What does "engineering accuracy" mean to you?

Discussion in 'Hydrodynamics and Aerodynamics' started by Leo Lazauskas, Feb 4, 2013.

  1. Leo Lazauskas
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    Leo Lazauskas Senior Member

    That's what they claim, and even better from some builders.
    The reason? Probably to save wetted area by making the lines as fair as
    they can. 1% extra drag is a lot in competitive racing.
     
  2. 65 N
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    65 N Junior Member

    Logarithm tables have been available (and used) for almost 400 years. You can do the same calculations than by a slide rule, simply but with an accuracy of more than ten digits.
     
  3. daiquiri
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    daiquiri Engineering and Design

    That's curious. So they say that +/- 0.5 mm difference in beam (how much is that - 0.15% of the rowing shell beam?) becomes 1% difference in resistance? Well, to me it sounds even more exaggerated, but I might be wrong... Did you try to verify that claim with your tools for analysis?
     
  4. jehardiman
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    jehardiman Senior Member

    I have a niece who is on her college's crew team and I suspect, just like there is in one-design sailing, that there is a lot of "mental" speed.
     
  5. Leo Lazauskas
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    Leo Lazauskas Senior Member

    I haven't done a formal sensitivity analysis on beam and fairness as such,
    but I have calculated the resistance of very many rowing shells. I agree
    that a 0.15% difference in beam will not increase resistance by 1%. Sorry,
    I was slipshod with my 1% figure: I was just using that as an indicator of
    what is important.

    I have found that there is less than about 4% to 5% difference between the
    best rowing shells on the market for the same displacement, despite
    differences in shapes and principal dimensions. It is true, however, that some
    hulls will suit some crews better than others. There is no point in giving a
    schoolgirl crew a hull designed for Olympic class men because the girls
    will not row in the same speed range as the elite class.

    I find it a delightful design exercise, especially when you consider squat, the
    effect of large moving weights, acceleration, and the energy available to
    propel the boats. Most NA's are conservative with their powering estimates.
    After Lance Armstrong's and other recent revelations, I should probably
    include a potential "over-clocking" factor. ;)
     
  6. DCockey
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    DCockey Senior Member

    Thirty years ago a friend had a good business repairing rowing shells for universities, schools and clubs. Most of the shells at that time had glass/epoxy skins and a wood framework made up of thin "sticks" in the cockpit. After a few seasons the framework would crack, the boat would become much more flexible. John would repair the cracks and add small reinforcements to prevent a reoccurence. Added weight to an eight was less than a kilogram.

    I wondered why the shell builders didn't add the reinforcements when the shells were built. Total weight on the water for a shell with a men's heavyweight crew is close to a ton so one kilogram is 0.1% of the total, and any speed loss due to the added weight would probably be more than offset by a gain in efficiency the delivery of the human power to the water. But there was probaby would be a psychological effect if another crew had a lighter shell.
     
  7. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Most modern builders can produce a shell that is very close to the minimum
    allowable weight. I agree that increasing the thickness (and strength)
    of the shell is worthwhile.

    I calculated the effect of weight on mean speed for 16 classes of rowing at
    the elite level. The rule of thumb I came up with is:
    "The percentage decrease in mean speed is equal to one quarter of the
    percentage increase in total weight".

    So, a 0.1% increase in weight makes 0.025% difference in average speed.
    Hardly worth worrying about.
    See: http://www.cyberiad.net/library/pdf/rsn07jul12.pdf
     
  8. DCockey
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    DCockey Senior Member

    And that assumes the difference stiffness does not affect speed.
     
    Last edited: Feb 7, 2013
  9. Ad Hoc
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    Ad Hoc Naval Architect

    Isn't that just a simple QA problem which wasn't highlighted by the QA dept, for whatever reason..and also not noted by any other dept when reading the QA reports?
     
