raideur a la toile - which unit?

Discussion in 'Stability' started by JensK, Feb 22, 2009.

  1. JensK
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    JensK New Member

    Dear all
    French boats give their righting monent (raideur a la toile) in a unit I cannot decipher.
    My Attalia is declared with 5,15 (units?) at 15 degrees' heel - but I cannot make that fit with other units of moment or force.
    At 10 degrees, I have calculated 7636 Nm or 764 kilopond.
    Can anyone help?
    Best regards
    Jens
     
  2. yoan.stephant
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    yoan.stephant New Member

    Hello

    Generaly, RM is given in Ton.meter, but it looks very important for Attalia

    But 7636N.m looks very small !

    Regards

    Yoan
     
  3. JensK
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    JensK New Member

    Dear Yoan
    So, the 5,15 would be Ton.meter?
    Then my calculation would be 7,636 Ton.meter - not too far off, I think.
    Best regards
    Jens
     
  4. Olav
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    Olav naval architect

    Hmm, from experience with sailing yacht GZ curves I would vote for kNm. But then - provided my assumption is true - your calculated value at 10° doesn't fit as I cannot believe a conventional sailing yacht to have its maximum stability at less than 15°, it should be at around 90°.:confused: By the way, 7636Nm is 0.778tm (7636Nm / 9.81m/s² = 778.4kgm = 0.778tm).
     
  5. JensK
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    JensK New Member

    Dear Olav
    Thank you. My value at 10 degrees is not the maximum - I put it in to be compared with the Attalia declared value of 5,15 at 15 degrees (and 8,7 at 30 degrees).
    So, tm is from kilpond, not from NM. Do you think the French declared values are tm?
    Best regards
    Jens
     
  6. Olav
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    Olav naval architect

    Hej Jens,

    of course the boat doesn't have its maximum stability at 10°. But you have to admit that 7636 Nm at 10° is a larger value at a lower heeling angle than 5150 Nm at 15°. ;) And this is confusing me. The course of the stability curve at low angles should be roughly linear. Maybe you could show us your calculation in detail?

    I still believe 5.15 kNm at 15° to be a reasonable value (although the resultant lever arm of GZ = (5150 Nm / 9.81 m/s²) / 3700 kg* = 0.142 m is rather small compared to boats of similar size); anything else like Nm, tm, kgm, kpm or lb-ft is either too large or too small to be plausible.

    Med venlig hilsen, Ole

    * I assume we're talking about a Jeanneau Attalia 32'; a quick google search produced said 3700 kg as its displacement.
     
  7. jehardiman
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    jehardiman Senior Member

    Reporting righting moment as a (unitless) number at a given list strikes me as regulatory requirement and tickled something in the back of my memory. Check the EU RCD and/or the ISO (12217?) standards.
     
  8. Olav
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    Olav naval architect

    jehardiman,

    interesting point, the only thing I found was the Stability Index (STIX) from the ISO 12217, but I doubt this is meant in this case. There's a table in the ISO that gives some minimum STIX values dependent on the design category (A, B, C, or D). Category D ("sheltered waters", 0.4 m significant wave height, wind speed 13 m/s) demands a minimum value of 5, while category C requires a minimum of 14, 23 for B, and 32 for category A.

    I assume the boat in question seaworthy enough to not fall within category D. ;)
     
  9. jehardiman
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    jehardiman Senior Member

    No, not STIX, maybe something older. IIRC there was at one time a requirement to have a minimum righting area at (?) degrees.<shrug>.

    Ok, here is a google...from US law, so i would go looking for a EU contemporary.

