Round bilge or multiple hard chine on Sailing yacht ????

[tang]

Hi everybody,
A simple question without answer at the moment : Why use hardchine or multiple hardchine indeed of a round bilge??
In my own opinion we have more WSA with the chine than with the round bilge, so we increase the drag....but why the new racing yacht as open 60 or Class 40 are biuld like that?? Is it more stable (not only on course keeping) ??? What about seakeeping ???

I want to have any opinion on the question and all experiences on hardchine/multiple hardchine SAILING YACHT are welcome....

thanks

[Guillermo]

Incorporating deep chines into the back section of the boat, provides greater power and stability when the boat is sailing heeled over. Drag increase, more relevant at low S/L numbers, is more than compensated by those benefits.
Chhers.

[MikeJohns]

chines modify the 3d flow field significantly, they can reduce resistance, entrain water increasing both roll inertia and damping and structurally they add to longitudinal stiffness. They need careful placement. While very sensible for sheet materials in composites the lateral stress concentrations at the chine need some thought at the design stage wrt trans framing.

WSA increase is often insignificant from a performance angle particularly when you already have flat sections.

[tang]

Thanks for your answers.
Guillermo, what do you mean by S/L numbers? Is it the same than Froud Number? OK I'm agree with you for a simple hardchine as on 0pen 60 but what about multiple hardchine??? (http://www.julienmarin.com/_gb_newslire.asp?num=21 ; http://www.julienmarin.com/_gb_newslire.asp?num=20) this boat was 5th at the last route du rhum with an unknown skipper....

Mike, can you develop on the position. For single hardchine (open60), the position will depend of the heel angle, but for multiple hardchine???? What will be the best, put the boat on the chine or on the "flat" plate (not really flat!!!) ????

Cheers

[Guillermo]

S/L means speed-length ratio, a number that serves for the same purpose as the Froude one, but in different measurement units.

I do not have enough expertise on this kind of boats, but in my opinion I think performance between a rounded hull and a multi-chined one like this, should be not relevant. I think only extensive tank testing can show up what the differences are, if any. Maybe Mike Johns or any other forum member with more expertise may bring us more light

[MikeJohns]

Originally Posted by tang

Mike, can you develop on the position. For single hardchine (open60), the position will depend of the heel angle, but for multiple hardchine???? What will be the best, put the boat on the chine or on the "flat" plate (not really flat!!!) ????

Cheers

You do a heeled analysis just as you would for the upright condition. All the same performance design rules hold. Really you should ask this question of the particular vessels designer since the resistance will be optimised for a certain heel angle.

In practice for the designer the exact chine positions can be a bit of a lottery, tank testing is essential for performance craft. Unfortuantely the whole underwater 3d flow field is dependant on a myriad of conditions that are only extant in a full sized model underway at sea.

[tang]

Ok, so if I well understand, nobody knows actualy how it works !!! I'm gonna do some tank tests during this year. I keep you in touch !!
Cheers

[MikeJohns]

CFD for us is Indicative rather than accurately predictive.

Knowing how it works and being able to predict it with certainty are not the same thing. I'm curious are you working on a student project or a real world design?

[tang]

I'm student in final year at Southampton Solent university (UK). I've the opportunity to use the towing tank for free, and that's a question that obsess me since my first sailing experience on muscadet and mousquetaire(designed by Mr Harlé) !!! Looking on these class 40 designed by Julien Marin, I was surprised of the performances of the boat. The harlé projects are realy stable (in course keeping) but are not realy efficient, to heavy I guess... (except the Muscadet which is realy a good compromise).... I was thinking that was an old school shape but infact, that's actual....I want to konw why !!!

[Stephen Ditmore]

You might find some relevant research done for workboats. Mostly it would be displacement and semi-displacement vessels that would be relevant - chines on true planing speed vessels are a somewhat different subject.

I would suggest grouping into three categories: chines that are aligned with the flow (probably there for ease of construction), chines positioned deliberately to break flow from the hull, and chines that are there to simply truncate, for example to achieve a wide, powerful bottom shape near the stern without adding unnecessary weight to the stern in the deck & topsides.

[Wynand N]

Tang,

Since you live in the land of "God save the Queen", I would recommend you look up this gentleman - Gordon Trower. He has his own design office and the last time I had contact with him (me studying the trade), he also lectured full time in Yacht and Small Craft Design at Cornwall College where I studied my long distance course in the early 90"s.

He has done extensive tank testing over the years at the University he lectures and have used some of that data and photos in one of his books on boat design. This guy is genius and if anyone can help you, it will be him. The last I heard of him he stayed in Falmouth, Cornwall....

[Guillermo]

Something on the matter:

"Hard Chine Versus Round Bottom-Comparison of Stability in Waves and Seakeeping Performance of Small Displacement Ships"
Zborowski, Andrew Chu, Hui-shen
SNAME Transactions, Volume 100, 1992 1/1/1992

[yacht371]

In the mid 80s I designed a line of steel hulled sailboats called Amazon, built by SP Metalcraft. These were not "experimental" but rather mainstream in design. The first Amazon 37 we built was a double chine design, but only the lower chine was underwater, and the upper chine was given a 24" radius for appearance sake. The second boat was built radius chine (basically a single chine shape with a 30" radius from stem to stern).

These two boats were otherwise identical with the exact same weight, keel, rudder, rig and sails. We had the opportunity to "race" them against each other. There was NO difference in speed on any point of sail. The chined boat seemed slightly stiffer at small angles of heel, but you wouldn't feel the difference.

My conclusion at the time was that a well designed chine hull has no performance disadvantage versus round bilge, all else being equal. Today I would modify this to say that a chine aft may well aid speed in fast light vessels by helping the flow to separate from the hull, just as it does in planing power boats.

If building in aluminum, steel, or plywood a considerable labor saving is possible by using chine construction. I have also seen fiberglass paels laid up on a formica table (gel coat down) then assembled stitch and glue fashion resulting in a decent finish with minimal fairing.

Grahame Shannon