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  #821  
Old 05-20-2013
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Re: Full or fin keel?

Quote:
Originally Posted by bobperry View Post
Paulo those are lovely photos you downloaded from the internet. I'm not sure what they have to do with the tandem keel though. I see no mention of the tandem keel in any of that downloaded material.

I have no issue with CFD at all. It's a wonderful tool when used correctly. My original question had to do with why was Paulo so certain CFD had been used to design those tandem keels?
Bob, I don't know if the several modern tandem keels designed recently by some major NAs where designed used CFD, a speed prediction program or tank testing.

Quote:
Originally Posted by bobperry View Post
... I wouldn't make the automatic assumption that CFD is being used for every new keel you see from a major design office....
Not all NA are using yet CFD or based speed prediction complex programs but big NA offices are (in general terms) and some smaller ones too. Those images that I downloaded from the internet were from the sites of two Naval Design offices : Ker and Berret/Racoupeau.

But I would find hard to believe major NAs making statements like these ones without knowing about what they are talking about:

“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”
Mortain/Mavrikios

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

Both have recently worked and used tandem keels on their sailboats designs. Do you think they don't know what they are talking about?

Both firms are well respected and credible, we are not talking about a boat manufacturer making publicity, they work with several different types of keels. I don't think they are making these statements without knowledge. Sure, there are several ways of testing to acquire that knowledge, from tank testing to testing in real size, but CFD or a based speed prediction complex program is the more inexpensive way.

Regards

Paulo
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Last edited by PCP; 05-20-2013 at 09:53 PM.
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  #822  
Old 05-21-2013
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Re: Full or fin keel?

Fine Paulo. That's exactly what I thought. Thanks for clarifying that.
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  #823  
Old 05-21-2013
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Re: Full or fin keel?

Paulo,

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.

For example, we agree that many of the larger yacht design firms do employ CFD. We seem to agree that the use of CFD is not always the case. It sounds like we agree that the forms of CFD that are used vary from extremely sophisticated versions, capable of reasonably fine grained analysis, to pretty course versions which are good at evaluating general data trends but not providing highly accurate quantitative data.

I should also note that I am very aprpeciative of your posts which talk about the broad range of 'experimental' or 'non-traditional' design taking place in Europe. I use the terms 'experimental' or 'non-traditional' only in reference to what is happening in the U.S. which, for better or worse, tends to be more conservative.

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.

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.

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.

I assume we are in agreement that when you look at the drag of any foil and bulb keel, there are a number of drag inducing elements. Wetted surface certainly is the big one at slower speeds. But as speeds increase, induced drag becomes more significant. And in that equation if we assume equal foil lengths and both keel types operating in undisturbed water, the bulbs on both acting effectively as end plates, then we can assume for the moment that for equal lift, the foils generate equal drag.

But there would be very different amounts of drag for the bulbs. The twin keel would have two smaller bulbs than the tandem, and the properties of those two bulbes mean that the tandem keel has two leading edges, two trailing edges, and more surface area, (since proportion of internal volume to surface area increases with size).

And if tandem keels could have inherently less drag for an equal lift, then its possible to partially use some of that difference to increase the lift on the tandem keel so that it offers more lift for less drag.

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.

What that statement cannot address are issues that cannot be defined in broad terms and for which we have no tools to adequately evaluate universally. For example, one advantage of a twin keel is that as it heels, the leeward keel becomes more perpendicular to the side force, and in flat water, the windward one reduces drag as it is lifted out of the water. that should offer some advantages to the tandem keel.

But in waves, the collisions of the waves with the windward foil would offer a large non-steady state drag which will vary with wave size, steepness and frequency, and which could return the drag advatage to the immersed tandem keel.

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.(And while I don't want to speak for Bob, I speculate that he may also come at this with a similar and definitely far more experienced based skepticism.)

Respectfully,
Jeff
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  #824  
Old 05-21-2013
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Re: Full or fin keel?

I asked a very simple question of Paulo when he claimed the tandem keels were designed using CFD studies. "How do you know that CFD was used to design those tandem keels?"
Getting a succinct answer was anything but easy.

