2) you trim the canoe properly. If you can't adequately trim the canoe by shifting the heavier pack or other gear forward, put some water ballast up front -- in the very front of the bow if possible. I do that when paddling solo or with my dog (who weighs about 60 pounds) if I'm out for a day paddle without gear to trim the canoe. I use a dry bag or two and just scoop up water at the put-in and jam the dry bag(s) up into the bow as far as they will go. You'll cut down on the bow paddler's leg room, but I can't imagine with a 60 pound paddler you need to worry too much about their leg room.

                      Now, as for measuring rocker (which I doubt is your problem), I don't see why this is such a mystery. Put the canoe on level floor. Stand back. If both stems and middle are all touching the floor up until the point the stems turn upwards, there is zero rocker. If both stems are touching the floor and there is space between the center and the floor (you may need to get down on all fours to check), you've got negative rocker. If the center is touching the floor and the stems are off the floor at all before they turn upwards, you've got rocker. Measure furthest distance from stems to floor before the stems begin to turn upward. Now you know how much rocker (if any) you have.
                      Last edited by alsg; 12-27-2019, 11:13 AM.

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                        #12
                        If the boat is supposed to have rocker and you have laid it on a flat surface and it doesn't seem to have rocker at all, then it is likely time to bring the manufacturer in on the conversation.
                        The problem with this is that there is no specification on where rocker is to be measured or how it's to be shaped. You can have rocker that runs gradually from the center of the boat to the stems or it can be flat nearly the whole way and then sweep up suddenly at the stem. The manufacturer can measure it 3" behind the stem or 3' behind the stem.

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                          #13
                          Plastic hulls are pron to flexing, ( oil canning), a lot ! Spreading the Shear width, is likely going to increase that effect.

                          It's OK to experiment ! Just try and make "Temporary" adjustments ! You might want to go back to square one.

                          Good Luck !

                          Jim
                          Keep your paddle wet, and your seat dry !

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                            #14
                            I have cut up some scrap lumber and forced it between the gunnels at the 3 thwart locations. The gunnels are now wider at the 3 thwarts by 1.625 &1.75 & 1.125 inches. Measuring the rocker it has been increased by about 7/16 inch. Alan nailed it in that foam cores don’t change much. I had falsely belief that I could push the gunnels out and greatly effect rocker.

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                              #15
                              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.

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