# raideur a la toile - which unit?

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

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### JensKNew 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

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### yoan.stephantNew Member

Hello

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

But 7636N.m looks very small !

Regards

Yoan

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### JensKNew 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

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### Olavnaval 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°. By the way, 7636Nm is 0.778tm (7636Nm / 9.81m/s² = 778.4kgm = 0.778tm).

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### JensKNew 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

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### Olavnaval 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.

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### jehardimanSenior 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.

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### Olavnaval 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.

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### jehardimanSenior 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]

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### JensKNew 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

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### fcfcSenior 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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