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post #825 of Old 05-21-2013
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Re: Full or fin keel?

Originally Posted by Jeff_H View Post

I would like to comment that very often it sounds like you and I are disagreeing with each other when in fact, we seem to have similar opinions.
I suggest that this conversation is one of those cases.

Where we sometimes go off the rails is in how we interpret what we read. For example: "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."

I think that statement is one that all of us come to general agreement on. But that statement does not describe the specific digital tools being used, and does not specify CFD. So while CFD may be used, it does not necessarily mean that Berret/Racoupeu does use CFD, or how they use CFD, or whether they analyze their keels and rudders using CFD.
Jeff, on their site there are several pictures of keels (and not from racing boats) going through CFD analyses. That seems to indicate that they use it on all their boats as they state on the site. I do not want to make this a discussion but if you look on the sites of European main NA offices many state that they are using CFD or high quality prediction speed programs that are based on CFD. Some don't say that explicitly on their sites but that does not mean they are not using it. In fact it its a lot less expensive to use that than tank testing.

Originally Posted by Jeff_H View Post

This is similar to the two Mortain/Mavrikios statements:

"Those looking for small draughts will be delighted to know that cast-iron tandem keels …offer almost the same sail stiffness and the same ability to go close winded as lead keels with far deeper bulbs”

"The tandem keel is an alternative to the twin keel, it increases lift while reducing drag "

I think that we could agree that there is a possibility that these statements could be true. But where you and I, and perhaps Bob might not agree is the pieces of those statements which are missing.

If we look at the first statement, my interpretation is that it says two things, a that cast-iron tandem keels can offer almost as much stability as a deeper lead keel with a bulb, and that cast-iron tandem keels can point as high (i.e. close winded) as a deeper lead keel with a bulb. Properly designed, both may be true. And it does not take CFD to prove that statement to be true.

What that statement does not say is that cast-iron tandem keels designed to offer the same stability and pointing ability offer the same VMG as a deeper lead keel with a bulb.

And my sense is that the cast-iron tandem keels offering the same stability and apparent wind angle, cannot offer the same VMG, because by its very nature being cast iron, and shallower, the volume of the tandem keel needs to be greater than the volume of the deeper straight fin with bulb, and therefore there is more wetted surface, and therefore there is more drag, and therefore there is less speed for a given sail area, even if there is equal stability to carry that sail area.
In fact it says the opposite. The word "almost" implies they are less performant than deep draft bulbed keels but that is not the point. Both designers are not defending or sugesting tandem keels as an absolute performance keel but as the better performance in what regards a shallow fixed keel options, or one of the best options regarding that.

Defline compares the performance of shallow tandem keels with the performance of twin keels implying that they are both the most efficient options in what regards swallow draft keels.

Both are not as performant as deep bulbed keels and they have to be heavier to produce the same RM. There are an interesting detailed comparison made by Marc Lombard using one of those programs I was talking about, between the performance of both keels (twin keel and deep keel) on the same boat (a RM) with different wind speeds and wind angles that can give a very approximated idea of the differences.

Originally Posted by Jeff_H View Post
The second statement can be viewed similarly. To me that statement says that properly designed a tandem keel offers greater lift relative to drag as compared to a twin keel. It does not take CFD to find agreement in that statement.
In other words, without CFD its pretty easy to see that this claim could be true. On the other hand, there was an assumption that the downstream keel foil of the tandem keel produces the same lift as the twin keels, and it is here that this statement may go off the rails. I do not believe that to always be the case, and it would be next to impossible to make that kind of universal statement since the reality of this is so dependent on the specific design, and conditions.
It is for those types of reasons that perhaps I sometimes view these kinds of designer statements with perhaps more skepticism than your comments appear to reflect.
Jeff, what make the use of CFD and high end based speed prediction programs is that while they are hugely expensive if you use them often they are much less expensive than tank testing with very close results. If an office designs a significant number of boats such a program is a very efficient, easy to use toll that can give very valuable help in what regards hull, keel and ruder design options/efficiency and that's why it is widely used by major NA offices.

Regarding the use of CFD or high end speed program analyses on the design of those keels it seems probable that they have used that but if not I don't believe they had done the design without a lot of studies and tests. That is a keel that only works well if properly designed and the design is more complex than the one of a normal bulbed keel.

Mortain-Mavrikios had used them in several designs and they designed them also for Etap that says:

"After thorough investigation and numerous tests, ETAP Yachting N.V. is pleased to introduce its ETAP tandem keel. The most important advantages of this keel are the excellent sailing qualities at a considerably reduced draft. This new design is the result of a co-operation with the architects' bureau Mortain-Mavrikios."
The two most important features to reduce drift, are the size of the lateral plan and its efficiency. The efficiency is defined by the proportion between the depth of the keel and the length.

So it seems a lot of testing, computer or not was going on.

Regarding Defline, that is one of the most talented new generation French NA and puts performance very high in what regards cruising boat design I know a bit more. I know that he uses a very high end prediction speed program as a toll to help to design his boats and I know also that before using a tandem keel on a 43 foot fast performance cruiser he had done not only computer speed testing but live testing replacing the keel of a First and tested the performance before and after.

This is the Defline 43, a fast voyage boat. The boat was tested by a French magazine and they said that with 10K of wind the speeds were not far from the wind speed. Not bad for a boat with a shallow keel


You can see the care with weight distribuition with the engine over the ballast.



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