Gerr's Roll Period Recommendation

Discussion in 'Stability' started by DCockey, Jun 20, 2011.

  1. dgerr
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    dgerr Senior Member

    I’m truly sorry that Alik continues to be so mistaken. His inaccurate critiques are doing a disservice to visitors to boatdesign.net by fostering erroneous information. More important, Alik could benefit by being more willing to learn. To respond to his mistaken critique:

    Regarding the roll-period formula he states is wrong because it omits the mass damping of the surrounding water:

    It does omit the mass damping effect, but why? The approximate formula for roll time discussed in the article is formula (75) in “Principles of Naval Architecture – Volume II,” SNAME, as shown in my post of June 20, 2011. This text further explains the derivation of this formula as follows:

    “By use of equation (22) the metacentric height may be computed from the observed period of roll provided that the mass radius of gyration is known and GM is not less than about 6 inches. It has been found that the mass radius of gyration is roughly proportional to the beam of the ship. Hence equation 22 may be written . . .”

    This is followed by an exact statement of the approximate roll-period formula in the article in The Masthead. So what is the parent equation 22?

    Equation 22 IS the equation Alik is finding fault with due to its omission of the mass damping of the water. In the context of this specific article, it would be incorrect to use the equation as Alik indicates because it is the equation without mass damping that is the basis for the approximate roll-period formula. To use a more complete version of the formula, which included water’s mass damping, would not be consistent.

    Of course, this formula 22 is only a secondary reference in the article in question. In fact, the article is titled: “Basic Criteria for Powerboat Stability.”

    The approximate roll time formula is provided and explained so that the reader has a convenient and proven method to make a good approximate evaluation of roll period and its effect on comfort as well as a possible warning indicator for further investigation if excessive roll time is identified.

    The bulk of “Basic Criteria for Powerboat Stability” deals with the criteria used by the United States Coast Guard for small passenger power vessels in commercial use in U.S. waters. Comments by Alik (and some others) indicate unfamiliarity with these criteria. Someone even politely stated that the information in the article seemed a sort of reasonable boatbuilder’s method. In fact—for this type of vessel—this is THE primary stability criteria under United States law under the Code of Federal Regulations (CFR), specifically 46 CFR. (The exact subparts of 46 CFR are given by number in the article.) Having solid knowledge of 46 CFR stability criteria for small passenger powerboats is important.

    The purpose of the article “Basic Stability Criteria for Powerboats” is to give small-craft naval architects, boatbuilders, and hobbyists a clear explanation of these criteria from 46 CFR and enable them to understand these criteria and apply them effectively to their own boat projects. To the best of my knowledge, this is the only such article available that covers this specific subject in one place in a simple, easy-to-understand form. Are there other important aspects of stability analysis? Absolutely. Are they relevant to this subject matter specific to this article? No. Will following the criteria in this article be a good step to ensure that a powerboat of normal form is reasonably safe with regard to stability? Absolutely. If you think otherwise, you should take it up with the United States Coast Guard.

    I’ll recap the mistakes in Alik’s critique:

    1) He apparently believes the methods presented in “Basic Criteria for Powerboat Stability” were invented by me, and calls them the Gerr method or Gerr formula. They were not. They are standard procedures found in standard texts and also specified under United States law under 46 CFR.

    2) He was apparently unaware of the approximate roll-period formula given and its utility and effectiveness.

    3) He was unaware that the coefficient of 0.44 for the formula is the correct one as recommended for general use in “Principles of Naval Architecture – Volume II,” published by the Society of Naval Architects and Marine Engineers (SNAME). (It also appears in other standard references as it is standard and basic.)

    4) He seems to think that because this is an older edition of “Principles of Naval Architecture,” the laws of physics have changed that that the formula won’t apply to new boats of normal form. Not so.

    5) He indicated that the discussion in “Basic Criteria for Powerboat Stability” was deficient because it didn’t deal with large-angle stability and roll. This is incorrect. Large-angle stability is not relevant to the specific concepts covered in “Basic Criteria for Powerboat Stability.” Designers who are familiar with the material covered in “Basic Criteria for Powerboat Stability,” and understand the underlying principles of the stability criteria in 46 CFR know this. (Large-angle stability considerations are, indeed, VITAL to other aspects of stability analysis, but not to the ones covered in the article “Basic Criteria for Powerboat Stability.”)

