designing a fast rowboat

Discussion in 'Boat Design' started by nordvindcrew, Oct 13, 2006.

  1. daiquiri
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    daiquiri Engineering and Design

    You guys got me interested in this stuff... :)

    Leo, is the last graph (speed variation during strokes) in the page http://www.cyberiad.net/library/rowing/stroke/sres1x.htm correct? It gives an average speed of around 5 m/s, with variations of +/- 1 m/s during a stroke cycle.

    I have reproduced it in the attached spreadsheet, interpolated the data with a polynomial curve (5th degree gives a nearly perfect fit) and derived the resulting equation (visible in the graph window) to obtain istantanteous acceleration dV/dt (yes, I like it difficult way... :D) As you can see, the maximum dV/dt is around 5 m/sec^2.

    Now an eight rowing shell is 60 ft long, 2 ft wide (please correct me if I got it wrong, I'm not a rowing expert), which gives an L/B ratio of 30.
    By using your formula for Rankine ovoid, an added mass of 8.9 kg is obtained, which gives an added resistance during the acceleration stroke of 45 N.
    That's 15% of the constant-speed resistance at 5 m/s speed! (which is 300 N, according to the resistance curve in your post #826)

    If these data and calcs are correct (but please check it out), then the influence of added mass should not be overseen in the calculations of resistance and required power, imho.

    But I do agree with you that, since boats of the same class are all nearly identical and hence their added-mass resistances are practically the same, there is no use including it in the comparative analysis of two designs.
    Or, as NoEyeDeer has put it, it's one of those things you can't do much about when comparing two boats.

    I'm looking forward to your peer-review of the above calcs, as it got me really curious... ;)

    Cheers!
     

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  2. Leo Lazauskas
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    Leo Lazauskas Senior Member

    That graph is for a single. A better graph for the eights is on:
    http://www.cyberiad.net/library/rowing/stroke/sres8.htm

    Your estimates seem fine to me. And yes, assuming the speed as constant in some calculations leads to considerable errors in estimating required power.

    There are also some interesting unsteady effects on the boundary layer. Unsteady effects on wave resistance are complicated, but because it is a small fraction of the total drag it's less important.

    If anyone loses a race by less than 1/10th of a second they should consider gluing their ears back next time to reduce air drag.

    Leo.

    Late edit: I'm not sure that I would justify the inclusion of added mass using the acceleration you used. Better to use the equations of motion as shown on:
    http://www.cyberiad.net/library/rowing/stroke/smodel.htm
     
    Last edited: Nov 12, 2010
  3. NoEyeDeer
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    NoEyeDeer Senior Member

    Ok, tell us about the boundary layer. Might as well. :)
     
  4. Leo Lazauskas
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    Leo Lazauskas Senior Member

    No, there's not a lot I can show yet. There are interesting structures behind the stern due to vortex shedding. The wake behind the stern has some unusual lumps and bumps, but because a boundary layer can't sustain shear forces without deforming, I don't yet know what effect it has on wave resistance.

    Shouln't you be heading for your shed to build something instead of procrastinating?
     
  5. daiquiri
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    daiquiri Engineering and Design

    I agree. I just wanted to illustrate quickly and in a straightforward manner (though approximatively) the influence of the added mass. Not everyone reading our discussions feels comfortable with differential equations. ;)

    The equations of motion are easy to modify. The only difference with those shown in the page linked by you is that the total mass on the left side of the equation becomes (mb + mr + madd). The rest is unchanged.
    I suppose that you have the original files (algorithms, programs) used for that research. I would be curious to see how this little modification influences the final results. Awaiting to see some news from you. ;)

    Cheers Leo!
     
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  6. Leo Lazauskas
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    Leo Lazauskas Senior Member

    That research is very old now, Slavi. I have advanced it a lot by using a multi-segment model for the rowers' bodies, incorporating squat, and a lot of other features. Unfortunately it is embargoed while I am under contract, which is why I have been a bit cryptic.

    Added mass doesn't make a huge difference to the results, but it is useful in providing some damping so that wild accelerations don't create numerical difficulties. I'm afraid you and other Olympic enemies will have to do their own homework :)

    Leo.
     
