Towing resistance – surface friction

Discussion in 'Boat Design' started by kolloff@get2net, Mar 1, 2011.

  1. Alik
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    Alik Senior Member

    We need to know more about source of this graph to make conclusions. Where it comes from - paper, technical report? There should be some notes of how it was obtained.
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    Alik

    I dont get your point.

    The values are tabulated, it is not a graph. Just a series of numbers from 3.28 to 26.59. These data are incorrect, since at such low Fn, the residuary cannot be that high, impossible! It has nothing to do with reports or papers, it is simple hydrodynamics.
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    In the strict sense, yes I am sure.

    Why?...well, we have the total drag…and we can calculate the frictional component. Thus whatever is “left” is considered the residuary/viscous etc etc….this can be broken down how you like. When breaking each item down, discrepancies between theory and measured are noted. So how does one account for them?....but this misses the point. We don’t care too much so long as they are accounted for in the analysis. Easiest way is to convert them into useful data.

    When analysing tank test results, you can easily obtain the residuary values, and then you convert them into kN/tonne. This kN/tonne is used for various geosims. Attached is one of my spread sheets for “simplified” results.

    basic calcs.jpg

    I have another more complex spread sheet for when we do detailed analysis from a tank test, taking everything into account. This simplified version, attached, is when we are just scaling up the design….it is not rocket science. It may not sit well with you and pure hydrodynamicists, but this is how naval architects establish quick reliable values of EHP.
     
  4. daiquiri
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    daiquiri Engineering and Design

    Slightly off-topic

    AH, I have a question regarding your spreadsheet.

    I see that the boat examined in your spreadsheet has two engines. And I also see that the power of each engine is calculated as half of the total required engine power.

    However, there is a general consensus in the boat design community that the total engine power of a twin-engine ship will be higher than the engine power of a similar (same EHP) but single-engine hull. Various authors claim various percentile increase, ranging from 10% to 25%, depending on hull type and speed.

    The difference is explained with the fact that a twin-engine installation will have smaller propellers (which are less efficient), which don't benefit the influence of the hull wake. It will also have a drag increase due to double propeller struts, shafting and (eventually) double rudders. Furthermore, two smaller engines will be somewhat less thermodynamically efficient than a single big engine.

    So the power of each engine in your spreadsheet should be 1/2 of the total required engine power, plus a margin (10%-25%) due to the twin-engine installation. However, I don't see that allowance in the spreadsheet.

    So the question is: did you intentionally neglect this power increase and why? Is it somehow included in the EHP of the hull, or in the propeller efficiency figure?

    Cheers!
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    HI D

    Good question.

    However, the question you posed is back to front.

    The spread sheet shown is calculating the bare hull resistance, at a given speed, the “Total Drag (kN)”. This is then converted into a power in kW, “Naked EHP kW”. This is the amount of power required, rather than presenting it in resistance/drag form of kN.

    So once we have the total power, how does this relate to “what size engine do I need”..?

    The “PC”, propulsive coefficient, is the delivered power, Pd, or SHP, divided by the EHP (naked). In other words, whatever “fudge factors” people like to attribute, but all rolled up into one ratio, the PC= EHP/SHP.

    So your next question will be….er….how do you know what the PC is going to be??

    These data comes from endless previous designs and all with similar hull forms, and drive trains and displacements etc. After you plot a simple graph of SHP v EHP..for each type of design….you end up with PCs.

    We know from sea trails and measurements taken on board with strain gauges etc, and whether one or two props, or jets etc…we arrive at a PC which is “common” amongst certain types of designs.

    So, the PC value shown in the attached of 0.45…is for a twin installation prop boat of “medium speed” of a round bilge semi-displacement hull. I changed the values (confidential), but the calculations across the sheet are correct.

    Hence all the debate about adding %’ages for XXX or YYY…to me, a naval architect, matters little. I am only concerned about the final values and values which can be validated by previous installations/designs to give confidence of “what size engine do I need”. Knowing what PCs i will get for real on sea trials, is all that matters.
     
  6. daiquiri
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    daiquiri Engineering and Design

    And a good answer.

    All clear now, thank you. :)
     
  7. Leo Lazauskas
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    Leo Lazauskas Senior Member

    How it sits is immaterial.
    What is interesting is the reliability of the estimate. For example, are you talking about plus or minus 20%, depending on which towing tank was used for the experiments and what protocols were used?
    What is an acceptable "reliable" estimate?
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    Not such an easy question to answer, but here goes:

    From a naval architects perspective the “tolerances” are relatively straight forward. If the results indicate the amount of power required is XXX kW, and on sea trails the speed is as predicated, this is considered a good result.

    The caveat being, in those great words by Donald…it is all about the unknowns.

    We mitigate the unknowns into it is known. For example, the hull, is built according to a proper QA plan, thus the frames are checked for dimensional tolerances. The finished fabricated hull is checked against the known 3D hull dimensions and the same hull is imported into a hydrostatic program. The hull is weighed prior to launch and once the vessel is launched, draft marks are taken to correlate the weighed weight with the hydrostatics. Thus, obtaining the correct displacement for sea trials from an unknown or suspect quantity, into a known and verifiable fact.