    From 46 CRF

    § 170.173 Criterion for vessels of unusual
    proportion and form.
    (a) If required by the Coast Guard
    Marine Safety Center or the ABS, each
    mechanically powered vessel less than
    328 feet (100 meters) LLL, other than a
    tugboat or towboat, must be shown by
    design calculations to comply with—
    (1) Paragraph (b) or (c) of this section
    if the maximum righting arm occurs at
    an angle of heel less than or equal to 30
    degrees; or
    (2) Paragraph (b) of this section if the
    maximum righting arm occurs at an
    angle of heel greater than 30 degrees.
    (b) Each vessel must have—
    (1) An initial metacentric height
    (GM) of at least 0.49 feet (0.15 meters);
    (2) A righting arm (GZ) of at least
    0.66 feet (0.20 meters) at an angle of
    heel equal to or greater than 30 degrees;
    (3) A maximum righting arm that occurs
    at an angle of heel not less than 25
    degrees;
    (4) An area under each righting arm
    curve of at least 10.3 foot-degrees (3.15
    meter-degrees) up to an angle of heel of
    30 degrees;
    (5) An area under each righting arm
    curve of at least 16.9 foot-degrees (5.15
    meter-degrees) up to an angle of heel of
    40 degrees or the downflooding angle,
    whichever is less; and
    (6) An area under each righting arm
    curve between the angles of 30 degrees
    and 40 degrees, or between 30 degrees
    and the downflooding angle if this
    angle is less than 40 degrees, of not less
    than 5.6 foot-degrees (1.72 meter-degrees).
    (c) Each vessel must have—
    (1) An initial metacentric height
    (GM) of at least 0.49 feet (0.15 meters);
    (2) A maximum righting arm that occurs
    at an angle of heel not less than 15
    degrees;
    (3) An area under each righting arm
    curve of at least 16.9 foot-degrees (5.15
    meter-degrees) up to an angle of heel of
    40 degrees or the downflooding angle,
    whichever is less;
    (4) An area under each righting arm
    curve between the angles of 30 degrees
    and 40 degrees, or between 30 degrees
    and the downflooding angle if this
    angle is less than 40 degrees, of not less
    than 5.6 foot-degrees (1.72 meter-degrees);
    and
    (5) An area under each righting arm
    curve up to the angle of maximum
    righting arm of not less than the area
    determined by the following equation:
    A=10.3+0.187 (30¥Y) foot-degrees
    A=3.15+0.057 (30¥Y) meter-degrees
    where—
    A=area in foot-degrees (meter-degrees).
    Y=angle of maximum righting arm, degrees.
    (d) For the purpose of demonstrating
    compliance with paragraphs (b) and (c)
    of this section, at each angle of heel a
    vessel’s righting arm is calculated
    after the vessel is permitted to trim
    free until the trimming moment is
    zero.
    (e) For the purpose of demonstrating
    acceptable stability on the vessels described
    in § 170.170(d) as having unusual
    proportion and form, compliance with
    paragraphs (a) through (d) of this section
    or the following criteria is required:
    (1) For partially protected routes,
    there must be—
    (i) Positive righting arms to at least
    35 degrees of heel;
    (ii) No down flooding point to at least
    20 degrees; and
    (iii) At least 15 foot-degrees of energy
    to the smallest of the following angles:
    (A) Angle of maximum righting arm.
    (B) Angle of down flooding.
    (C) 40 degrees.
    (2) For protected routes, there must
    be—
    (i) Positive righting arms to at least
    25 degrees of heel;
    (ii) No down flooding point to at least
    15 degrees; and
    (iii) At least 10 foot-degrees of energy
    to the smallest of the following angles:
    (A) Angle of maximum righting arm.
    (B) Angle of down flooding.
    (C) 40 degrees.
    [CGD 79–023, 48 FR 51010, Nov. 4, 1983, as
    amended by CGD 85–080, 61 FR 944, Jan. 10,
    1996; CGD 95–028, 62 FR 51218, Sept. 30, 1997;
    CGD 85–080, 62 FR 51353, Sept. 30, 1997]
     
  10. JensK
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    JensK New Member

    The Jenneau Attalia 32 was first made in 1984, so it is an older measure.
    I found a 2008 ORC international certificate on the boat on Google.
    There it is says: RM measured/Default: 74,9/56,7. This could be kilopond at 1 degree. I do not know what the difference between Measured and Default would be.
    My calculation, based on the naval architect drawing and calculations from Principles of Yach Design says:
    BM: 1.53 m
    VCB: -0,33 m
    VCG: 0.058 m
    GM: 1,15 m
    Weight: 3920 kg (with the extra loads).
    From that I get:
    GZ= 0.02 at 1 degree (RM 771 Nm - 79 kilopond))
    GZ= 0.20 at 10 degrees (RM 7636 Nm - 778 kilopond)
    That seems allright compared to the ORC.
    Now I want to make a VPP for the boat and for this I need RMs at higher angles (20 and 40 degrees) and as far as I know these values cannot be calculated. Therefore I got interested in the declared data from the old owners's manual.
    Best regards
    Jens
     

  11. fcfc
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    fcfc Senior Member

    If you are speaking of "raideur à la toile", from french sailing magazine "bateaux" used in their tests, it is better translated to "sailing stiffness" or "ability to carry sail". Not righting moment.

    It is a NON dimentional coefficient, which can be computed by rigthing moment divided by heeling moment.

    All constant factor are simplified:

    It give raideur à la toile = m*GZ / (H * cos (heel) * S)

    m mass of boat.
    GZ righting arm at considered angle of heel

    H = Heigh of geometrical center of Sail Area
    S sail Area = mainsail + fore triangle.

    I do not know the units, and I think the revue "bateaux" technical writer P Gutelle multiplied the result by a fixed coefficient (not published) to have human readable figures between 1 and 10 for typical boats.

    That, you can only compare "raideur à la toile" from "bateaux" magazine with "raideur à la toile" from "bateaux" magazine only.
     
    Last edited: Mar 19, 2009
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