front rowing system for canoe

Discussion in 'Boat Design' started by jyoder111, May 3, 2011.

  1. ancient kayaker
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    ancient kayaker aka Terry Haines

    Those are all good points that I will have to take into account. I don't think the canoe outboard system will be in danger of dropping in the water but it will certainly need something to support its outboard weight otherwise the downward force on the handles will be huge. I think I will finish the canoe for paddling first so I can evaluate it like that as it is a new and somewhat experimental design that is being reported in another thread. It will likely have a shallow keel and has little rocker so it will track well and may not need a skeg. The gunnel is very low, but the linkage and outboard weight support arrangement will raise the rowing height a bit. The canoe system doesn't have to be removable from inside the boat but getting in and out of the canoe version will require some thought - there’s not much room.

    The balance of the inboard system for the sailboat should be about the same as an oar, and it should fold up conveniently. The gunnel is a bit low for ideal rowing so any added height there due to the linkage may turn out to be a plus. The oars already tuck away nicely in the sailboat so I will only have to make provision for storing the linkage, but it may be OK to just leave it if it will fold up compactly in place - the sailboat has wide side decks.

    Currently I am wondering if the same link lengths will do the job for both the inboard and outboard versions: OK, I’m a bit lazy . . .
     
  2. ancient kayaker
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    ancient kayaker aka Terry Haines

    The linkage arrangement for the front rowing system can be either inboard or outboard per posts #46 & #51. I have determined that the virtual center of rotation (or virtual oarlock rotation) is where the angled links would meet if they were extended; this is the same for both configurations The biggest design challenge is now to obtain adequate range of motion, angular - for the oar, and linear - for the handle.

    [​IMG]
    [​IMG]
     
  3. Squidly-Diddly
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    Squidly-Diddly Senior Member

    AK, here are some dimensions for sliding seat and 9' sculls

    that I've been playing with.

    the rectangle is 4' x 2' for reference, and the rows of squares on the gunnels are 1" and 1" apart, again for reference.

    The sides of the boat are flat just to make it easier to make "mates" in Solidworks.

    A sliding seat stroke seems around 4', and 'cruising' sliding seat oars seem to be at a ratio of 3/1 to 4/1 sweep/stroke.

    I haven't finished with detail like the pivot at the "tee" to allow dipping, or the lower 'a-frame' supports for the two bars from gunnel to "tee".



    WHOA! position of rower in last pic all wrong in relation to rig, or rig needs to be positioned much further forward, but other oar stroke geometry seems about right, and feels workable when I wave my arms around.
     

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  4. Squidly-Diddly
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    Squidly-Diddly Senior Member

    one niffy and useful feature could be locking oars in place

    when one of the bars comes to a "stop" at parallel to the gunnel-bar.

    It would be easy to include a little pin to hold it, and maybe a little fold-up leg from the gunnel-bar to the oar-handle to dip the oar into the water in a feathered position, thus stabilizing the boat as done by rowers with those 12" wide racing boats but 'hands free'.
     

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  5. magnus
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    magnus Junior Member

    High friction at joints

    I may be missing the concept but it looks like under normal loads we have magnified loads on the pivots/bearings and arms which requires heavy hardware and large losses in power delivery. I am an advocate of front rowing but this solution seems flawed. Please explain why I am an idiot, which is possible/probable:D
     
  6. Squidly-Diddly
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    Squidly-Diddly Senior Member

    Magnus, I'm not seeing more than 5X load of any standard

    oarlock on any of the pivots.

    Modern pivots can take very large loads and do so for thousands(if not millions) of cycles.

    A few extra pivots wont increase friction or drag enough to notice.

    Remember the loads we are talking about (from one puny human) are all but non-existent in mechanical terms. You would have to shop around to find hardware that you could wear out, much less break, for this application.

    The factor determining the size of hardware wont be the load it must withstand, but that it be large enough to be easily assembled without tools by human hands(maybe even gloved hands).



    But I will run a Solidworks Stess-Simulation on the whole shebang once I get all the actual parts in place. The bars in my drawing are from old steel bedframes. I plan to also do frames using recycled playground steel tubes with flatten ends and their matching pivot hardware. I can't find pics of those foot powered trapezoidal playground double swing chairs. I guess they don't make them any more. I'll do drawing/simulations using 6061 T6 tubes with flattened ends....3/4" OD and 1/16" walls should be plenty. 1/2" holes to take 1/2" white plastic bushings/washers and 1/4" bolts.
     
    Last edited: May 23, 2011
  7. ancient kayaker
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    ancient kayaker aka Terry Haines

    Squidly: looks like some good information on rigging here although I haven't had time to study it yet http://www.stivesrowingclub.com/Rigging.htm

    I discussing the linkage design with a mechanical engineer* as I noted in Post #62. Your design goes a lot further with the angular range than my efforts to date; I need to check on the linearity over such a wide range but it looks good. You also show that the linear range at the handles is (as could be expected if I had thought about it) proportional to the linkage lengths.

