theoretical displacement hull shape for min drag

Discussion in 'Software' started by Padava, Jun 21, 2009.

  1. apex1

    apex1 Guest

    What did I miss???? Has RW gained some real world knowledge I did not mention?
     
  2. Guest625101138

    Guest625101138 Previous Member

    Leo
    Rowing shells have evolved over a period of about three hundred years to be close to the most efficient hull form for their power and displacement - whether they be 1, 2, 4 or 8. The fact that GODZILLA can improve on their shape to produce a lower drag hull in a matter of minutes is powerful validation.

    Even if GODZILLA was only to get close to the optimum it reduces years of effort in trial and error to arrive at the lowest drag hull. My own experience in the single person class, probably the most used form of craft, indicates it produces not just close to but, indeed, the lowest drag hull for the design power and displacement.

    Rick W
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    No, sadly "normal service" is resumed...sigh!

    Was an intelligent thread, for once....oh well, good things don't last for ever!
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    Rick

    If you have read and if you have actually understood any of the postings above, you would not make such a vacuous statement as thus:
    " ..The fact that GODZILLA can improve on their shape to produce a lower drag hull in a matter of minutes is powerful validation..."

    You have no concept of validation and verification, this is clear.

    Without verification all work is meaningless, until otherwise quantitatively proven and verified independently.

    Leo quiet rightly acknowledges this, with his own work, yet for some reason you ignore this and claim it to be something it is very clearly not.

    Leo's work is very interesting and potentially exciting, but in the hands of those that do not understand, very dangerous!
     
  5. yipster
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    yipster designer

    good read Leo Lazauskas
    have the patience attaching file Ward's Optimum Symmetric Ship here?
    loads in 8 and 20 stations version rite?
     
  6. Leo Lazauskas
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    Leo Lazauskas Senior Member

    I think this should work in version 8.x

    1. Rename the file to in.mlt
    2. Run Michlet
    3. Look at the wave drag curves and marvel at how low the drag is at 1.223m/s
    4. Think about why the wave drag is so low :)

    Optimum symmetric ship shapes at higher Froude numbers have smaller bulbs at the ends of the ship, but sometimes have large midship bulges... like some of the older readers here.

    All the best,
    Leo.
     
    Last edited: Mar 27, 2015
  7. yipster
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    yipster designer

    works and with the 8.5 manual next to the keyboard marvelling, thank you
     
  8. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Hi Rick!
    I'm not sure that Godzilla results are really a validation of
    anything, but I'm not that worried about confirming results
    for the thin smooth hulls and speed ranges you usually deal with.
    Michell's theory does a pretty good job with the wave resistance
    and that's all that really concerns me.
    Form drag, of course, is tough to estimate, but everyone has that
    problem. I'm not sure what CFD guys do - maybe they just wave
    their hands around like the rest of us.

    I can get pretty good agreement with experimental rowing data,
    but I have to make a few biomechanical assumptions along the way.
    However, with those assumptions I can get good agreement for Froude
    numbers between about 0.2 and 0.75. As far as I am concerned, there's
    no real need to repeat the verification for every "suitably thin"
    hull. (I doubt that you, personally, will ever get much over F=0.75 in
    your human-powered events!)

    Another point to remember is that there are some very subtle features
    of rowing shells that aren't captured by the mathematical series I use
    as examples in Godzilla. The human designer I worked with was much
    better than Godzilla at fitting two 2metre tall brutes into a 10m shell,
    and making the hull efficient and stable for its design speed range.

    The graphs in the attached pdf document illustrate a few of the factors
    I used to model the velocity and acceleration of a rowing stroke.
    From what I remember of your human-powered events, you tend to move at
    a more constant, slower speed, so some of the following is probably more
    than you need to consider.

    Measured data comes from a trial crewed by Olympic Gold medallists.
    I've used data for 5 strokes at 36.1 strokes per min for sake of this
    example.

    The first page shows the rowers' body segment angle regimes and the
    trajectories of their segment centres of mass.

    We also need to know the oar angle regimes and the forces exerted by
    the rowers. Good measurements are usually available for these
    quantities as shown on the second page.
    I'd guess that your pedal drives etc are probably easier to model than
    oars, but then we both have difficulties with the propulsors - me with
    what happens at the oarblades, you with propellers.

    I need to know the hydrodynamic forces and moments on the hull
    that induce sinkage and trim (aka "squat") because these change the
    underwater shape of the hull. The location of the bow and stern of
    the hull during a single stroke are shown in the top plot on the
    third page.
    These curves (red bow, green stern) were calculated using the
    combined effect of the rowers' centres of mass and the hull squat.
    Of course, these results depend on the location in the shell of the
    crew members, and the exact shape and proportions of the shell.
    Matching the crew, the forces they are capable of producing, and several
    other factors, make for a very tricky design problem, moreso given that
    races are sometimes won by a bee's dick.

