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Excess 14 - Winglets to the rudder?

Excess 14 - Winglets to the rudder?

I noticed that the new Excess 14 has tumblehome bows. Is the Excess team contemplating adding winglets to the rudder to help the bows power through small to moderate waves without hobby horsing as much? What are your thoughts about adding winglets to the mini-keels to increase lift when going to windward?

4 comments
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Herve
Herve
January 5, 2023
1 answer
Dear John,
The keels on the Excess 14 are not additional pieces glued of bolted to the hull, they are infused 'one-shot' together with the hull laminate, so the integrity of the structure is maximum.
Of course, the deeper and narrower fins have requested additional calculation and local reinforcement within the laminate but, even on traditional shapes, the keel-hull attachment is always a scrutinised area.
On cruising cats, generally speaking, the efforts generated while landing the boat are greater than those issued by hydrodynamic side-force are.
This answers your last question: Yes the Excess 14 can land on its fins, provided the ground is flat, stiff and reliable enough. This can be very useful for sand beaching of course, but also during winter maintenance, when the owner can leave the boat on a dry dock without dedicated berth…
I hope this answers your interrogation.
Best regards and happy 2023 onboard the Excess Lab!
R
Roy
January 6, 2023
Herve can the Excess 11 also rest on its fins ?
Thanks
Roy
J
JohnH
January 4, 2023
One point I was curious about regarding the keels rather than the rudders.
As the keels are shorter chord and deeper depth than conventional (lesser ;-) ) cruising cats, as well as being more efficient (developing more resistance to leeway) presumably this means the forces where the keel attaches to hull is correspondingly greater, also with a smaller area available to secure the keel to the hull. I’m thinking of issues some monohulls have had as keels have got deeper and narrower. How has the design of the fixings been adjusted from conventional cat keel fixings to ensure a secure keel fixing? Also is it still possible to dry out the Excess 14 on a sandy beach for example?
Herve
Herve
January 3, 2023
Dear One-prop,

Thanks for these inputs, good to see we have passionate sailors / hydro-dynamists on the lab !
The two suggestions you make both make sense, although we decided –up till now- not to have them on Excess

The rudders winglets could indeed help preventing bow movement. Such study was run by VPLP on CFD software, for a Gunboat 80, and showed very positive impact !

Unfortunately this impact is major at a speed range of 15-20 kts, but at lower speeds, the efficiency of the solution (which is related to the square of the speed) falls dramatically.

We feel that, at the speed range our Excess boats are sailing (let’s be realistic !), the additional drag of such appendages would not be compensated by sufficient dynamic gains.

On the keels winglets, in reverse, the efficiency upwind is positive even at low speeds (probably around 5kts and above)

This solution (among others) was on the table when we investigated the draft and upwind performance (our article on the subject can be found here : https://lab.excess-catamarans.com/the-draft).

Our conclusions were that adding winglets was a good solution if we wanted to stick with the 1.20m draft, but it was costly, potentially fragile, and adding a lot of drag downwind.

We eventually choose to stick with classic and economical fins, while increasing their draft and improving their design for upwind performance…

But, no subject at Excess is closed to consideration, and this is the kind of investigation we may very well put back on the drawing board on a future model, if the entry data are different…

Thanks again for your inputs,

Best regards
Hervé
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Bonjour One-prop,

Merci de vos propositions, quel plaisir de constater que notre lab compte des marins passionnés et amateurs de mécanique des fluides !

Bien que vos deux suggestions soient pertinentes, nous avons choisi – pour le moment – de ne pas les mettre en œuvre sur le modèle Excess.

Ajouter des ailettes au gouvernail permettrait effectivement de prévenir les abattées. La simulation réalisée par VPLP sur le logiciel CFD pour un modèle Gunboat 80 souligne son efficacité !

Toutefois, son efficacité est maximale pour la plage de vitesses comprise entre 15 et 20 nœuds. En dessous, cette solution perd considérablement en efficacité (qui est liée au carré de la vitesse).

Nous sommes d'avis que, à la vitesse de navigation de nos voiliers Excess (soyons réalistes !), la résistance due à ces ailettes serait plus importante que les avantages liés à l’hydrodynamisme.

En revanche, les ailettes sur la quille facilitent la remontée au vent, même à faible vitesse (dès 5 nœuds).

Cette solution comptait parmi les options étudiées lors de l’atelier sur le tirant d'eau et les performances au vent (notre article sur le sujet se trouve ici : https://lab.excess-catamarans.com/fr/le-tirant-deau).

Nous avons conclu que l'ajout d'ailettes était une solution pertinente pour un tirant d'eau de 1,20 m. En revanche, elle se révèle coûteuse et potentiellement fragile – surtout, elle offre une grande résistance au portant.

Nous avons donc choisi de rester sur des ailerons classiques et économiques, tout en augmentant leur tirant d'eau et en améliorant leur design pour optimiser leur performance au vent...

Toutefois, Excess n’oublie jamais une solution efficace : ces idées pourraient tout à fait être reconsidérées dans le cadre d’un futur modèle aux objectifs différents.

Encore merci de vos suggestions,

Bien cordialement,
Hervé