I shortened my Peterson 34 keel by 1 foot. It was a fin keel covered in fiberglass.
Everything I read said to get an expert to do all the calculations.
I looked around the yard, saw what other keel fins looked like and decided to just copy the design. So I got out the electric chainsaw and sliced off the bottom off the keel. Then sliced and sliced and sliced until I could haul the 1200lbs of brick sized peices home, plus lots of lead shaving.
I contacted Mars, they surprised me with a many month lead time and didn't want the old lead. So I rented a large barbecue, bought a big steel pot, and melted the lead myself (in my driveway). I molded the old keel into 12 pointed 2x4 by about 18" ingots. I made the molds from 2x4 aluminum construction studs as . Then drilled holes all the way through each ingot. I stacked the 6 ingots on both sided of the existing keel with a slight offset between each, and through bolted with 6 1/2" stainless steel threaded rods. In between each piece I used plenty of 5200. I filled in the gaps and faired, then re-fiberlassed the whole bottom of the keel. There were lots of curious onlookers and many critics. The boat was fast before, and placed second in the next race she competed in on Lake Winnipesauke, NH. She also had less weather helm after.
Maybe I was just lucky, but it worked out great and that nagging rock at the entrance of our harbor no longer posed a threat.