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

See Fast on water – slow on paper | it is not thousands of design, but "we are talking about tenths of thousands of design-variants for a hull"

 

Wouldn't you conjecture, though, that each designer has an interest in bringing their learnings forward, where some projects have big budgets and others don't? Each project must have a starting point, and it seems to me the concept of a TARGET sectional area curve is a very useful one, if only as a control as one plays with other variables. So what I'm wondering is whether there have been any useful updates to Carl Scragg's 1988 work on the topic.

Also, each boat is going to have a unique LCB (and that must also be the LCG of the boat as a whole). In the case of the XR-41, seems like they knew in advance they wanted a full bow. In that scenario, does it make sense to initiate the design process with a symetrical sectional area curve with LCB midship, or with LCB a little farther aft, as has been the norm (in which case there is a particular way to shape that skew according to Scragg)?

 

In my opinion, no, absolutely not.

 

So initiate the design process with the LCB a little farther aft? Is the bow's "fullness" mostly a matter of breadth forward, then (not volume)? Three reminders:

• Many sailboat designers continue to constrain the problem such that there is no, or very little, immersed transom, even if they are using a high-ish Cp, so that the boat will perform well at a wide range of speeds.
• The possibility of moving ballast forward or aft, as well as auxilliary engine, tankage, etc, helps in making adjustment of the LCG match the LCB. So in sailboat design I tend to assume that designing to a target LCB is possible.
• There are classes, such as Class 40, where full bows are now the norm. Class 40 is a development class - a box rule, not a handicap rule. Recent designs are arguably scows.

 

Hold on here....we need to define which lines exactly we are looking at and why. A sailing vessel actually has two sets of lines. The "construction lines" to build the hull and the "sailing lines" that she actually sits while underway. A modern high-performance sailing vessel rarely sits her "construction lines". This allows for significantly different area distribution than say a mid-century CCA vessel which the rules rewarded a pinched stern. Much like the "cod head, mackerel stern" of the 1840's fast packets the idea is to but the buoyancy where it can resist the sail forces. I know there have been model tests since the ANTELOPE test but everyone is holding them close. You need to go survey and understand what the sailing lines look like before trying to wring anything out of the construction lines.

 

Good point, and many of the ocean racers designed for reaching & downwind do immerse their transoms significantly when crew and water ballast are aboard, unless the forces on the rig are pushing the bow down.

When straight keels were the norm, construction lines were often with the structural keel blocked level, irrespective of whether that was parallel with the flotation waterline.

The way I'm describing LCB location might get changed up with certain bow shapes, as Fcfc pointed out. I'm thinking of the Manuard IMOCA design, for example. The LCB might appear to be farther forward relative to where the waterline "entry" is because it has a spoon, not a cutwater, bow. But that boat is foil-assisted, making it a different animal for a lot of reasons.

These caveats have always been with us. Some boats deflect significantly with headstay tension (though this might be countered by "hogging", depending on the weight of the ends). If deemed necessary, the answer is probably to design it deformed, then loft the unstressed shape for construction. Anyway, I'm supposing most designers design on what they take to be the sailing lines, or whatever is stipulated as measurement conditions if designing to a particular rule. I favor designing on something close to the full load condition, which I suppose is common practice in ship design (though it's been a while).

 

"Theorists favor midships, while empirisists favor 8/15 of the waterline length (53.33% measuring from the bow)"

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(!?)

LCB 53.5% is typical for 0.35 Froude (Yacht Upwind) since 1930

 

"In the case of the XR-41, seems like they knew in advance they wanted a full bow"

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Because it's the trend.

Since 1850 ... the cutting edge of high-level competition has set the trend

other yachts copy it, and so on down the entire scale, down to the ridiculous imitations in cruising yacht

In the first edition of Skene's book in 1903, the author was already complaining about this iron law. Nothing new under the sun.

 

The funny thing is that the trend is heading towards the classic world, that is: reconciling LCF with LCB, haha.

Either we move the LCB aft or we have to reduce the wetted area at the stern.

In the end, we the defenders of classic tradition are winning, as is logical, and the troglodytes are losing, haha

 

One hundred years going around the same wheel

Either we draw a stern that matches the bow ... or ... we move the center of gravity aft of the center of Flotation (!):

L_CG => L_CB => L_CF

CG: Center of Gravity
CB: Center of Buoyancy
CF: Center of Flotation

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Denys Rayner

Denys Rayner - Wikipedia https://en.wikipedia.org/wiki/Denys_Rayner

Thomas Harrison Butler

Thomas Harrison Butler - Wikipedia https://en.wikipedia.org/wiki/Thomas_Harrison_Butler

 

If a very wide stern is going to be wet, then everything needs to be moved aft.

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I don't know why it's so hard to understand.

There are five fundamental centers, five

(1) CF, The Primordial Center
(2) CB
(3) CG

(4) hydroDynamic Center of the Keel
(5) aeroDynamic Center of the Sails

And the Longitudinal position of these Centers should be more or less the same.

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The Reconciliation

The Reconciliation can be done around 53.5% LWL ... or around 58-60-62% LWL

You can draw a stern as sharp as the bow, or you can draw a huge, wide stern.

The point is not to cheat yourself.

 

And

And you could also reconcile around 48-50% LWL, which would be a yacht with a very full bow and a thin stern.

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three/four large families
48-50
53
55
58-60

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What makes no sense is the absurdity that troglodytes consider normal simply because it's commonplace, for example:

hydrodynamic center of the keel at 40%
Center of Buoyancy close to 50%, and Center of Flotation close to 60%

 

In this case, the aerodynamic center of the sails is around 40% since the yacht sailing upwind has a massive 30° heel.

However

LCF: approx. 52%
LCB: approx. 53%
LCG: approx. 53%
Keel hydroDynamic Center > 6° Leeway/Yaw: approx. 50%

 

L_CF: 58% like a 1979 IOR yacht; Buuuuut ...
L_CB: 60% (!)
L_CG: 62% (!!)
Hydro Center Keel: 62% (!!!)
Aero Center Sails: 62%

 

The Brandon Dobroth designed Canada's Cup winner Coug was said to have its LCF forward of its LCB, and some believed this helped it hobby-horse less. I designed a catamaran with the LCF deliberately forward of the LCB. Both of these had transoms (and neither transom was triangular).

Theoretically symetrical and skewed curves of areas are both defined in Carl Scragg's paper. It seems to me one sees both in shells and skulls, with some top competitors being symetrical fore & aft while others are longer at the bow than at the stern.

I think 53.33% (with some variation according to Froude number) comes from the Delft Systematic Yacht Hull Series (DSYHS). There's a graph showing this in Principles of Yacht Design. People are welcome to check me on this. Fcfc's discussion of CFD results having superceded use of the Delft series are interesting; I wonder what can be said about the effects of that.