My point was this:
The tandem keel (if we are going to call it that) Has been around for about 20 years now. It was tried on an AC boat with some success but other AC boats did not use it. There's probably a very good reason for that. In my consultations and work with Laurie Davidson and his AC effort I was exposed to some experimental AC design ideas. Most, the "hula" for example, were abandoned along with the tandem keel and the forward rudder. And certainly by any criteria the AC boats had restricted draft. It was a rule. One of the things they found with the tandem keel is the distance between fins was critical. The aft fin is operating in the "bad air" of the forward fin, i.e. turbulence off the forward fin and an increased angle of attack. Think about racing and trying to sail straight up the stern of the boat ahead. It NEVER works. So in order to get some clean flow of water with a reasonable angle of attack on the aft fin the two fins have to be separated. The basic rule, as I recall, for the aft fin to have "clean water" was 7 times the tip chord of the fin. You can clearly see that in the photo Paulo posted of the AC tandem keel.

But in the new, tandem keels the two fins are squeezed together with barely one chord length separating the two fins. With the two fins this close together I can't See how the aft fin can have any clean flow over it. It has to be operating in turbulence and with an increased angle of attack, i.e. increased drag. And, unlike a sail, you cannot trim this fin. You cannot change the angle of attack.

So if these new "compacted" tandem keels are working as well as the promotional material claims, then I would like to learn more about them.

Paulo said the tandem keels were the product of CFD studies but he was assuming that. I'd like to know if they were and I'd sure like to see the evidence so I can learn something. I am always looking for ways to reduce draft while preserving windward ability.

"Breakthrough" keels come along regularly. The Scheel keel was an attempt as was the Reijo Salminen wing keel to name two. Both worked, sort of and mostly because they lowered the VCG. But they are clunky keels and not good for boat speed. If the tandem keel preserves stability and windward ability on an equal level with a keel of reasonably deep draft then I would like to know more about it.

I am not going to buy into every piece of promotional material I read from any designer or any builder, even if I do read it on the internet.

I am waiting this morning for a new custom design client. He wants a variation of the 62 SLIVER project I have going now. Undoubtedly the question of draft will come up. I need to be prepared to present him with workable, efficient options. Talking the talk won't work for me. I have to be be ready to walk the walk.

Right now I believe I'll go walk my dogs.
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  #825  
Old 05-21-2013
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Re: Full or fin keel?

Quote:
Originally Posted by Jeff_H View Post
Paulo,

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.

Quote:
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.

Quote:
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

[/QUOTE]





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

Regards

Paulo
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  #826  
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Re: Full or fin keel?

Feel some level of responsibility for this back and forth given I was the first to mention a 20+ yr. old tandem keel design. Having said that and noting how much useful information I have gained from Jeff and Bob and how little from this distraction concerning CFD let's please agree CFD exists - it it used by some NAs and my Aunt Tilly - as with all technologies it continues to be devloped and doesn't answer all possible questions to be asked. Please move on and continue to educate me and the others following this thread.
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  #827  
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Re: Full or fin keel?

Oh come on Outbound. I swear I saw the horse twitch a little bit.
His eyelid moved.

Morelli and Melvin, designers for two of the big AC cats does not use CFD.
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Re: Full or fin keel?

As regards CFD went to engineering school for a while before doing what I do now. Remember a professor talking about materials design saying : 'we do the math, then build it and see if it breaks'.
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Re: Full or fin keel?

Call me old fashion, but I prefer the beauty and grace of an old design over the extra knot or two I would gain in light air from the boats whose esthetics are.....well actually they aren't.
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  #830  
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Re: Full or fin keel?

Quote:
Originally Posted by bobperry View Post
....

Morelli and Melvin, designers for two of the big AC cats does not use CFD.
If you say so, but on the design team EMIRATES TEAM NZ DESIGN TEAM, and on Morelli and Melvin design team for EMIRATES TEAM NZ they have specialists in VPP and CFD and I don't believe they are there only for the ride. The same with Luna Rossa design team, in what regards CFD and VPP specialists and all the other teams for that matter.

http://www.sail-world.com/multimedia...GN%20TEAM1.pdf

http://www.morrellimelvin.com/emiratesteamnz/team.html

http://luna-rossa-challenge-2013.ame...en/team#design

Regards

Paulo
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