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raideur a la toile - which unit?

2074 Views 11 Replies 5 Participants Last post by paulk, Apr 1, 2009

JensK

Discussion Starter · Feb 21, 2009

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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billyruffn

· #2 · Mar 7, 2009

Unit = [(hectopascals / gigawatt^2) X LOA] / kilopounds of the beam coefficient

It's a secret formula used only by French engineers to confuse foreign yachtsmen.

-----
Sorry, Jens, I know it was a serious question, but I couldn't help myself.

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paulk

· #3 · Mar 26, 2009

J'irai chercher la réponse pour toi. Ça prendra peut-être un jour ou deux.
À bientôt.

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paulk

· #4 · Mar 27, 2009

Hi Jens-
Checked on a French website and got an answer from a guy in Brittany who pretty much agrees with our friend Bellerophon, above. He says the number is a coefficient; a relative number that has no "unit" associated with it - the same way a length to beam ratio has no unit: it's a ratio. My contact suggests that the French magazine "Bateaux" developed it in order to help describe to readers the relative stiffness of different designs under sail. He suspects that it involves screwy computations involving various square roots and trigonometry functions, and is only useful in comparison with other boats for which the value has been calculated. He admits he may be wrong, and that Naval Architects actually are aware of this formula and use it, but he has his doubts. I've been sailing for a good number of years, and this is the first time I've ever heard of this thing, so... the best thing to do is perhaps to simply go out sailing, and if the boat heels too much, reef.

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tdw

· #5 · Mar 27, 2009 (Edited)

Hey...I didn't see that....Bellerophen.....BillyRuffin.....Damn that is clever....double damn I hate it when I miss a good pun.....

Yesterday I did a google search of

1. French Force and it came back with Majeure which was kind of cute but not of much use.

and

2. raideur a la toile. Thought I'd struck gold until I realised I was in a thread on the Boat Design Forum. Sound familiar Jens ? I figured there was nothing I could add that you didn't already know.

Jens.......the Attillia.....it is almost identical to a Van de Stadt 34. Do you know who designed it ?

paulk said:
Hi Jens-
Checked on a French website and got an answer from a guy in Brittany who pretty much agrees with our friend Bellerophon, above. He says the number is a coefficient; a relative number that has no "unit" associated with it - the same way a length to beam ratio has no unit: it's a ratio. My contact suggests that the French magazine "Bateaux" developed it in order to help describe to readers the relative stiffness of different designs under sail. He suspects that it involves screwy computations involving various square roots and trigonometry functions, and is only useful in comparison with other boats for which the value has been calculated. He admits he may be wrong, and that Naval Architects actually are aware of this formula and use it, but he has his doubts. I've been sailing for a good number of years, and this is the first time I've ever heard of this thing, so... the best thing to do is perhaps to simply go out sailing, and if the boat heels too much, reef.

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Valiente

· #6 · Mar 27, 2009

TDW, I strongly encourage you to read the entire Aubrey-Maturin series by Patrick O'Brien.

http://en.wikipedia.org/wiki/HMS_Bellerophon_(1786)

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tdw

· #7 · Mar 27, 2009

Valiente said:
TDW, I strongly encourage you to read the entire Aubrey-Maturin series by Patrick O'Brien.

http://en.wikipedia.org/wiki/HMS_Bellerophon_(1786)

Hayman, I'm not THAT dim. I knew there was an HMS Bellepheron (in fact there have been a number of them) but I hadn't heard the BillyRuffn thing. Thats nice, I like it.

As for O'Brien....I did try. Got through five of them I think. Each one took longer than the previous.......never could build up enough enthusiam to open the sixth.

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billyruffn

· #8 · Mar 28, 2009

Totally, OT -- but if the marsupial is still watching this thread, have a look at...
The Billy Ruffian: The Bellerophon and the Downfall of Napoleon (Hardcover)
by David Cordingly

...available on Amazon or at your friendly neighborhood bookstore.

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tdw

· #9 · Mar 29, 2009

billyruffn said:
Totally, OT -- but if the marsupial is still watching this thread, have a look at...
The Billy Ruffian: The Bellerophon and the Downfall of Napoleon (Hardcover)
by David Cordingly

...available on Amazon or at your friendly neighborhood bookstore.

Thanks for that. I'll winkle it out.

Great quote "If it had not been for you English, I should have been Emperor of the East; but wherever there is water to float a ship, we are sure to find you in our way." NB

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JensK

Discussion Starter · #10 · Mar 29, 2009

A French explanation

Dear Paul K
Thank you for your effeort.
Meanwhile I have found it at:
La raideur à la toile : elle est déterminée par le rapport de deux couples : le couple chavirant, qui est le produit de la surface de voilure par la distance verticale entre le centre de voilure et le centre de dérive ; le couple redresseur, qui est caractérisé par le poids du bateau et la distance horizontale entre le centre de gravité et le centre de carène à un angle de gîte déterminé.
On prendra, pour étudier la stabilité, la voilure maximale (V) et, pour les dériveurs, la position maximale des équipiers au rappel ou au trapèze. Pour les habitables, les équipiers seront installés dans une position de rappel normale et compatible avec la croisière , c'est-à-dire : assis sur la banquette au vent pour 2 ou 3 équipiers, les autres étant, le cas échéant, répartis sur les couchettes du bord. Comment interpréter les résultats ? Plus le coefficient est élevé, plus la raideur à la toile sera élevée. Si le voilier A est plus raide à la toile que le voilier B à 30° de gîte, cela signifie que, pour atteindre le même angle de gîte, il faudra un vent beaucoup plus fort au voilier A.

So, you are right: it is a ratio (no unit).
I did not manage to get it fully translated yet, so I have not tried the calculation for my boat yet.

Best regards
Jens

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paulk

· #11 · Apr 1, 2009

Bonjour Jens
What it says is essentially to outline how they propose to calculate the ratio for various types of boats -- to use the maximum sail area and to spread the weight of crew (in the case of cruising boats) between the windward cockpit and bunks. Since you already have the ratio (15.5?) there's not much left to do- no need to translate any further, really. One comment on the Hisse et oh website suggested that your boat, at 15.5 , was pretty stiff because the sail area was pretty small. Bon vent!

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paulk

· #12 · Apr 1, 2009

or in paperback: ISBN 1-58234-468-x

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