In what respect is the X-Yacts XR-41 more optimized than its competitors?

In what respect is the X-Yachts XR-41 more optimized than its competitors? I'd like to conjecture the answer might be found in its longitudinal curve of areas.

Please correct me if I'm wrong, but it seems to me there has been a gap between what the theorists have been saying and what the empirisists have been indicating, for decades, about optimum LCB placement for a displacement hull (constrained to be double-ended or to carry its transom above the waterline). Theorists favor midships, while empirisists favor 8/15 of the waterline length (53.33% measuring from the bow).

The theorists I'm thinking of are Carl Scragg, who worked with the Stars & Stripes '87 design team, and Alan Skinner, an Australian. Surely, though, there have been others, and what top designers consider an optimum curve-of-areas has doubtless evolved beyond my understanding.

What I'm wondering is whether the XR-41's hull designer, Thomas Mielec, has found something, perhaps in a theorist's work, that is giving his boat an edge, and if that something might be LCB placement more nearly midship.

Does anyone have information on this?

 

Says who?...an independent unbiased body/institution of peers?

Define optimised.....it can mean anything to anyone, as noted in this case.
So..optimised for easier crewing?...or optimised for easier build, or optimised for a budget to build, or optimised for easier ...well, pick your poison.
Optimised is a terrible word to use for any design, as it does not describe the boat - as a whole. It is a word used by PR/Salesman....

 

What does "midship" mean to you?. But in any case, the longitudinal position of the Center of Buoyancy is important in relation to the CoG or the barycenter of the waterline area. Its position relative to "midship," whatever that may be, I don't see as relevant at all.

 

There is a long rant on sailing anarchy. It is essentially cwpa (coefficient of waterplane area) that is higher (outside) the validity range used for resistance prediction. Hence the VPP goes mad, and give a artificially high resistance, so a lower rating. And that makes a winning boat. Seems that X-yacht has more money to check the limits of the ORC VPP than the ORC council itself.

 

The answer to AdHoc's question is: race results. The ORC (formerly IMS) handicap system relies on a VPP, but because this new model sailboat has been so dominent in this summer's racing, ORC has controversially imposed an additional handicap on that particular model until the VPP can be scientifically corrected.

I worked with one of the principle authors of that VPP, Jim Teeters, briefly.

Fcfc, thank you for your informed answer.

I'm using "midships" to mean station 5 of 10, the midpoint of the measured waterline length of the volumous hull.

That the theoretically optimum curve of areas is symmetrical about midships is discussed in Carl Scragg's paper, which is linked to if you click on his name in the initial post. The sailboat designers on the Stars & Stripes '87 design team then asked Scragg to recalculate an optimum area for each station based on a CB farther aft. In a later paper, also published by SNAME, one of those same designers, Bruce Nelson, discusses using the symmetrical version of Scragg's curve to design a rowing shell.

The idea that 8/15, give or take, of the waterline length is an optimum from empirical data is consistent with a graph from tank tests in Principles of Yacht Design.

 

I am not sure ORC has imposed an additional handicap on THAT particular model, but on ALL models that have a CWPA coefficient outside the ORC range. And it seems it is not a fixed handicap, but they reverted to a former method of computation. The XR-41 is the most impacted, but it not the only one that have been recomputed.

 

All handicap racing rules eventually get beaten. Those boats are optimized to beat the rule, as it has happened to all rules. The committees then have serious discussions and meetings to modify the rules, which get beaten again. These are designs driven more by lawyers than NAs.

 

Just like Cascade did to the IOR back in 1972.

 

I agree about handicap classes. It remains to be seen weather Thomas Mielec figures into development class competition. Certain designers dominate those classes as follows:

• I-14 and Moth: Bieker
• Fast 40+: Carkeek
• Class 40: Manuard
• TP52: Botin
• IMOCA: Verdier
• Multihulls: VPLP, Verdier, Neyhousser
• Volvo 70 (past, but still highly competitive ocean racers): Juan K
• Honerable mention for Maxi 72 class (and IACC, past): Judel/Vrolijk

I suspect all those designers, and all pre-hydrofoil America's Cup winners from '87 on, design / were designed to a target longitudinal curve of areas. Has anything been published on the subject since Scragg's 1988 paper, or is there informed discussion of the topic published? I've only seen this:

• The Electric Launch Fusion and the Component Waveform Theory | WoodenBoat https://www.woodenboat.com/online-exclusives/electric-launch-fusion-and-component-waveform-theory
• https://mcgowanmarinedesign.com/mcgowanmarinedesign.com/Home_files/Hull Speed_ 2017.pdf

 

Excuse my ignorance, but what is a longitudinal curve of areas?. Where can I find literature on this topic?. Thank you.

