"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.
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.
They survived till today were they are designed not by trial and error but by seeing their comparative performance with other types of keels in CFD studies. If they are still used by major NAs it is because they are a valid alternative to other options, specially in what regards low draft options.
For instance, Mortain&Mavrikios, the designers of the line of Harmony, Etap, Lockwind, Passoa, Nautitech Feeling and some Dufour said about tandem keels regarding their use on the Harmony:
“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”.
And Etap said some years ago regarding the use of tandem Keels on the Etap 30i:
"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. Also a wing section is a classic aid to improve the efficiency.
For a strong reduction of the draft neither a wing keel or a bulb keel were sufficient. The solution was found in placing two shorter keels behind one another, linked by a wing-bulb profile : the ETAP tandem keel.
The ETAP tandem keel gives a better aspect ratio, thus generating more lift.
In addition to increased stability, the wing-bulb also provides better hydrodynamic characteristics. "
Tandem keels are also used by J&J yacht design in several Bavarias.