  7. daiquiri
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    daiquiri Engineering and Design

    I did the homework. Resolved numerically, with Excel the equations from your site. The curiosity was just too big. :)

    You were correct Leo, a decrease in average speed due to added mass appears to be neglectable. We are talking about something like 5.794 vs. 5.793 m/s. :)
    The velocity peaks and lows are somewhat smaller too when added mass is included because it acts as a brake, both during the acceleration and during the deceleration phase of the stroke cycle. So it dampens the variation of velocity.
    But again, the differences are extremely small - probably smaller than the numerical error due to the primitive method used for integration ( U(k+1) = U(k) + (dU/dt)*Delta_t ).

    I'm including the spreadsheet used. In order to find the average speeds, I've plotted the velocity vs. time in two cases (U1 = without added mass, U2 = with the added mass), and have interpolated with linear regression line. The visualisation of regression-line equations is switched on.

    Then I have played with the initial speeds (U1init, U2init - green fields), untill the regression lines were flat (multiplier of the "X" in the regression equation has to be zero). The average speed is then given by the constant part of the regression equation.

    Not sure anyone might be interested, but anyways... I wouldn't fall asleep tonight if I didn't have the answer to my question. :D

    Cheers!
     

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  8. NoEyeDeer
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    NoEyeDeer Senior Member

    Hey, that wont stop me procrastinating. I'm an expert. :p
     
  9. ancient kayaker
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    ancient kayaker aka Terry Haines

    All this talk of procrastination so I looked it up in the dictionary and now I feel guilty ...
     
  10. NoEyeDeer
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    NoEyeDeer Senior Member

    Yeah it's dangerous. I can't really do any serious building until after Xmas so until then it'll just be minor prep work. Lofting to inside of planking, test panels for a few ideas on scantlings, etc. I am determined to have this thing finished for my next birthday though. That gives me until early March. Feel free to give me hell if I get behind schedule. :)
     
  11. magnus
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    magnus Junior Member

    Magnus

    Procrastination, I think I have bragging rights here; I have been sitting on this bow facing rowing craft for 15 years. Correction, it has been hanging overhead in my shop for 15 years as a reminder that I or someone should be sitting on the water with it. Oh, I have good reasons to avoid building more hulls. Two female clamshell open molds (plus two more for the outrigger), razor trimmed at the precise green stage timing/ cured/ attachment of further mold sections to clamshell flanges / construction of inner bondable
    flanges on both clamshell halves (2 inch boat tape installed by the braille method) / removal of extra mold sections/ then a carful application of filled epoxy resin/ 100 bolts and nuts to hold the flanges together/heat tape wrap and insulation blanket wrap/ 10 hour cure/sand seams and fine sand hull/spray with PPG epoxy and then finish coat/attach rowing hardware which totally ignores the KISS (keep it simple stupid) montra. It was a large a time commitment on top of the time spent on the molds. On the other hand it was a good ride and fast so I have resurected the project of late and am infusing (VARTM) very lightweight outriggers in carbon and kevlar with a glossy skin finish which is done in the mold (no paint or gel coat). I will give an update in 15 years or less.

    Keep it constructive

    Jim Magnuson
     

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  12. ancient kayaker
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    ancient kayaker aka Terry Haines

    I can't wait, Jim, so hurry up!

    No, seriously, I can't wait; I'm 71 next week!
     
  13. magnus
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    magnus Junior Member

    Jim Magnuson

    Terry, I will try to have something finished for your next birthday. It will be wide enough to sit in rather than sit on and I might make it stable enough to dump the outrigger.

    I am curious if anyone has had experience with stabilizing foils mounted on skinny fast row boats, like shells. My thought was mounting foils fairly wide, somewhat astern to reduce squat and wetted area slightly and increase stability. But is the drag penalty likely to be to high and what other concerns should I have? I am considering relatively flat water conditions. Thank you.

    Jim.
     
  14. NoEyeDeer
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    NoEyeDeer Senior Member


  15. magnus
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    magnus Junior Member

    Thats fast. I had not seen that, thanks. I am still thinking essentially displacement with just a little foil help if practical.

    Jim
     
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