    6) He indicates that Westlawn Institute does not teach large-angle stability. This is simply false. Good engineers check their facts before jumping to conclusions.

    Regarding Westlawn Institute, here are the facts:

    For over eighty years, Westlawn Institute has done only one thing—teach boat design. Westlawn has trained more practicing boat designers than any other school in the world. Many of Westlawn’s alumni are among the most famous and successful boat designers in history—see:

    http://216.119.80.31/who/success.asp
    http://216.119.80.31/gallery/gallery.asp

    Westlawn Institute is and has been accredited in the U.S. for decades. Accreditation is a lengthy and rigorous process, which reviews all aspects of a school’s operation and particularly its curriculum. The accreditation process is repeated every five years. Just recently RINA reviewed the curriculum of Westlawn’s Yacht & Boat Design Program and accredited it as well.

    If you are looking for practical, reliable, tested information about boat design from an internationally accredited institution, Westlawn Institute is an excellent place to go.

    Dave Gerr, Director
    Westlawn Institute of Marine Technology
    www.westlawn.edu
     
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  2. Alik
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    Alik Senior Member

    Not 'mass damping of surrounding water'. In formula marked by You as 'exact' two components are missing : a) moment of added inertia and b) damping, those are different components of different origin. This is if we look at linearised equation of ship roll on flat water, otherwise other components would be present.

    Not really; basis for formula includes added moment of inertia; see extract of Molland's book I posted.

    So why do You call that formula 'exact' then? Seems You were unaware of added inertia, that's it.

    Again, what is 'mass damping'? Not term used by me and in ship hydrodynamics either.


    These are not Gerr's formulas, but their interpretation and calling some of them exact is Gerr's misunderstanding of phenomena.

    Well aware of this formula; I was teaching it to ship operators and used it during sea trials :) I have publication dated back to 2003(?) where this formula is used to define C coefficient for different craft.

    Lie. I have nothing opposed to that '0.44 formula' and never said anything about it. Just read carefully.

    No comment on such insinuations.

    Lie, I never said that. I only said that formula for roll period is only valid for small angles of heel (i.e. linear area of GZ curve).

    Lie again. Can You quote where I said that?

    Your usual marketing... at the end of every post.

    In general, reading some books before start writing them is good practice especially for those having no degree in the field of naval architecture.

    And I am really upset by Your lying and emotional posts, David. It could be discussed in more professional manner. Every professional can make a mistake; just accept it would be right thing to do.
     
  3. Alik
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    Alik Senior Member

    MustRead

    Special post for those interested, explained in simple words.

    The case of measuring natural roll period of ship is roll on flat water. Such roll is described by this type of equation:

    Inertial forces + Damping forces + Hydrostatic forces = 0

    Where first term - inertial forces - consists of two main components:

    • caused by Own inertia of boat and;
    • caused by Added inertia of so-called added masses.

    The second component is often referred as ‘inertia of water moving together with boat’ - not exactly correct definition but gives some understanding. For most of ships this added inertia comprises 20…40% of own inertia of ship. For small craft it tends to be much higher due to higher B/T ratios; for sailing yachts with appendages can exceed 100% of own inertia of boat. Added inertia is the reason for so-called hydrodynamic forces of inertial nature.

    Damping forces are hydrodynamic forces of non-inertial nature and are caused by viscous effects (friction, vortex making), and wave effects (wavemaking).

    Hydrostatic forces in this case are related to transverse stability of boat, and at small angles of heel can be described by metacentric formula.

    Transforming the equation above, one can get expression for roll period as:

    Roll period = Function of (Own inertia, Added inertia, Damping factor, Hydrostatic component)

    So now we come back to formula from Masthead claimed to be exact by Gerr (see p.8 bottom) (NOTE: this is the only formula/claim criticized by me in this thread):

    Roll period = 2 * Pi *((Sum of lr)/(GM*Displ))^0.5

    As stated in my post #2, in this formula one can see only own inertia and hydrostatic term, but other important components - added inertia and damping - are missing. Transformation of formula from Masthead, for clarity:

    Roll period = 2 * Pi * (Own inertia / Hydrostatic term)^0.5

    Moreover, on top of p.9 Gerr says that own inertia of boat is calculated by summarizing all elements of the boat. If so, why Gerr provides exact value of own inertia but neglects added inertia that can add 20…100% to total inertial component? It will definitely effect roll period.