    Once we know the correct displacement, we can measure the engine rpm and power from strain gauges and tachos. This is compared against the test bed data from the engine supplier, again another cross reference.

    So we build up more and more data of facts which are used to establish what the true values are and in this way the “unknowns”, as a percentage, become smaller.

    Finally we arrive at the total SHP and displacement and speed (from measure trails runs). Assuming the tank test data is “correct”, the EHP values are used to estimate the speed with the now “known” data.

    If a brick, when towed indicates an EHP of 200 kW, with a drive train that has the props in say true open water efficiency, we use those efficiencies to arrive at the required installed power. If the speed, from tank data suggest a speed of say 10knots…and on trails we achieve 10 knots, as NAs we are happy, assuming all the measurements for hull/power etc have been taken and using calibrated instruments. If however we achieve 15knots..clearly something somewhere is wrong, similarly if we only achieve 5 knots.

    The percentage of reliability comes from your own sources, such as hydrostatics, build tolerances, measuring the power on sea trails etc etc….and then comparing against the predicated test tank data. If you can satisfy yourself that the data you have is 100% correct, or at least confirm every data independently as close as, you have mitigated all the usual percentage errors, and it then comes down to the reliability of the tank data.

    If the speed achieved was 5knots, for the brick, clearly something somewhere his wrong with the data from the test tank. In a simplistic approach, the total drag (or power) is just friction and residuary. These are easily calculated, from the total drag, measured using the tank test instruments and methodology. Ergo, one of these values “as calculated” is wrong. By how much, doesn’t really bother us, we know it is wrong and hence investigate where and why, since “our” data has been validated by several methods. We look for obvious systematic errors, not 0.05% type stuff arguing which wake fraction to use.

    This actually occurred to use some time ago…predicated speed was 32.5 knots, we got 37 knots. Conclusion, tank testing was in error. This was confirmed by running same model in several different tanks, with different result.

    So, going back to your original question….we use the same test tank facilities and methodologies. When on sea trails everything appears to “work as predicated” we don’t bother scrutinising the data with a microscope. And we use the same tank and company for our data again and again. The reliability comes from their previous estimates of EHP. Since “we” can confirm the data on “our side” easily enough to reduce the amount of known’s to a minimum.

    So, many things can go wrong when designing…we don’t care about absolutes, only trends. How we understand and mitigate this data comes from experience and trust with the test tank facility. If everything we have done indicates XXX and everything the test tank data suggest is YYY and the result is ZZZ and we get ZZZ or close enough….we are happy. We only care about the end result, does it satisfy the speed as per the SOR. If 0.1 or 0.2 knots over…who cares…job done, client happy.

    The amount of systematic errors in measuring data can easily account for up to anything from 2-5%. This is not the tank test faculties fault, it is simple metrology. The accuracy of the data presented is directly influenced by the accuracy of the instruments being used. How accurate can you measure your desk with your standard school boy 30cm ruler compared to a say a laser ruler and which is correct??
     
  9. Alik
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    Alik Senior Member

    Single value PC works well for initial design stages.

    A bit different from Ad Hoc, we used to split PC into components such as: hull efficiency, wake factor, transmission efficiency, etc. This way we can account for single/twin installation.

    In spreadseet posted by AdHoc seems power margin is included in PC value. We used to include it in resistance prediction. Say, I use at least 2-3 methods for resistance prediction for every design we make; use PC to evaluate power; use tank testing data or sea trials data to validate the result. Then, I give a bit lower speed number to customer :)
     
  10. Alik
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    Alik Senior Member

    Another important issue is: how many HP is actually delivered by the engine. There are 3 factors:

    - climate; engines are rated for certain atmospheric pressure, humidity, temperature; here in tropics the HP delivered will be less.
    - marketing factor - we found that some engine manufacturers specify higher power than actually delivered. Taking some less popular brands we decrease their power in calculations;
    - installation and maintenance factors - back pressure, etc.
     
  11. Leo Lazauskas
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    Leo Lazauskas Senior Member

    I'm with you on this one, Alik. We really need more information from MARIN. Imagine going back to the client to give them a progress report...

    We now have results from some expensive tests at one of the best-known towing tanks in Europe, and we have some good news and some bad news.

    The bad news is that we don't know what the column headings stand for.

    The good news is that the results could be in error by a factor of 10 or more. If they weren't so bad, we never would have suspected there was anything wrong with the data!
     

  12. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Thanks, John.
    Believe it or not, I am in almost total sympathy with your approach.

    About 10 years ago I received a batch of papers written by some high profile Italian Naval Architects and ship-builders. For some reason, the papers (translated into perfect English) came via the Italian Embassy in Canberra.

    The papers were absolutely scathing about towing tank tests. For these guys, sea trials were the only reliable measure of performance. They were particularly critical of self-propelled model tests which they regarded as expensive and a complete waste of time.

    If I find them in the morass of papers I have, I'll scan them for you as I believe they are quite rare.

    Leo.
     
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