Join Date: Dec 2004
Location: Portugal, West Coast
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Re: Full or fin keel?
Originally Posted by Jeff_H
Paulo, I am not sure that your assumptions are right. I attend a lot of presentations on the development of yacht design software and how it is being used. Clearly CFD is out there and being used on specfic projects by the high visibility high performance shops. But for the most part, what seems to be way more common in design offices are a battery of VPP's (velocity prediction programs) that are based on historical data and prior CFD and towing tank studies. These VPP's are pretty good but they do not analyze down to a level of detail which would validate the performance of a particular choice of keel profile, keel section, or keel layout.
The better of these programs produce quasi CFD data and the kinds of graphics that replicate what you would expect out of a full blown CFD analysis, but these programs do not appear to be at the fine grain level that would be needed to analyze whether a specific tandem keel worked effectively or not.
This is not to say that RANS and CFD analysis is never performed. I think that really high budget projects get that level of attention. Farr's office showed amazing graphics from their analysis of the new Volvo boats.
I am talking with Quantum about doing a paper exploring the trade off between sail area and sail shaping for cruising boats (i.e. more powerful sections vs. larger sail area) and in the course of the preliminary conversation, it was suggested that ideally we should use the VPP for a specific design, including the drag, leeway, and stability characteristics of a real hull and rig.
The designer at Quantum and I spoke to one of the partners at Farr about the likelihood of getting basic data on one of thier newer production designs, and while it was clear that Farr ran moderately sophisticated performance analysis on all of their designs, it was not clear that it was run on a fine grained enough level to allow a precise enough computer simulation for the sail study.
As recently as only two years ago, there was a paper presented on a study to validate a CFD analysis of flows around a keel. It was one of the first times that they were able to instriment a full sized keel in action to measure the points at which laminar flow became turbulent. They found that there was almost no steady state laminar flow, and that the flow even in calm water was discontinuous, shredding in much the same way that spinnakers have discontinuous flow. It largely invalidated the CFD which assumed way more continuous flow than existed, and it was only once factors were added for discountinuity that the CFD could be validated.
So, while I do not want to talk for Bob, and I do not want to be dimissive of what you are saying, my sense of this is that CFD, especially for keels is still in its infancy, and that while better design firms have access to CFD for their higher budget projects, it is not in general use and perhaps fine level analysis not accessible enough to acertain that a tandem keel really does work as advertised.
I did not said all NA, I had said major NA firms use CFD to help designing boats. As you have pointed out Farr use it as other major Na firms and even some minor ones. Look at these computer printouts from Ker. The first one refers to the final hull for the Sydney GTS 43, designed with the help of CFD, the second one is a beautiful study on a keel and ruder flow.
You can read on the site about the CFD they use (Numeca).
Some advertise that on their site, others not. Have a look at Berret/Racoupeu (the designers of Allures among other sailingboats):
As part of our research and development efforts, we made the deliberate choice, some time ago, to incorporate totally digital calculation and analysis procedures.
As genuine decision-making tools, these different digital simulation programs enable us to confirm our design ideas in a flexible and efficient manner and at an acceptable price. In contrast, to achieve comparable accuracy, experimental processes such as towing tank tests, wind tunnel testing or mechanical trials are significantly more laborious and costly, less “state-of-the-art” and ill-suited to systematic use.
Digital tools enable us to optimise hull shapes, sail plans, appendages such as keels, and rudders for example, and to predict their performance based on outdoor conditions such as the wind, its strength, the direction its blowing from and the state of the sea. All of our yachts systematically undergo these simulation steps to guarantee optimized performance and handling. Structure is high on the agenda too, using finite element method (FEM) calculations. This equipment is systematically incorporated into our design procedure, for all of our creations. Over the years, thanks to our experience, we've developed and optimized their use.
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Last edited by PCP; 05-20-2013 at 07:51 PM.