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Hull length / weight / sail area ratios.

Hull length / weight / sail area ratios.

Hull length / weight / sail area ratios.

We know that there’s a “hull speed” related to the waterline length of the hull (I think in relation to the wavelength of the bow wave generated by the displacement of the boat): the longer the hull, the faster the boat, but a shorter boat can go faster because it is lighter and/or carries more canvas... What are the incidences of these different ratios between themselves, and what were the thoughts of the naval architects in the design of the 14? Moreover, the longer a cat is, the more comfortable it is in bigger seas... so why did you choose this hull length? What is the impact of the beam? And more specifically, the asymmetric shape of the hulls?

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Herve
Herve
November 3, 2022
3 answers
Bonjour,
La vitesse de carène est en effet directement liée à la longueur de flottaison.
Sur un catamaran de croisière, les deux paramètres majeurs pour la performance sont la longueur à la flottaison, et le rapport voilure/déplacement.
Si l’on réduit la longueur du bateau en se disant : Mon bateau sera plus court, donc plus léger, donc plus rapide… On a de fortes chances d’être déçus !
En général en réduisant la longueur, on réduit la masse mais aussi la voilure, donc le ration V/D reste inchangé, la carène est plus courte, donc on va juste moins vite…
Pour simplifier, si l’on veut aller plus vite il faut soit augmenter la longueur, soit augmenter la voilure, soit réduire la masse.
Les deux premières pistes augmentent mécaniquement le prix du bateau, seule la troisième peut (parfois) être obtenue à iso-coût…
C’est tout l’exercice qui a été fait sur l’Excess 14 : la longueur des coques est globalement liée aux besoins d’aménagement, elle est volontairement limitée à 44’ pour garder un bateau abordable.
C’est le rapport voilure déplacement qui a été travaillé au maximum, pour réduire la masse sans augmenter le coût, et conserver un budget ‘voilure’ significatif.
L’asymétrie des coques, et toutes les études aéro/hydro qui ont été menées par nos architectes, n’est pas de lien direct avec les ratios ci-dessus.
Ce sont des efforts supplémentaires, tels qu’ils sont faits couramment dans le monde de la course, pour faire ‘encore mieux’ avec une longueur de coque et un ratio V/D figés.
Ils apportent bien sûr un plus, mais ne pourraient ‘sauver’ un bateau qui serait trop lourd ou pas assez toilé… !
Hervé

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Hello,
Hull speed is indeed directly linked to waterline length.
On a cruising catamaran, the two major parameters for performance are the waterline length and the sail area to displacement ratio.
If you reduce the length of the boat, saying to yourself: My boat will be shorter, therefore lighter, therefore faster... you’re likely to be disappointed!
In general, by reducing the length, you reduce the weight but also the sail area, so the SA/D ratio remains unchanged: the hull is shorter, so you just go slower...
Simplifying things somewhat, if you want to go faster, you need to either increase the length, or increase the sail area, or reduce the weight.
The first two lines of thought will mechanically increase the price of the boat. Only the third can (sometimes) be obtained at similar cost...
This is the process that was carried out with the Excess 14: the hull length was mostly linked to the layout and interior requirements, being voluntarily limited to 44' to keep the boat affordable.
It was the sail area to displacement ratio that was worked on to the maximum, to reduce the weight without increasing the cost, and to maintain a significant ‘sail resource’.
The asymmetry of the hulls, and all the aero/hydro studies that were conducted by our naval architects, was not directly related to the ratios above.
These were extra efforts, as is commonly done in the yacht racing world, to do ‘even better’ with a fixed hull length and sail SA/D ratio.
They are of course a benefit, though wouldn’t ‘save’ a boat that is too heavy or carrying insufficient canvas...!
Hervé
R
Ruppmic
November 4, 2022
Dear Herve,
Thanks for sharing the theoretical background. I would be very interested to understand the real performance of the Excess 14. So far the the Polar is still not available / published. Even in the published video there are no shots from the plotter that would allo some understanding of the actual performance.
Can you please share some more information.
Michael
Claire - The Excess Team
Claire - The Excess Team
November 4, 2022
Dear Michael,
I am sending the Excess 14's Polar to your email address.
Don't hesitate if you need more information!
Claire
P
Philippe
November 7, 2022
Bonjour
pouvez vous m envoyer les courbes svp ?
bien cordialement

Philippe
Herve
Herve
November 3, 2022
1 answer
This comment has been deleted by the user.
O
Oudrup
November 3, 2022
But reducing weight, will reduce stability, and if you want the same safety, you will also have to reduce the sailarea. Its important not to reduce overall beam of the boat, and to make the hulls a bit assymetric like the Excess 14 also helps!