Reverse rocker in a small canoe?

Discussion in 'Boat Design' started by kengrome, Jun 17, 2008.

  1. kengrome
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    kengrome Senior Member

    Last night I came up with the bizarre idea of creating a very small one-man canoe.

    At first I was going to make it a flat panel design for simple plywood construction with minimal joints. Then I clicked the wrong button in my design software at precisely the right time -- and the hard chines 'disappeared' leaving me with the design shown below (after I made a few minor modifications).

    It's still only 8 feet long and since I read somewhere that reverse rocker helps tracking in short boats by keeping the ends firmly implanted in the water, this design still has a bit of this reverse rocker in it.

    Will reverse rocker actually help or hinder the performance of such a short and small boat?

    I tried to find some existing small canoes on the web and some information about how and when to use reverse rocker in a canoe/kayak, but I didn't find anything that might help me answer these questions, so I'm hoping someone here can give some pertinent guidelines ... or at least their logical thoughts on the matter.

    As far as the details of the design are concerned, I attached a FreeShip hydrostatic report which provides most of the info. Note the longitudinal asymmetry and the tumblehome to make it easier to paddle. My thoughts on construction are to make a one-off plug so I can mold it in epoxy and fiberglass. Naturally the plug would be destroyed to remove the hull.

    Comments on the design are welcome of course, but I'm mostly interested in learning whether or not the reverse rocker makes sense. Thanks.
     

    Attached Files:

  2. eponodyne
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    eponodyne Senior Member

    Looks like it would form a big vacuum seal. If you wish for this thing to fly, that'd be the first problem to go after.

    As for your other questions, I have no idea. I'm just a deck ape.
     
  3. duluthboats
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    duluthboats Senior Dreamer

    Ken,
    I can give you no hard numbers but I can reason from many years of building and paddling canoes that a hull with a hog in it like that will be far less efficient and be harder to control than having the keel line straight.
    Gary
     
  4. Knut Sand
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    Knut Sand Senior Member

    Eponodyne is pretty accurate, and very close to my experience here...

    A zillion years ago, in the scouts, we made some fiberglass canoes, a small keel, but flat bottomed. When put into the water the bottom were coming slightly in (no promlems really), creating this shape (concave?), when paddeling, the experience was that these canoes "stopped" in the water, immedeately, when we stopped paddeling, other canoes (Slightly v bottom/ slightly canvex keel line) were still continuing gliding, after paddeling had stopped.

    This of course led to some fighting over available recourses.... Also known as the great "Stampa war"....:rolleyes:

    On the pluss side; they were experienced to be somwhat more stable than the other canoes... (Initial stability, that is.... Made newcomers relax....:D )
     
  5. kengrome
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    kengrome Senior Member

    Thanks for the replies so far folks. I've created a new version with a flat bottom because I think the boat should not stop moving the instant the paddler stops pushing it. It's going to be hard enough to get it to glide as it is, I don't think I need to make it any worse, and I think I may try a slight v-bottom version next ... :)
     
  6. rwatson
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    rwatson Senior Member

    I have less reservation about the performance issues. I dont think it would be any slower than any other 8ft canoe, and would overcome the inherant lack of directionality of these flat bottom boats.
    The trick would be not to make the ends so deep they would make turning hard. A bit of trial and error might be required.
    The other 'feature' would be that the ends would ground before the middle, but that could be good or bad.
    Overall, I think on a short canoe, the reverse rocker might be a good idea if you want a bit of directional control.
     
  7. kengrome
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    kengrome Senior Member

    rwatson, it sounds like you're thinking the same way I was when I created this thread. I was thinking that since the reverse rocker results in depressed ends, those ends might behave like skegs to keep the short little thing tracking straight. In such a short boat I cannot image turning ever being a problem ayways, and it's never going to be a 'fast' boat either ... so it's the straight line tracking I'm mostly concerned about.
     
  8. Knut Sand
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    Knut Sand Senior Member

    If the "belly" area is lower than the rest, it will turn (swing :) ) easy.
    If you will have somewhat directional stability, add a small (long) keel. It's pretty easy to twist the paddle outwards on the last part of the stroke to keep the direction (like a mirrored "J" figure, if you're right handed, and hold it a sec or two at the lower part of the letter), once learned it's like riding a bike. (then you don't have to paddle 3 strokes on the left side, and 3 strokes on the right side, and 3 str... ok, you get my meaning?

    I do not know why the one with concave bottom was slower, but we had some discussions on the theme, I stick to the theory that the boat will "feel two hills" in the water, first the bow area must push down the water, then there will be a rising area, behind there again, creating a small suction, pulling back, then it needs to rise again for the aft to pass, then at the arse you get drag/ suction there too... I think that was the theories we developed, but then, it was always by the campfire, and the wilder the better the theories always were....

    Take it with a tblsp of salt.... :)

    I still have some connections to the scouts, so anyone passing by here, is welcome to try the real thing.;)
     
  9. diwebb
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    diwebb Senior Member

    Knut Sand's idea is on the right track I feel. A boats speed is proportional to the square root of the waterline length and by creating a hollow in the center you are effectively creating two hulls one following the other and so halving the effective waterline length. The wave making created by a hull moving through the water follows the shape of the hull so any rise at the center of the boat forces the wave to asume an unnatural shape when speed progresses upwards from half speed for that length of hull, which will take much more effort to overcome. This shape effectively halves the length of wave that the hull creates.
     
