Resistance factors, planing hull at low speed

Discussion in 'Hydrodynamics and Aerodynamics' started by Mr Efficiency, Dec 6, 2010.

  1. Perm Stress
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    Perm Stress Senior Member

    And what kind of final hull form was?
     
  2. Perm Stress
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    Perm Stress Senior Member

    My guess would be long range speed ~10-15 knots?
     
  3. Alik
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    Alik Senior Member

    That is a secret :)
     
  4. Perm Stress
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    Perm Stress Senior Member

    Series 62 accuracy at displacement speeds

    In order to clarify the matter, it is quite proper to consult the author of original data. So I sent an e-mail Mr. D. Blount:
    ______________________________________________

    Subject: Article on Series 62 in Profesional Boat Builder Dec/Jan 2011

    Dear Sirs,

    In PBB isssue Dec 2010/Jan 2011,

    An Interview with Donald L. Blount is published.

    On page 24, four figures with resitance/speed graphs for full size Series 62 hulls are published.
    Graphs of Fig 2 start at Fn(L) 0.25.
    Normallly, planing boat resistance data is published, starting from much higher speeds, significantly above hull speed of Fn(L)=0.4.

    Quick analysis show that pure displacement hull (ex. as used for sailing yachts) has up to about 50% less resistance than Series 62 at displacement speed Fn(L)=0.3.

    As it appears from the article, Series 62 models were extensively tested at displacement speeds too. So resistance data as published in the article, looks sound and credible for me. It correlate well with what I know from my experience.
    ...
    To clarify the matter, I would like to ask a couple of questions:

    Is Series 62 data at speeds below Fn(L)=0.4 published somewhere?
    How accurate resistance data, as published in Professional Boat Builder Dec 2010/Jan 2011, page 24, is for displacement speed mode?

    Best regards,
    .....

    Here is the answer (highlitings in RED are mine):


    ...

    Thanks for your interest in Series 62. The models were tested from the lowest volume Froude number, FnV = 0.25 to high planing speeds and the model scale test data are available and tabulated in the original paper published in SNAME Transactions 1963 (note that there are some scattered typographical errors in the tables). Graphs of the low speed data are also given in the original SNAME publication as R/W versus FnV for speed values from 0.25 to 1.0 in Figures 21 to 25 for 100,000 pound displacement. The accuracy of Series 62 data is about +/- 2% at high planing speeds and about +/- 6% at displacement/hull speed, FnL = 0.4. In addition to the Series 62 low speed, hard chine data published by SNAME, Keuning, et al has published similar data in the International Shipbuilding Progress in 1982 for 25 degree deadrise hulls and 30 degree hulls in 1993.

    For the lowest speeds for the five models, the Reynolds Numbers ranged from
    3 to 12 million depending on model length, thought to be high enough to avoid laminar flow.
    This was confirmed graphically by plots of residual resistance coefficients, Cr versus FnV which were devoid of low or wildly scattered experimental data points at low speeds.


    As the model hull lines were optimized for very high planing speeds, their low speed resistance characteristics and values were expected to be higher than a well designed hull for displacement speeds. However, it is necessary to understand the low cruise/loiter speed resistance tradeoff for designing planing craft for a variety of operational speeds and missions. Thus, at hull speed, FnL = 0.4, a hard chine, high-speed planing is likely to have double the hull resistance of a displacement vessel optimized for hull speed.


    This topic of hull form relative to design dimensionless speed has been discussed in some detail in the recent technical paper, “Resistance Characteristics of Semi-Displacement Mega Yacht Hull Forms,” by Blount and McGrath, RINA, IJSCT Volume 151 Part B, 2009. An unpublished figure for hull speed, FnL = 0.4 leading to findings in that reference is attached showing the relative resistance to weight ratio values for a number of 500 mt vessels having round bilge, double and hard chine hull forms. Clearly at hull speed a round bilge displacement hull form is appropriate when the preponderance of operations are at this speed and “no higher speed is required”.

    I trust that these comments provide satisfactory answers to your questions.



    Best regards,



    Donald L. Blount


    ....

    __________________________________________
    Mr. D. Blount here talk about hull speed, i.e. area of very steep rise of resistance for displacement hulls. So, at lower speeds, eg. FnL =0.3, resistance difference could be even bigger.

    I hope, it is now proven, that planing hull at displacement speeds COULD have 2 times resistance as compared to purely displacement hull?
     

    Attached Files:

  5. Alik
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    Alik Senior Member

    And how about 3 times? :)

    I got similar reply: 'Reynolds Numbers ranged from 3 x 10^5 to 12 x 10^5'
    To me, those Reynolds numbers are quite low for reliable prediction without turbulence simulators, especially considering transom terms.

    This way You can prove it to Yourself, but not to experts :)
     
  6. Perm Stress
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    Perm Stress Senior Member

    About 3 times:
    In attachment (It was sent by Mr. Blunt) "cross-section..." at L/(D^1/3)=4.25, extreme points are at 0.021 and 0.064. That is 3.05 times difference.
    More so, from about FnL=0.4, and and below FnL=0.3, displacement hull resistance drops dramatically (close to 2-3 times again :) ), at much much higher rate as that of planing hull. So, at FnL=0.3, difference could be close to 3 times easily.

    As was already posted in this thread, turbulence simulators are two-edged swords. However, as far as I remember, "For the lowest speeds for the five models, the Reynolds Numbers ranged from 3 to 12 million depending on model length, thought to be high enough to avoid laminar flow."

