Progress, progress
For the early 70's type boats, the 27-29-32-35-II, etc, these were considered modern keels AT THE TIME. This was at a time when many boats still had full length keels, and the "fin" keel was the "latest craze". The obvious advantages were lower wetted surface, lower CG for more stability, more lift from a more efficient foil shape. The thinking of the day in terms of hull shapes were more full and rounded shapes (look at this vintage of hulls compared to the newer boats-the new ones have finer entry, less wetted surface, and flatter bottoms).
Partly as a result of the emerging IOR rule for ocean racers, and the marketing "cache" surrounding them, Ericson and other builders introduced new models designed to take advantage of the rule-which means using shapes which yield good performance for the rating. Because of the way a boat is measured under the IOR, there evolved a certain style of boat-the "IOR" boat, which in those days meant a tall, high aspect masthead rig, fine bow sections with a flat forefoot, and a pinched transom. Many of these features were not ideal for overall speed and seakindliness, and some were, but it did yield the best performance FOR THE RATING. Most boats designed to the IOR rule-regardless of the designer, shared many common features and shapes.
Reducing wetted surface is key to speed, and having the most optimal foil shapes in the keel and rudder help with windward ability. Add to this the need to maximize stability and you can see why keels got deeper and narrower-and the modern fin keel became the norm. Just like a Darwinian evolutionary chart of man, you couild draw a chart beginning with a full keel boat from the 50's, the early fin shapes of the late 60's and 70's, and finally the keels of the late 70's and 80's. You will see the transition from heavy boats with full keels and low aspect rigs to progressively lighter boats with deeper keel and taller rigs.
In the early 90's the influence of the IOR rule had faded, and boats got better again. We kept the good stuff: deeper keels with good foil shapes, but smoothed out the hull shapes since they no longer were concerned with certain measurement points to "fool" the rule into thinking the boat was slower than it was. Now boats could benefit from the gains in some areas without the nasty handling characteristics that some (not all) IOR and IOR- type boats had. We even started adding bulbs to the bottom of keels to get the weight lower while keeping area to a minimum!
Nowadays, we are seeing water ballast and canting keels-another step on the evolution chart, and boats get better anad faster all the time.
There is a lot more to this story-the advances in construction, engineering and materials had a lot to do with this process.
Long way to say that deep keels with nice airfoil shapes are faster and more weatherly than shallow fat ones. Progress and science drives all of of this, and the more scientifically we were able to look at a boats' design and evaluate the good and bad points the better we got at developing newer and better designs.
The "delta" keel is really BK's variation on the fin keel of the late 70's, and I think the forward sweeping trailing edge had something to do with reducing end plate effect at the bottom of the keel, and keeping area down while maintaining a low CG. Sort of like an elliptical keel, rudder or airplane wing (Supermarine Spitfire!).Martin may recall some more details about the delta shape, but it is essentially BK's take on the concept-the Holland/Peterson fin keels (which later became elliptical) were most common, but King's "delta" in actual fact was probably not much different than any other similar fin keel-there is more than one way to skin a cat!
Hope this is not too boring!
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