  10. tom28571
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    tom28571 Senior Member

    Ken,

    You got someone to agree with you but you can add me to the list of people who think that it is a bad idea. It will likely carry more water in the boundary layer in the middle than a boat with normal rocker. More thickness to the boundary layer means more mass to move with the paddle. No wonder it stops quicker when power is removed.

    Of course, it's a bit like an Atkin tunnel, so maybe there is some of that magic there:p :D I'll bite my tongue:p

    An 8' canoe is so hard to paddle fast in the first place that adding drag is the last thing you want to do.
     
  11. Knut Sand
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    Knut Sand Senior Member

    I partly thought of that, but I feel this will not be true unless the middle section is as high as or close to the surface. But to some extent, I think you are right. The boat will also have to push some litres of water down, two times, as it passes...

    Believe me, the canoes the scouts had (We had/ have both types), with this bottom, they caused some "fighting" (to avoid), everybody wanted the canoes with the convex keel line, slightly v-bottom.
     
  12. diwebb
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    diwebb Senior Member

    double wave pattern

    Knut, once the water has changed to an upward direction then you have formed a back to the first wave, so you have created a double wave system even though the intervening crest is lower than the crest of the preceding and succeding waves. Not all waves are the same height at sea, it is the length of the wave that determines the speed and hence the displacement speed of a boat.
    Interesting theoretical discussion!!
     
  13. EStaggs
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    EStaggs Senior Member

    Correct me if Im wrong, but wouldn't extrapolation of the two hill theory work something like this:

    You are splitting the water forward, as in a conventional boat. Splendid.

    The water then follows the hull, much like the aft rocker of a flatiron skiff. Nice and efficient.

    The water then must be forced back down to clear the second point, which does two things. It generates a separate set of waves from its own depression (as noted above). It ALSO creates a small amount of lift, not unlike the wedges placed on semi-planers in the 1940s. If you introduce that lift (however small it may be, its still lift) and you are centered in the boat mass-wise, it would begin to act like a pirogue with the bow buried. If you haven't been in one paddling like that, they root and are very uncontrollable, with the aft end swinging all around.

    If you want to chase this idea out a little, leave the forefoot a little deeper, and keep a straight run aft, possibly with a little keel back there. That might give you the extra directional stability fore and aft that you seek, without the bad manners of having the lowering sections aft.

    E
     
  14. kengrome
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    kengrome Senior Member

    Wow, this has actually turned into an interesting discussion, I never would have predicted this, thanks guys!

    I have decided to eliminate the reverse rocker and put a little bit of normal rocker in the bow, and probably none at the stern. I'm not saying this to discourage our theoretical discussion about reverse rocker, but I've seen and considered your comments carefully, and I think this is the better way to go with this particular design -- so now you all know where I'm headed ... :)

    I've posted some new pictures of a more recent version which illustrates the boat with zero rocker at either end and also a very slightly arched bottom. It is very close to being a flat bottom but I decided to go with a very shallow convex curve side-to-side simply to help the boat slip over the water more easily:

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    After I created these images I made another modification -- which was to add 0.9 inches of rocker forward -- so although the pictures do not illustrate the forward rocker it is noted in these new specs which haven't changed in any other way from what you see in the pictures:

    Hull length - overall 7' 0"
    Hull width - at gunwales 25.5"
    Hull width - maximum 28.0"
    Hull width - at 4" waterline 27.7"
    Hull depth - bow 16.0"
    Hull depth - middle 11.3"
    Hull depth - stern 15.0"
    Hull rocker - bow 0.9"
    Hull rocker - stern 0.0"
    Average weight 30 lbs.
    Capacity - maximum 200 lbs.
    Capacity - efficient 150 lbs.

    I've noted your comments about tracking too. I plan to put a full length keel on the boat, or install a slide-in skeg that's maybe the same depth as the boat's bottom but 12 inches long extending rearward, or both. My guess is that with the skeg I probably won't need the full length keel, and this will make the boat faster and easier and cheaper to build. My thoughts right now are also to make the skeg 'stiff yet flexible' so it resists the yawing motion induced by paddling, but it won't break off if it hits a rock. This will probably an interesting challenge in and of itself ... :)

    I have actually gone so far as to consider something I've never seen on a canoe before -- a forward mounted skeg. If I can put one on the rear I can just as easily put another one on the front end too. I'm not sure what kind of new problems this might create, but if it didn't affect the boat's safety or handling that much, I'll bet it would eliminate virtually all the concerns I have about the boat's tracking -- even without the full length keel -- and I would really like to eliminate this keel if I can. With a skeg on each end the boat it will be harder to turn of course, but is any 7 foot boat really that difficult to turn???

    I've also (briefly) considered a rudder like the one shown on 'Dubber' in this thread at the wooden boat forum:

    http://www.woodenboat.com/forum/showthread.php?t=79102

    ... but a rudder is far more complicated and time consuming to design, create, build and install than a skeg, so right now I'm thinking that "just a skeg" -- and probably just an aft-mounted skeg -- will be more than enough to enhance straight line tracking to the point that it will never be a problem, even without the keel.

    Comments anyone?
     

  15. diwebb
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    diwebb Senior Member

    Hi Ken,
    the revisions look good. Regarding skegs, have you thought about the skegs used on sailboards?? These slot in and out easily and I believe that there is a track made that could be screwed to he bottom of the boat that would allow installation of skegs in multiple numbers and positions. This would allow for experimentation and flexibility without too much work. As regard a forward skeg I would suggest no further forward than one third from the bow, this should allow balance for side winds and also improve tracking without making turning too difficult.
    Best of luck with the project.
    David.
     
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