Back in the 80's I designed a series of cat-ketches with tandem keels very much like the Collins Keel shown in the photo
. People who sailed the boats could not believe it when we showed them pictures of the keel after their demo sail. There was no reason to suspect any kind of wierd keel from the way the boat sailed. This keel was so popular that we only sold one conventional keel.
There were several positive benefits noted. The horizontal plane of the big delta
plate at the bottom of the keel seemed to damp pitching. The center of gravity of the wing keep was so much lower than the conventional keel that we added 5' to the masts after hull
#4. Of course the main reason to use it was to gain shallow draft
I never heard any complaints from the owners. I owned one of the 36 footers myself for many years and never thought about the keel when out sailing. I was actually sort of surprised when we hauled her out and I saw it again....
Cruising the South East