    So what is the reason for calling the formula in question ‘exact’, making item-by-item calculation of own inertia and – at the same time – not accounting for added inertia? Evidently, the reason is ignorance of the writer of Masthead. I can understand why it happens: this is special issue of ship hydrodynamics studied by naval architects; not at boat design school.

    I realize that some readers were not able to dig into differential equations I posted before. Believe my explanation is now clear enough for Westlawn ‘graduates’ with basic maths.

    I would say: it is never late to study. For all of us, even those appointed themselves as ‘guru’. And all the rest is just Dave’s own insinuations.
     
  4. dgerr
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    dgerr Senior Member

    To grab three quotes quickly:

    It's interesting that the success of a school's alumni are "advertising" and not a good indicator of anything else. It's an unusual perspective. You decide.

    I won't be replying further on this. I wish Alik all the best.

    Dave Gerr, Director
    Westlawn Institute of Marine Technology
    www.westlawn.edu
     
  5. BMcF
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    BMcF Senior Member

    Already decided Dave; I too will stick with what I've been doing wrong for that last 25 years in building lots of HPMVs that work right.

    And my Chief NA has just too darned many formulae extracted from your various books and developed in to fancy spreadsheet apps to go back and change any of it now. *chuckle*

    Cheers.

    Bill
     
  6. Alik
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    Alik Senior Member

    And what? Anything is wrong in those quotes/formulas I posted?
    Evidently You have nothing to say other than accept Your mistake.

    Keep posting more 'success stories'; hydrodynamics is not Your strong point.
     
  7. Alik
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    Alik Senior Member

    Formulas? I have checked some (not my hobby).
     

    Attached Files:

  8. BMcF
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    BMcF Senior Member

    A comparison of predicted vs. measured roll periods would be more germaine..methinks.;)
     
  9. Alik
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    Alik Senior Member

    Just pls use those formulas with care, OK? ;)

    Your idea is interesting; I will record free roll once we test some suitable boat.
     
  10. BMcF
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    BMcF Senior Member

    We do that all the time; accurate determination of the natural periods of pitch and roll is a fundamental part of setting up new stabilization systems.

    But we engineer the same systems for each vessel using analytical prediction tools.
     
  11. Alik
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    Alik Senior Member

    For us free roll is of not much interest; one of fields of our research is assessment of vertical accelerations underway, for high speed craft. We do those measurements and calculations regularly, as well as tank tests for some projects.
     
  12. BMcF
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    BMcF Senior Member

    For us..that is also a business and has been that for over 25 years. Active stabilization is all about the reduction of vertical and lateral accelerations underway. Tank tests, free-running models (both manned and semi-autonomous) and full-scale trials..and the opportunity to do enough data correlation between all of the above and prediction tools that I'll retire happy knowing I won't do any more of it after that.;)

    But you cannot design an active stabilization solution without at least a rough idea of the natural periods of heave, pitch and roll..and in some cases, also yaw. Nor can you accurately predict the effect such a system will have on any new vessel design if you do not have good tools to do that. We do..and have had for decades now.

    But more research is always good...as are better and better predictive methods and tools.
     
  13. Alik
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    Alik Senior Member

    Well, we do not design stabilization systems; we have to rely on experts opinion in those issues. Actually my primary degree is about manoeuvrability simulation and unsteady performance of sailing craft.
     
  14. BMcF
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    BMcF Senior Member

    To close this circle, if that is possible, we have always had to rely very heavily on analytical methods to predict the natural periods of motion (including roll..where this thread started), and the motions (4- or 5-DOF) in a seaway at any relative wave heading, for every vessel we provide stabilization systems for (that would be ALL vessel types...except for sailboats and large, slow displacement ships;).)
     

  15. Grant Nelson
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    Grant Nelson Senior Member

    Alik, please share the well accepted design books, or other publications you have produced, then we can decide if we will consider your thoughts in equal weight to that of Dave, who has a good reputation in the business for distilling complex engineering requirements into insight and formulas that we all can use, AND THAT ARE PROVEN TO WORK!

    I tried your website, but the information is far too limited to judge the contribution you made to these design / builds.

    I only want to balance the scales, and stick to the facts. So far, Dave has provided much more to back up his apporached than your criticisms or defenses of his arguments.

    Grant
     
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