Hollow Sections aft on ULTIME Trimaran Amas

SAC Sectional area curve. CSA typing mistake, sorry, now corrected. Wetted surface area

I said "tends to" because many new trimarans don't have those features, but the better ones do.

RW

 

Thanks, RW. I understand it but I would like to know why. I do not want to think that it is a fashion, as some have pointed out, or at least what is the theoretical basis in that fashion. Copying by copying does not make any sense to a responsible designer.

 

During AC34 with the the 72 feet catamarans, ETNZ had the same hull shape as earlier mentionned for the C Class Groupama C (same designers) while Oracle Team USA had more rounded hulls.

The difference for Oracle was reported to be smoother transition between floating & foiling mode.

The same evolution seems to occur in the foiling Moth serie with the "old school" Mach 2 (ETNZ hull style) and the new rounded hull made by Exocet.

Probably, despite its little dynamic lift advantage the ETNZ hull style might remain a bit stuck in the water during the take-off phase.

In your bath tub just try to lift a flat plate floating on the surface, and try the same experience with a more rounded shape of the same wetted area, I guess you might feel the difference.

Happy week end & fair winds.

E

 

two ways to look at it:
1) water flows fastest in a straight line
2) a rockered hull has the same profile as a wing. However, it is of very short chord. How much this sucks the hull down is a moot point, but it is near enough the same as an assymetric hull, with the below water part turmed on it's side. This has enough "lift" to prevent a lot of leeway, so presumably the sucking of a rockered hull has some too. Whether this offsets the 'at rest' reduction in wetted surface from a rockered hull, I have no idea.

Cats and tris, with aft mounted rudders are hard to turn in a tack. The argument is that the rocker (in the main hull of a tri, both hulls of a cat) gives them something to rotate around and ii reduces wetted surface.

None of my boats have rocker, does not have any effect on their speed or their turning, but they are all very shallow draft (partly due to no rocker). No rocker makes them a hell of a lot easier to build and fit out,

 

... as visible on this photo of G itana 17

 

The chubby Nicol main hull with its flat run definitely gets dynamic lift..... Saying narrow boats can't is like saying water skis don't rise to ride on the surface....

 

On this interview of G.Verdier in 2017 by Voilesetvoiliers, just after the launch of Gitana 17, there are 2 answers in relation with hulls shape. Here the proposed translation of these two Q/A + the full text in French with small sketches about the cylindrical versus U hull sections issue :
Voilesetvoiliers.com : U-shaped hull shapes are very pronounced. This drawing is very original ...
G.V. : "Not so exceptional eventually : it looks like what we did on the AC72. We know that a spherical or cylindrical shape offers less wetted surface but it has a huge drawback : under 10 knots speed, it is sucked down. At 25 knots we doubled the mass of the boat in suction down ! So let's get rid of it and adapt what we know, from a water ski for example. Finally flat bottom shape avoids this sucking. This makes much lower dynamic mass. "
(see sketches in the interview here below) : "These sketches explain the interest of flat bottom floats. According to the creators of the trimarain (?), they create a lift, avoid being sucked down, increase the ability to fly and provide excellent stability in rough sea."

Voilesetvoiliers.com: And also allows more easily the lift ...
G.V. : "Exactly. And even when he does not lift, he sinks less! This has the disadvantage of more soliciting the structure. There, we took a step compared to what was done before. One of the peculiarities of the trimaran is that we think that, to have a better control of the boat and the appendages, a rather rigid platform is necessary. We started from what had been initiated in multihull by Nigel Irens with very flexible shapes that allowed to well pass in rough sea. It was the « flexible-soft ». When we imagine foils of such size, we can no longer afford to have structures that are poorly controlled. Although it is very interesting to be flexible when you start flying. But, if we want to transform the propulsive energy into speed, with shapes that generate such deformations this propulsive energy will go into a twist and that's not good ! We departed from this approach for that reason. Which explains why the floats are big enough between the arms. We have arms that are closed rectangular boxes while before it was I-beams more sensitive to torsion. We wanted to minimize the unknowns because it is very difficult to calculate a flexible structure. And as we are not very smart we went to what we master in terms of calculation.
Below, we have a slightly inverted hull line, for hydrodynamic reasons. When we get closer to 50% of the displacement of the boat we end up with a more appropriate hull. It's to help take off."

Guillaume Verdier : «On défriche !» - Guillaume Verdier - http://www.guillaumeverdier.com/guillaume-verdier-on-defriche/

 

Given that the latest Moths+foils+rig etc. with the best sailors on board lift of at about 6kn of boat speed, minor differences in hull shape are insignificant to the point of becoming irrelevant. Other factors have a much greater impact on performance (e.g. ability to foil early is as much about technique and sail trim as foil package given reasonably current equipment).

Both the Mach2 and Exocet hull shapes have been around for some time, very similar hulls for both can be found in pre-foiling low-riders. The recent rise of the Exocet in performance can likely be put down to the greater development in rig and foil shapes by a number of small, competitive teams vs the (more or less) one man band behind the Mach2 whose resources have been divided by attention to the WASZP over the last couple of years.

The combination of the Exocet rocker and round hull means the "wing" section has a 3.3m chord but almost zero span, so likely produces very little "suck" at 6kn or so. Water skis would not work so well if they were round on the bottom. ;-)

 

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Most excellent info in your post and link for those who want to know "why" !

 

Thank you all for your kind answers, some of them valid to clarify things. And sorry, Doug Lord, for asking "why". I am not able to admire something without knowing what problem it solves or what improvement it introduces into a design, without knowing why and how it is achieved.

 

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Asking "Why?" is good-no need to apologize.....

 

Gitana 17 and Banque Pop ama pix on first page--Macif here:

 

That is what I had always thought but it seemed to me that, to reassure everyone, it was preferable to ask for forgiveness. It seems that, once again, I was wrong.

 

On a surfboard a concave creates lift by effectively pushing broken water ie small bubbles through the void ,creating less wetted surface area. it has its limitations when speed drops and with slow directional change ,but it is very common. I should ad that this is my interpretation of how forces react and respond but I think that most boardmakers see it that way.

 

Looking at this thread again and reading Dolfimans Verdier quote above I figured this illustration by Guillaume Verdier should be part of this thread: