rowing rigs, forces of nature

Discussion in 'Boat Design' started by brucegseidner, Feb 15, 2014.

  1. brucegseidner
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    brucegseidner Junior Member

    I am working on a sliding rowing rig and would like to know who has figured out the forces found at the oarlock with a 280cm pair of sculling oars and a strong human motor?

    There are all kinds of ways to establish some connection of the oar lock to the boat. I have seen retro 50's looking laminated wings on the Glen-L site, all manner of braced and unbraced arrangements out of aluminum or steel. But I don't know the forces involved and am stuck. This is for a narrow boat 18'x33" the Gentry Ruth or on a SUP board.

    So rigging solutions aside, how many pounds of pull will the rigging at the point of the oar lock be expected to resist?

    I imagine a creosote 4x4 timber would be stout enough. I was just hoping for something that was more attractive and didn't stink.
     
  2. messabout
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    messabout Senior Member

    Bruce; Agreed the 4 x 4 is stout enough, albeit a bit of over kill. The strength of the oarsman is not the deciding factor. Oar traction is. That gets into all sorts of mathematical complexities in the realm of physics. I'll wager that several of our members can come up with a realistic assessment of the forces that one might apply to an oar lock. Leo L. where are you? Nordvincrew may also have some empirically determined input.

    Meanwhile just build something light and see if it breaks. If so, then build another outrigger set that is stronger.
     
  3. brucegseidner
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    brucegseidner Junior Member

    You, sir, are an empiricist. I respect this and may well resort to a levered set up pressing on a bathroom scale. I also have an easy way to fix a rig to the uprights of my back porch and fill buckets of water until it breaks. But lets fish a bit longer and see who pipes in with numbers. Surely, I have failed to find the right numbers. There is so much gardening, and pre-Spring cleaning that such science projects can wait on the results that will have emanated from great minds start showing up. Mine is a small brain that appreciates pulling on stuff until it breaks. But NASA and others have found less material intensive ways of discovering these verities. Thank you for your support. I have been on forums for years. At some point there will be an authority that upbraids me for not knowing about some study performed in Iceland during the 1930's that established these numbers and can't believe I am that ignorant. G-d love the Internet.
     
  4. upchurchmr
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    upchurchmr Senior Member

    I read once that 40# at the blade was a good maximum number. There was no proof offered but it worked for me.
     
  5. latestarter
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    latestarter Senior Member

    If x is the distance of the middle of the hand(s) to the oarlock pivot
    and y is the distance from the oarlock pivot to the centre of pressure of the oar blade
    then if P is the greatest force the oarsman can generate
    the force on the oarlock is = P(1 + x/y)

    That is a very simplistic analysis, I have read if the oars are too stiff you can get shock loads and if you catch a crab it could be greater still.
     
  6. messabout
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    messabout Senior Member

    Bruce; LateStarter has done this for us. And Upchurch has given us a number to begin our search for reality. It is true that I am a bit of an empiricist. "If it ain't broke don't fix it and if it is broke them make it stronger". On the other hand I do enjoy fiddling with the arithmetic of such problems seeking an answer. We have, basicly, a problem that resembles a playground see-saw. How much pressure is on the pivot joint? Simple physics.

    If the outrigger is to be a triangular arrangement, probably aluminum tubing, one of the legs will be in compression while the other is in tension. The compression leg should, from a purely structural point of view, be larger in diameter on account of slenderness ratio and all that. But wait..... maybe it should be allowed to buckle slightly so that it can absorb some of the shock load as mentioned by LateStarter.

    With all the indeterminent variables, not worth the exploratory hassles, to hell with the math. Just look at the outriggers from Piantadorsi or Alden or whomever has made them work satisfactorily. Do it like they do.

    That of course brings us back to the empirical. If this is stricly an academic exercise then we can continue to explore the matter. That might be fun too.
     
  7. upchurchmr
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    upchurchmr Senior Member

    While this seems like a simple problem in mechanics, it really isn't.
    If you fix an oar lock to something solid, then see how much force you can apply it will be a great deal. Basically if you can hold yourself up while pushing with your feet and the blade on something fixed, you can get a great deal of force, probably more than your weight.

    But if you are in the water, when you apply a force the boat moves, the blade slips while pushing on the water, and you have to stop the stroke with control while going into the recovery.

    The static calculation is just a bare start.

    Here is the cross arm on my home made row boat.
    3/4" Ash (before sanding), about 2 inch width at the oarlock, and about 4 inch at the center of the boat. As you can see much of it is tapered.

    Works fine, doesn't appear to bend at all noticeably in the horizontal direction. Vertically it might be a little flexible.

    Sorry for the picture, all I have right now. The boat is hanging on the fence for the winter. This is my wife's boat, but the one I use has the same size crossarm. FYI this is a sliding rigger system, not a sliding seat.

    [​IMG]
     
  8. brucegseidner
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    brucegseidner Junior Member

    That is great. I too am building a sliding rigger unit. You have done a great job!

    Are you using 290cm oars?

    And to be clear, this stock Ash lumber and not laminated plywood?

    I think Ash is great and the most cost effective hardwood in my area. Baseball bats and now rigging. Also makes great work benches without the expense of Maple.

