Calculating friction resistance, drag coefficient?

Discussion in 'Hydrodynamics and Aerodynamics' started by dustman, Jul 11, 2019.

  1. BlueBell
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    BlueBell Ahhhhh...

    Agreed.
    Buy a premade, predesigned sailboat.
     
  2. jehardiman
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    jehardiman Senior Member

    This is where a lot people who want to design a vessel do themselves a singular disservice, thinking that un-schooled boatbuilders have full knowledge of WHY the designs are like they are, or that modern science has all the answers. Primitive craft did not spring forth, fully formed, like Athena....a lot of people had to die first. Primitive craft evolved to fit a set of certain local specific functions and environments, by having the environment expunge, with the utmost prejudice, those that drifted too far from the nominal. Survivors copied the vessels that returned and over time improved on them...the historical Polynesian didn't care what the drag of his outrigger was, but he was taught by an elder to ritually bind the ama to the aka in a specific pattern not knowing the actual engineering explanation behind the pattern that gives it strength and flexibility. Even in the early days of scientific naval architecture (~1600 CE), a lot of it was builders rules of thumb, and still a massive number of people died. Even as we moved from wood to iron to steel to composite, vessels were lost and people died even as we improved the engineering of hydrodynamics and mechanics of structures. Even the best talent money and "higher math" that can be thrown at the problem killed someone in the 2013 America's Cup! As professionals, we know what we don't know, and predicting the actual drag of an unbuilt vessel down 0.0x% is one of them...don't expect a laser sharp cut on a line that can only be marked with a piece of chalk.
     
  3. Doug Halsey
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    Doug Halsey Senior Member

    There's nothing wrong with using simplified methods for preliminary design & performance estimation purposes.

    Famed hydrofoil developer W. S. Bradfield (Dr. Sam) used a simple formula for the drag of a Tornado catamaran, and then conducted towing tests for comparison. These are described in his 1971 AIAA Ancient Interface paper "Comparative Performance Of The Flying Fish Hydrofoil And The Tornado Catamaran." Here's the result:
    Like you, he assumed the form & wave drag were negligible, and simply found a skin-friction coefficient at some assumed value of the Reynolds number. For what it's worth, his value of Cd was 0.00369.

    As the other posts emphasize, this is just part of the process of designing a boat, but you can get some general idea how various boats compare by using these kinds of approximations.
     
  4. jehardiman
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    jehardiman Senior Member

    Doug, thank you for that graph...
    There is an excellent example of the type of engineering needed to solve dustman's question.
    A tornado minimum weight is 155kg(342 lb) it has 2 crew at 147kg( i.e. 2 at 165 lbs each not wearing weight vests [as I said N-dimensional])
    So from the theory line and Cd*A = D/(1/2 rho V^2) where rho =1025 kg/m^3 and converting kgf to N at 1 kgf = 1kg*9.8 m/sec^2 where Cd*A comes out in meter^2

    calculating-friction-resistance-drag-coefficient-table.png

    Notice the Cd*A is higher when going slow (as expected) is pretty stable in the nominal Froude Number ranges, and lowers again in the high Froude Number ranges (again expected).
    But really, did the theory line actually match the data? From the points plotted...not so much, slower speed drag should be higher and no comment can be made above ~14 mph.

    Edit I can't get any flavor of BBs code to work <sorry, replaced with image as table bbcode is not currently supported.>
     
    Last edited: Jul 15, 2019
    fastsailing likes this.
  5. jehardiman
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    jehardiman Senior Member

    Thank you...
     
  6. Boat Design Net Moderator
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    Boat Design Net Moderator Moderator

    No problem at all. (could also attach as an excel file as another alternative.)
     
  7. dustman
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    dustman Junior Member

    Yeah, spend lots of money on a poorly performing boat that requires constant maintenance and monetary input. Sounds wonderful. I think I'll stick with building my own boat.
     
  8. dustman
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    dustman Junior Member

    Pretty much exactly why I'm going towards a more proven seaworthy design. I have been looking heavily into the wharram designs to try and determine what makes them so seaworthy. I plan on doing plenty of benign sailing before taking my boat anywhere dangerous.
     
  9. dustman
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    dustman Junior Member

    Thank you for for this, even though a hydrofoiling boat is going to have drastically lower drag at speed than my design it shows I don't have to stick to classical thinking. Anyone happen to know the drag coefficient for a hobie 16? Could be a good jumping off point.
     
  10. Doug Halsey
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    Doug Halsey Senior Member

    I think you're misunderstanding something. The plotted curve applies to the Tornado catamaran without foils. It was used as a baseline for comparing to the foiler.
     
  11. Doug Halsey
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    Doug Halsey Senior Member

    I don't understand what you're getting at by calculating Cd*A. Shouldn't it be constant? Isn't any variation just a result of inaccuracies in picking values off the curve?

    I agree with you that the agreement shown in the curve is questionable, but I think the towing data looks worse than the simple theory.
     
  12. BlueBell
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    BlueBell Ahhhhh...

    Awesome.
    Tenacious, I like that.
    A used, well designed and proven boat is... never mind.
    Best of luck to you in your design and build.
    I hope you get it finished and it works to your expectation.
    Guys like you are good for the economy!
    Cheers!
     
  13. jehardiman
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    jehardiman Senior Member

    In the graph, we have the drag, we have the velocity, and we can assume the fluid density. What we don't have is a reference area. A true Cd is dimensionless, but the actual value will vary on the reference area selected. Modern ITTC convention is to use the LWL^2, but you will see other conventions like Heorners Cd(dot) which uses the frontal area or Cd(square) which uses the wetted surface. As I said in my first post, Cd itself is really not a relevant concept, might as well use Circle C, the admiralty coefficient, both are meaningless without context.

    Edit to add: variation in Cd is actually just the effect of residual resistance (which itself is a pressure function of the hull and fluid interaction). At any velocity (v) the total resistance (R) is the skin friction (Cf|v) and the residual (Cr|v): R= Cf+Cr. This is why Cd is only of interest between like reference areas or geosims.
     
  14. dustman
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    dustman Junior Member

    I've been watching and reading the experiences of many sailors for a few years now. If there is anything I've taken from this it is that practically all their (well designed and proven) boats are constantly requiring money and maintenance, especially cheap used boats. My aim is to minimize as much as possible, in the design phase, the potential problems. Not to mention, most of these cheap used boats are quite slow, heavy and inefficient.
     

  15. Doug Halsey
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    Doug Halsey Senior Member

    I should have emphasized more strongly that Bradfield's simplified method was considering the skin-friction contribution only. That's why I was saying that Cd should be constant for the theoretical method (solid curve on the plot). I'm pretty sure he's using the wetted-surface area for the reference, but I only have part of the paper (at the moment), so I can't verify that or see if he gives a value.
     
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