  10. Petros
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    Petros Senior Member

    It is much more complicated than just that: the assembly of the strut was designed "properly" (i.e. meeting company standards for sheet metal parts), the assembly was done according to the drawings, but the specification for the seal installation could not be met since there was no way to make adjustments to it after all the parts were assembled, the problem was not found until after the strut installed on the wing and the engine was installed on the strut. This was about two years after the drawings were approved, and about one year after all the tooling was built.

    It should have been recognized from the beginning that a +/- .080" tolerance was not possible and the original design engineer should have built-in the ability to make field adjustments. But he did not, nor did the plan checker, nor his suporvisor that approved the release of the drawing to production. There were $50 million aircraft to deliver per the contract and they could not stop a VERY costly assembly line to wait and see why the parts are not fitting. someone on the shop floor had to figure out a fix fast and get the aircraft delivered, even if not quite within spec. There was some adjustment they could make by reworking the parts, but the variation was more like 0.60" from one to the other.

    It was not known by engineering that these parts were being delivered out of tolerance until a year or two after the first deliveries when the struts were discovered to having damage from fatigue cracks. The cause of the cracking was traced to this seal, and it was given to our department (customer support) to make a retrofit kit to fix the problem. It was assigned to me and I got the kit and installation procedure approved and to the operators in 3 or 4 months (very fast compared to the rest of the company). I thought to stop the incorrect installation of the seal there had to be a change in production process (not my department, but I proceded to do it anyway). The engineering department just assumed the parts were being built wrong, the assembly department assumed the tooling was off, the tooling department assumed the engineering was wrong, and so fourth.

    It is just what happens to any operation when you have too many people and too many different departments, trying to accomplish one thing. Millions of parts on an airplane, each designed by a different department, fabricated by another, assembled by yet another. And they all have to come together and work so they have a product to deliver to the customer. In this case one poor design decision cost the company about $2,000,000 a year. If they had the ability to fix it in one year instead of four, it would have saved them about $6,000,000. BTW, I was "awarded" $600 for my efforts in saving the company so much money. I also got a certificate of appreciation.

    It is just a very frustrating process to try and get something changed. One of the reasons I do not work for large companies anymore. too many frustrations from things are out of your control. Sometimes your own best efforts and good work get sabotaged by a large and clumsy process, when someone second guesses your work and makes changes without checking first. Been there, done that, do not want to do it again.

    Like I posted earlier, it is amazing that companies this large manage to make a profit at all. It is just the cost of doing business when you have an operation this large. I have seen it also in the automotive business, I have seen in heavy construction, and I am sure it has also happens in ship building too.
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    Well, I think we’ll beg to differ then. Since once flagged up by QA that the installation could not be met, that’s when action should have been taken, not after cracks were found post delivery.

    Ohhh..i hear you there. Been there done that, read the book, bought the T-Shirt got the video….ggeesssss. Drove me nuts…and also, singing from the same hymn sheet now too :p


    It’s probably a bit higher now, as nearly 10years ago…but when we did a joint project with LM..they told us they needed to make $1m /day just to break even!! :eek:
     
  12. Petros
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    Petros Senior Member

    when I was working at Boeing from 1986-94, the cost of a 747 just sitting on the ground, no crew, no fuel, no maintenance, etc. was $80,000 a day in interest payments alone! Some of the larger airlines have fleets of 300 or more large commercial aircraft!
     
  13. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Correct. It's a comparison of the effect of deadweight and/or increased crew weight.
    Trying to quantify the effect of shell stiffness is on my list, along with the effect
    of lucky red socks some of the rowers swear by. :)
     
  14. DCockey
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    DCockey Senior Member

    The lucky red socks may have more effect than either a 1kg increase in mass or the difference in shell stiffness. There is probably more money to be made in sports psychology than in analysis. :D
     

  15. yipster
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    yipster designer

    ah a struth, i've seen a failing engine pin [​IMG] as in '93 on red socks having the dog for a walk i gave captain Fuchs a last wave [​IMG]
     
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