    As to experts.

    From what you did post in this tread, it appears, that you was newer interested or did extensive calculations of planing hull BELOW hull speed. Because your designs are normally so speed-full, that no-one is interested to know anything about efficiency below hull speed. Hence much smaller differences in resistance you find out -speed range is simply different and has little in common with displacement sailing as such. And, quite expectedly, displacement-type hulls do not shine ABOVE hull speed.

    Lets take the last example:
    Originally Posted by Alik:
    We do not deal with 45-50m boats; I was talking on 15-25m interceptor craft that we design. Those are pure planing hulls.


    For 15-25m LWL hull, an end of displacement sailing will be at 9.4-12 knots.
    As far as I know, some boats of this kind have MINIMUM speed ~12knots, with engines just on tick-over.
    Also, as far as I know, for military patrol craft cruising speed is well above 10-12 knots, otherwise controlled area became too small, or transinion time from base too long (here we assume boat do not use her top-speed capability for transit, to extend patrol time), or whatever.
    This, again mean semi-planing speeds for long range sailing. And quite naturally, resistance calcs for lower speeds are not interesting to anyone; quite naturally, displacement hull is not an option.

    __________________________________

    So, the point is back to square one:
    BELOW HULL SPEED, defined as FnL=0.4 planing (not semi-planing like Series 63, NPL, Groot, or SSPA) hulls are catastrophically ineffective, because they have 2, sometimes 3 times more resistance.
    Considering :
    a) low speed data for Series 62 IS PUBLISHED in SNAME Transactions 1963
    b) Mr. Blunt agree, that planing hull could have two times resistance at displacement speeds
    c) it correlate quite well with what I see in my sailing and engineering practice,

    I consider the point as proven.

    If someone consider otherwise, please post more detailed arguments as "Re =3 -12 milllion is not enough for me"
     
  7. Alik
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    Alik Senior Member

    For Your info, my Ph.D. included a section on acceleration tank tests of sailboat hulls - i.e. from zero speed to developed speed. (The purpose of research was added masses of hull-keel combinations and its dependence on acceleration and configuration). Besides that, I was involved in testing of other sailboat hulls and planing hulls.

    So Your insinuations are not appropriate here.
     
  8. Alik
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    Alik Senior Member

    I believe You have got original paper; so look at tables p.543-561. Reynolds numbers at lower speeds were 0.3*10^6 to 1.2^6 that is not "3-12 million", but a bit less. Now try to answer question: what is lowest Rn to guarantee turbulent flow without turbulence stimulators?

    You can also look at trend of CT to find out that laminar flow exists in most of tests, at low speeds.

    It is great that these raw results are included in report (at least we have some reference point), but I understand why they were not included in series.
     
  9. Perm Stress
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    Perm Stress Senior Member

    Quote:
    Originally Posted by Perm Stress View Post
    From what you did post in this tread, it appears, that you was newer interested or did extensive calculations of planing hull BELOW hull speed.
    For Your info, my Ph.D. included a section on acceleration tank tests of sailboat hulls - i.e. from zero speed to developed speed. (The purpose of research was added masses of hull-keel combinations and its dependence on acceleration and configuration). Besides that, I was involved in testing of other sailboat hulls and planing hulls.

    So Your insinuations are not appropriate here.

    There are no insinuations: I was talking about PLANING HULL at DISPLACEMENT SPEED, and nothing more. Even in this post you write about sailboat (i.e. displacement) hulls accelerations tests; and planing hull tests, but WITHOUT ANY MENTION OF MINIMUM SPEED of these tests. So, again, there is not a single insinuation. Only accurate (as far , as I could manage) statement:
    "From what you did post in this tread, it appears, that you was newer interested or did extensive calculations of PLANING HULL BELOW HULL SPEED."
    Only this.
     
  10. Alik
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    Alik Senior Member

    Raw data is published, but not included in series. At low speeds, the data represent low Rn and models (excerpt 4665) are not fitted with turbulence stimulators.

    I agree with this also; meanwhile I believe that 2 is already extreme case. If we take real planing hull and real-world round-bilge displacement powerboat hull with same DLR, the difference will be less.
     
  11. Perm Stress
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    Perm Stress Senior Member

    Here is a contradiction between what was written in an e-mail from D.Blount and what you quote here. I will check.
     
  12. Alik
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    Alik Senior Member

    But this is wrong again :D

    I was talking about lack of practical interest of research of plaining hull at slow speeds as they usually are not operated at such speeds.
     
  13. Alik
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    Alik Senior Member

    I can post screenshot if You want :)
     
  14. Perm Stress
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    Perm Stress Senior Member

    Quote:
    There are no insinuations: I was talking about PLANING HULL at DISPLACEMENT SPEED, and nothing more.

    Nothing wrong. I do not say there IS much practical interest. (not often, anyway). I DID say, that "planer" has 2-3 times more resistance below hull speed, as "displacer". Than, some distance down the discussion, I am opposed with an argument "there is not much practical interest".

    So it is now not quite clear, what you want to say:
    a) resistance difference is not 2-3 times, but much less
    b) there is not much practical interest on the subject (and nobody care about it, or have reliable data, or...)

    ...:confused:
     

  15. Alik
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    Alik Senior Member

    No, this is Your misunderstanding, read it clearly: yes sometimes we do calculations/tests for planing hulls at very slow speeds; but it is not of much practical interest for most of engineering tasks.
     
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