 

From Google AI (aka Gemini):

The longitudinal distribution of immersed volume, also known as the curve of areas (or sectional area curve), is a fundamental graphical representation in naval architecture that plots the cross-sectional area of the submerged hull at various points along the ship's length.

Resistance and Wave-making: The shape of the curve significantly influences the ship's resistance (drag) and wave-making characteristics. A curve that is too "bluff" (rises too sharply) at the bow can push excessive water, while one that drops too quickly at the stern can create turbulence, both increasing drag. Designers aim for an optimal shape, often approximating mathematical forms like sine curves, to achieve the desired speed and performance for the given hull dimensions.
​

You might be able to "download full text" on a paper at https://www.researchgate.net/figure...ficient-of-the-reference-hulls_fig3_312034760

What Scragg and (separately) Skinner have done is to reverse engineer wake theory to find minimum wave drag values at any given length Froude number - going beyond finding the correct Cp by finding the correct area at each station's section.

 

Thank you for the clarification. I know perfectly well what the curve of cross-sectional area along the length is; it's not the same as the longitudinal curve of areas. But perhaps my English is failing me again, although my technical English, both reading and writing, is quite good.
Thanks again.

Another thing I don't understand: " ..... going beyond finding the correct Cp by finding the correct area at each station's section."
I believe, and correct me if I'm wrong, that a hull can have the same Cp even if the area curve changes. If the length, beam, and draft remain unchanged, any shape of the area curve, if the area enclosed by it does not change, will result in the same volume and therefore the same Cp. Am I wrong?

 

You are not wrong and the paper he cites normalizes the Curve of Areas via Cp while varying the Block Coefficient trying to extract data on optimum bulb shape. I am aware of some theoretical studies I was involved in (on fully submerged bodies) where we looked at the rate of area change of the forebody, midbody, and afterbody trying to minimize total drag for a given volume or cargo block (basically to get a better/optimum TMB EPH). There was a lot of questionable math and much entrail readings (much like CFD).

 

I accept fault on terminology. I should have said "Sectional Area Curve".

There are articles of a speculative nature, such as:
Curve of Areas, Versed Sines, Trochoids, and the Wave Theory – US VMYG https://usvmyg.org/articles/design/curve-of-areas-versed-sines-trochoids-and-the-wave-theory/
Those by Scragg and Skinner are different in that they show their derivation from wake theory.

The Stars & Stripes campaign built three 12 meters: Stars & Stripes '85, '86, and' 87. I never saw a disclosure of what Stars & Stripes '86 looked like underwater, but she was thought to be their bulbed design. She was the slowest of the three, but led to enough data collection to make Stars & Stripes' 87 one of the first boats to be designed with a CFD developed bow shape by iterative analysis.

The dude below, with blue eyes and mustache, is Carl Scragg.

 

Below are my personal thoughts and may not reflect reality :

What i call "analytical" methods, a la Colin Archer , are things of the past. Now is the reign of brute force physical simulation, aka CFD.

For me, the switch has been acknowledged when ORC switched from Delft systematic series to CFD derived series some 15 years ago.

For instance, sectional area curve (or LCB, or LCF, or Cp or ...) does not say a single word of what overhang is above water.

X-yachts claims to have CFD tested thousands of hull forms, but does not state in how many sailing conditions (speed, leeway, heel, waves...) each of these hull forms was tested. Not how the different sailing conditions were weighted to give the final hull form ranking.

It is my understanding that a racing sailboat design begins with deep weather analysis of major regatta the sailboat is expected to participate. This will give the sailing conditions the sailboat will be CFD checked/ranked.