    For my fixed seat design I think I could reduce the linear range and hence the linkage lengths by 50% for a handle movement of 18" - assuming arm movement only, no back work. My oars are 6.5' compared with the 9.7' ones in your design which reduces the sweep from 12.5' to 8' and changes the ratio to 8/1.5= 5.3:1, which sounds a bit higher than usual. It looks like reducing the crossbar length slightly, perhaps by 10% would bring the ratio closer to typical value.

    I will feed those values into my drawing and see what comes out. Thanks for the ideas and efforts. These dimensions, if they work out, put the crossbar about the same place on the oar as the current oarlocks when used on the sailboat which is comfortable ofr me, so I will put the up/down pivot there. A design suitable for my canoe seems to be emerging. With the links as far forward as you show and the modified crossbar length, it may be possible to get in and out of the boat without additional arrangements.

    I have a fair amount of T6061 tubing 0.058" walls, left over from another project: sounds perfect.

    * checkout the double rocker linkage animation at http://www.brockeng.com/mechanism/index.htm
     
  8. Jeremy Harris
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    Jeremy Harris Senior Member

    That linkage looks good, but what's the physiological impact of having the handles move outward and inward that much? I'm guessing that our arms can apply maximum pull when the handles are pretty much in line with our shoulders and that we're probably better equipped to manage a hard pull when the load is inside our shoulder width than outside it.

    BTW, I've based my linkage on the old designs (and modern variants) illustrated here: http://rowingforpleasure.blogspot.com/2010/04/this-extraordinary-mechanism-is-fine.html
     
  9. upchurchmr
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    upchurchmr Senior Member

    Interesting comment in the "rowingforpleasure" site.
    "the linkage was developed in aluminum so it obviously wore out quickly".

    That would get a tremendous laugh at work (aerospace bidness). Anything that wore out quickly in aluminum just wasn't designed right. There is always a question about what weight you ended up with (in different materials) and linkages are more difficult due to space constraints (if you have any), but aluminum typically beats everything but Titanium and Composite (only in some specific cases) for weight of the system.

    We're not talking about aircraft and higher strength alloys so I agree this reply may not be the most practical comment, especially given Squidly's cost targets.

    Sorry, but the aluminum comment just isn't true in my profession.

    It is interesting to me that the "new" offerings never acknowledge the value or existance of the old designs. There is lots of value in old designs which worked without the benefit of modern metallurgy, analysis and computers. Typically there are some not so obvious features which made the design work, that are easily overlooked in the computer age.

    This is a great thread, actual technical details are always more interesting to me rather than "he said, she said".

    Thanks

    Marc
     
  10. Jeremy Harris
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    Jeremy Harris Senior Member

    I chuckled at the comment about aluminium, too (my background is aircraft design and test). The front rowing system I'm putting together is mainly aluminium alloy (6082-T6, pretty much the same as 6061-T6 that's more common in the US). I don't have any worries over strength, particularly as even a modest strength alloy like this is pretty close to being the same strength as mild steel.
     
  11. ancient kayaker
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    ancient kayaker aka Terry Haines

    Squidly: I did a rough-and-ready plot of handle and blade paths and I found that over a ±30º range the handle motion curves excessively as Jeremy predicted. Handle motion improves when the range is reduced to ±20º

    In my case, using a 6.5' oar and keeping the link and oar length proportions per your design I get ratio 3.6:1 and handle travel 1.8' which should suit a fixed seat design. In practice I will probably use only ±15º for arm motion only, which will help to further straighten out the handle path. Blade/handle ratio variation over the range is of the order of ±10%.
     
  12. NoEyeDeer
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    NoEyeDeer Senior Member

    Just a note: for standard fixed seat rowing the handle travel is usually in the 2.5-3 feet range, depending on the height of the person in question. 1.8 feet is a very short stroke.
     
  13. ancient kayaker
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    ancient kayaker aka Terry Haines

    I was basing it on what I can do in my sailboat when rowing it, as the oars are very short and there is only 2' between the handle and oarlock. Squidly's design has 4' travel with a sliding seat so I though I could safely use half that or less. The design will likely have to be revisited in any case as it is not ideal, the handle path is not what I would prefer, something straighter would be better IMHO.
     
  14. NoEyeDeer
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    NoEyeDeer Senior Member

    Ok, it's largely person perference, but even with a 2 foot inner I'd still be doing a 2 1/2 foot stroke (done it heaps of times). I'm 5'8" tall.
     

  15. ancient kayaker
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    ancient kayaker aka Terry Haines

    Structurally Aluminum is a good choice especially for the swinging arms but if plain holes are used as bearings it would only be suitable for a trial run or so . . . not much better than plain holes in wood! The swinging arms only have to deal with tension/compression stresses so they can be whatever minimum section will prevent buckling, but may have to be more substatial than that for general handling and robustness.

    Actually for the crossbar which carries the oar, wood might prove a better choice; unlike the swinging arms it has cantilever stresses as well as tension/compression, and it will need to have some kind of attachment device on it for the oar and to permit the oar to pivot.

    There is a fair amount of force building up in the linkages especially on the base pivots which are spaced by only 20% of the handle-to-blade length, and they also have to support the weight of the oar on the recovery stroke so they will need to be fairly substantial. However it amounts to no more than a roller blade bearing handles.
     
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