    Thanks so much for the picture. I am clearly over thinking this issue. I will probably use the left over 1/2" Okoume ply that I cut the ribs of my Gentry Ruth from. I'll just double it and epoxy the halves. That will give me an inch thick laminated wing. More weight than the $800 carbon Pocock rig but less than a 4x4 creosote post. Maybe I will round over the edges on the router table to make it more "aero." LOL

    I am inspired.
     
  9. brucegseidner
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    brucegseidner Junior Member

    Would you be so kind as to describe the aluminum rollers you used?

    It looks like one roller bearing per side and the roller rides on top of the support rail. I was thinking this would pull up a bit without a roller beneath as well. But then with the pressing down of your feet maybe there would be no pulling up off the rail.
     
  10. upchurchmr
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    upchurchmr Senior Member

    Bruce,

    Yes they are Concept 2 (whatever brand they sell) carbon hachets. We picked them up unused from a man who didn't want to change from wood (they were a gift from his son).

    My crossarm was solid ash, left over from another project, but I also like it. At the "kick" I laminated in a spline on the centerline, then another on each side. West system epoxy, no glass. I would guess the plywood at 1" will be more than strong enough and it will not sag vertically. If you wanted to overkill the strength you could put a 1/2" solid cap on the leading and trailing edges. That would give you full wood strength for the bending in the horizontal direction.

    As you say wood will never match the low weight of the carbon, but it is reasonable priced. You can always change it out later.

    You could make the shape a NACA foil shape :p:D

    Actually I didn't use aluminum for the rollers.
    The tube is a 1" moderately thick walled section.
    The rollers were supposed to be Delrin, but I think I was sold nylon.
    These were home built with roller bearings inset into the roller on each side.
    This may be overkill and the bearings loosened up - I believe the nylon gave a little. They still work with low friction but I can see the bearings are no longer a press fit.

    I looked for a long time to find something "stock" like a sailboat sheeve but never found it. Another thought was a roller from exercise equipment - I could never find where to get replacement parts.

    Good luck and I would like to see what you end up with.

    One suggestion, for a slliding rigger, all the force to push the boat comes out thru your seat/ butt contact.
    Rowing stores sell a Vibram pad - these are comfortable, but also have a very high friction.
    My son pushed himself off the seat when he had on slick shorts and no pad.
     
  11. upchurchmr
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    upchurchmr Senior Member

    I can't help myself, just remembered a better picture.
    This is an earlier version of the crossarm with splice plates at the kick top and bottom.

    The boat is heavier than I wanted at 50#, could be lighter.
    It does slip thru the water with very little wake.

    [​IMG]
     
  12. brucegseidner
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    brucegseidner Junior Member

    I misspoke. Perhaps I should have asked, "Whats in YOUR wallet?" But have you got nylon (or whatever) rollers top and bottom? I am trying to figure this out from the bolt patterns. But this is like ancient Chinese doctors, unable to touch a female patient having her point to a small female mannequin. I will be direct.

    How did you construct these?

    And I am now wondering if a bit of a recumbent style bicycle sling seat wouldn't be a comfort for a long day out on the water.

    I grew up with fixed seat plywood row boats, Cedar strip canoes and Mahogany sail boats in Northern Ontario lakes. This sculling is all new to me. But I love the Concept2 row machine in the gym and I like to build stuff especially if it floats.

    Thank you for sharing this delightful picture. I can't wait till spring.
     
  13. upchurchmr
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    upchurchmr Senior Member

    If you look at the last picture.
    The aluminum case is a 1.5 x 6 extruded rectangular beam.
    The two "holes" on the side are ~1/2" dia tubes which support the foot plates.
    The three other features are bolts acting as axles for the wheels. So there are 2 on top of the 1" longitudinal tube for the wheels which do all the support for the rigger, oars, and feet. One additional wheel is under the longitudinal tube more toward the back of the aluminum box. This insures the rigger does not fall off while you handle the boat. At first I was concerned that the rigger would lift up during the recovery, but that has not been true at all. Perhaps the rigger, etc., is too heavy for that to happen.

    Note that I made an eccentric cam adjuster on the aft most upper wheel axle, so that all 3 wheels would fit snug. Might not be necessary.

    When rowing your body will rock back and forth. Leaning aft on the boat during the recovery, fwd on the boat during the power stroke. You might want to have a short seat back with a little clearance when sitting upright. Personally I don't think you want full support of your back as in a sling seat.

    The reason I went with the sliding rigger is that the boat was to be transported in the back of a pickup. So 11' was as long as I could go. My wife wanted stability of the boat, so it is a catamaran. The fine ended catamaran would not allow the typical sliding seat, the moving weight would have caused it to dive up and down in the water, much worse than a monohull boat.
     
  14. brucegseidner
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    brucegseidner Junior Member

    Thats very helpful. I also thought I could get away with just one bearing on the bottom to guide the rig, between the two weight bearing ones on top. I also decided on a rolling rig because of the issue of bobbing. I plan to be able to use this unit as a drop in for the Gentry Ruth and a bolt on for an SUP. Thanks for all of your help.
     

  15. Skyak
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    Skyak Senior Member

    UP, nice rig! I have often wondered why seats slide and not the oarlocks because the thin small boats flop back and forth out of trim each way. I think if they took the oars off and just had the crew go through the motions he would produce more waves than if the hull were moving smoothly at speed. Fixing the seat greatly improves trim because you lean back at the same time you kick out. Am I missing something? Is there any good reason to move the seat?
     
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