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post #823 of Old 05-21-2013
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Re: Full or fin keel?


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.)


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