Can somebody pro/con a full vs. fin keel for a newbie (will learn to sail on said boat) and taking it thru the Caribbean? All I can seem to come up with so far is fin keel is better to the wind, and a full keel will protect your rudder.
I described my Mac 26X, Bossa Nova, as a 26-foot, sleep-aboard sailing dinghy, that can also serve as a funny-shaped travel trailer. It's a very good first boat, for someone who hasn't decided between sailing or power-boating, or who wants to take it on a camping vacation to some big lake or bay, or other sheltered waters; and I know a few people who have taken their MacGregors to the Bahamas, and had a great trip of it.Even living at the dock and not taking my boat out I still exercise my engine.
The MacGregors with enough power can get up and plane to get you back to the marina in time so you don't miss cocktail hour. As much as I dislike them, I have noticed one thing about MacGregors.....the owners do tend to use them a lot more, and that what it's all about right.
It is possible for me to get my boat over 20kts....but that would involve putting it on a container ship.
Sorry, I guess I was not been clear. They are around as you say for a long time, from the time CFD studies were not the norm to study keel, hull and rudder efficiency, but things were done by try and error, experience and a good eye."I believe they can design them in an effective way otherwise they would not be using them."
What a novel thought. I wonder how many design features have been tried over the years that have proven less than effective but initiated with that same optimism? The "split keel" has been around for quite a long time.
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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.... I wouldn't make the automatic assumption that CFD is being used for every new keel you see from a major design office....
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.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.
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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.
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.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.
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.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.
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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.
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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.
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.....
Morelli and Melvin, designers for two of the big AC cats does not use CFD.
Bob, I was not doubting. M&M have a specialist in VPP in their design team but then VPP is strongly based on CFD. They are very complex program with its roots in it. Some work directly with CFD, some use programs that are based on. Both are very powerful computer analytic tools that can predict how a boat will behave and can help boat design. On AC teams they have both, CFD and VPP specialists.My comment about M&M was made because Jody Culbertson who does my 3D rendering work has also been working with M&M on a cat project (not AC). We discused this today, this morning. He asked M&M about doing some CFD work on another project he was involved in and Gino told him they did not use it. They use VPP's. This is not first hand information. But it is second hand. I did not read it on the internet.
The horse is really still now. Doesn't look good. Probably playing possum.