    Once we have all that, we can estimate the hydrodynamic drag as shown
    at the bottom of the third page. We must also burn some offerings
    to the air drag gods. After collecting some empirical drag coefficients
    for the riggers, oars, human bodies and hull we might get a reasonable
    estimate. Or maybe not, air gods can be fickle.

    Having assembled that morass, (and a few things I've left out) we can
    now estimate the instantaneous hull acceleration and velocity as shown
    on the last page.

    For long distance events, squat is not going to be an issue, and for
    fixed-seat events you aren't going to bob the hull around much, so
    Michlet should give reasonable estimates for your applications.
    If you are actually going faster than Fr=0.45, squat might become
    important. On the other hand, if you are moving at a constant
    Froude number, you can move the cg a bit to reduce adverse effects.
    It's not so easy with rowing shells that operate over a wide range of
    Fr.

    In short, I wouldn't be surprised if the predicted performance of the
    Godzilla hulls you found agreed as well with your experiments as I have
    shown here for my more complicated problem. Whether your hulls are truly
    optimal (whatever that means) for your "missions" is another matter
    completely.

    All the best,
    Leo.
     
    Last edited: Mar 27, 2015
  9. Guest625101138

    Guest625101138 Previous Member

    Leo
    As you point out I have simplified the analysis significantly by using steady thrust.

    Knowing what the lowest drag hull looks like for any set design conditions is a good start in optimising the design. Whether the end result is an optimum boat is a function of many variables.

    The pedal powered boats are primarily my large model boats but give me endless fun. I am moving on to electric powered now with the aim of building a boat for coastal cruising using solar and wind power. The target is design cruise of 8kts with 9.5m hull using energy collection from both sun and wind so well within Michlet range.

    Although squat is not an issue for what I am currently doing I am still interested to know if you intend to make Flotilla publicly avaialable. I am sure there are a few other tools like me who wouls appreciate it and be prepared to pay for it.

    Rick W

    wand .
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    Leo

    ".. As far as I am concerned, there's no real need to repeat the verification for every "suitably thin hull"..."

    But for verification to show the solution converges consistently, a large sample is required. Just one or two samples statistically isn't enough validation.

    What B/T ratios are these thin hulls?

    For better correlation, you really need 'naked' resistance value. Since there are far to many variables, in the example given, that could under or over estimate your values, or even introduce some other unknown quantity. But therein lies the difficultly, getting a 5m model to be "propelled" in someway where you can reduce the variables to just 1 or 2 and have these measured, such as via strain gauging and cavitation expts for the power delivery and prop characteristics, at your budget.

    Then to also repeat this on large hulls, say in the 50m range, just to confirm that there are no real scale effects or, your 'model' is able to predict the scale effects with sufficient accuracy.

    Since from a naval architecture point of view, those rowing hulls, are not much larger than some models i've used for ship/boat expts.
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    Rick

    "..Knowing what the lowest drag hull looks like for any set design conditions is a good start in optimising the design..."

    You're missing the point, again. The results are not verfied in any form, so to claim as you consistently do that you ahve the perfect hull, is extremely misleading at best, and in competence at worse.

    Incase you misread it, like you appear to have continuously done so far , here it is again.

    "Hi Rick!
    I'm not sure that Godzilla results are really a validation of
    anything, but I'm not that worried about confirming results
    for the thin smooth hulls and speed ranges you usually deal with..."
     
  12. Leo Lazauskas
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    Leo Lazauskas Senior Member

    B/T is about 2 to 3

    Unfortunately, hardly any published experimental data gives error bars on resistance results which makes them almost useless for validation and verification purposes. Futhermore, the small model sizes are a real worry, as you and Don M have noted.

    If there was more good published data it would make life easier for us academic types.

    As I said earlier, I'm happy with how my model works for "sufficiently" thin hulls. The rowing shell used in the comparisons on the attached Excel sheet was measured at full-size and the agreement is good enough for my purposes.

    Leo.
     

    Attached Files:

  13. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Unfortunately I don't have time to put out a new version of Michlet, let alone other codes I have that are scattered around the garage floor.

    I'm only on here because I am doing some computer runs that each take about 30 minutes. That's too short to do much else except to make a few moves in CivIV on another computer and post crap to the internet.

    All the best,
    Leo.
     
  14. Guest625101138

    Guest625101138 Previous Member

    Leo
    I note your earlier comment about running out of a supervisor. Have you made progress on your PhD being awarded.

    My son submitted his thesis two years ago and is still waiting for the formal recognition. Having put in all that work it is unreasonable to let it slide.

    Life takes people in interesting directions. In some cases formal qualifications count for little but in others they can be critical to opening doors. I know people who have not pressed the last little bit to get the formal recognition and they have come to regret it at certain times in their career.

    Rick W
     

  15. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Yes, I finished it and had it marked just before Ernie Tuck died in March. It was a very sad and quite sudden loss for his family and the hydro community. We still had a lot of interesting and useful stuff to do. Maybe, like him, I'll find a nerdy clot to do some of the work.

    Leo.
     
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