Rowing Rigging

Discussion in 'Hydrodynamics and Aerodynamics' started by Tallman, Jun 13, 2016.

  1. Tallman
    Joined: Jun 2011
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    Tallman Junior Member

    I'm not certain this is the right forum but I have a rigging puzzle I need solved--

    I've got a finished hand-built recreational double with Hudson slides and stretchers, and Wintech wing riggers. It's a two-man sliding seat rowing boat, tapered at the end, 37" max beam, 24.5' long.

    The hitch is that the centerline was mismarked before the internal structure was installed. Therefore, the seat center, rigger centerpoint and stretcher center are all biased, uniformly bow to stern, to starboard about 2 cm. Later this will be fixed by replacing some of the structure, but for a while I need to row it in some races and cannot fix the underlying structure.

    My question: how to best compensate in the interim? I went out in it and we went continually to port; I am thinking it's because the starboard oars were that much farther out from the centerline (like adding 2 cm outboard? Or actually net 4cm?). If we move in the starboard oarlock pins 2cm, the "center point" of the oarlock-to-oarlock spread will be in the correct boat center. But then the oar handle overlap gets odd, biased to port, I assume (I don't have a lot of time for many outings to try multiple tweaks, unfortunately).

    Is there a way to split the difference by moving in the starboard pin less, and tweaking oar inboard/outboard on either side to help even the load/dynamics? Moving port pin?
    Any and all ideas appreciated.
     
  2. gonzo
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    gonzo Senior Member

    If you are competing, 2 cm off is way too much. The net difference is 4 cm (+2, -2). Those outriggers are a single unit. They look like they can't be adjusted individually. Even if you adjusted the oar locks, the seat will be offset. How much work is to unscrew the whole rig and shift it?
     
  3. Tallman
    Joined: Jun 2011
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    Tallman Junior Member

    Moving the riggers is unfortunately not possible without building new supports -- so that has to wait until the off-season.

    To split the difference, I was thinking if I bring starboard pin in 1 cm, and take off 1 cm from the starboard oar outboard, the off-center overlap might be bearable and the outboards effectively equal. However, the question then is how do the starboard and port loads differ with that change, and how to equalize them so oar blade pressures are equal -- or would they be as a result?
     
  4. messabout
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    messabout Senior Member

    A difference of a centimeter or two at the thrust point is not very much. Does it matter? Yes, but the differential force vectors are very small. Easily assigned to the difference in human force differences in right /left muscle capacity or delivery.

    Not to be the voice of doom but.........Make a careful analytical examination of the boat. Are all the sections absolutely symmetrical? If not, minor differences can induce a turning tendency. If the 2 cm offset applies also to the seats then there will be a small lateral weight transfer with a resulting heeling moment applied to the boat. That can make a boat tend to turn away from a straight line. Kayakers use the heeling technique routinely to help turn their boat.

    This is an interesting set of possibilities..... Tell us more.
     
  5. gonzo
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    gonzo Senior Member

    On a rowing shell a centimeter is the difference between winning and loosing.
     
  6. Tallman
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    Tallman Junior Member

    There are some asymmetries -- the boat is hand-built, using wood strips like a canoe, sandwiched in fiberglass. But they are quite minor (at least for those portions actually in the water). I have been pondering this and think I may try to cheat over the seat slides and foot stretchers to port to align with the revised, correct rigger centerline, which will help, but still leave the overall rigger offset.
     
  7. Steve Clark
    Joined: Jul 2004
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    Steve Clark Charged Particle

    I hate it when that happens!
    Short term:
    Forget where the structure is, get the things that matter where they belong
    Can you get the sliding seats to centerline?
    If not quit.
    Can you get the pins to be equal distances from centerline?
    Forget where the structure is, one rigger can be full out and the other full in as long as they are correct relative to the center of the boat. None of these things is so heavy and the distances are small enough that the moments of unbalance will be trivial compared to the bodies, or the oars.

    You may have to compromise on spread, or build some lock plates to adjust beyond what is normal. If you can get the seats on center and the pins rioght, it may look funny, but it should row just fine.
    SHC
     

  8. gonzo
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    gonzo Senior Member

    If you are not competing a small twist to the skeg will compensate for the tendency to turn. However, that will introduce a small amount of drag.
     
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