Magic Keel

BOAT REPAIRS & MODIFICATIONS

Truncating the keel & adding a bulb on Cambria 44 MAGIC

The owner of Cambria 44 hull #2 did not like the 8’ draft of his keel, and he called Sponberg Yacht Design to see what could be done to reduce draft without adversely affecting performance. Cutting off the tip and fashioning an equal amount of lead into a bulb was the answer, and this has become quite a common modification for deep draft boats.

Having done most of the naval architecture on the Cambria 44’s for David Walters Yachts when they were built, I had all the necessary information regarding the keel design and the keel bolt specifications. I modeled the keel lines on the computer, and made the alterations to reduce draft by one foot. The resulting volume happened to be equivalent to 1,100 lbs. of lead. Again using the computer, I modeled a bulb of the same volume and weight which would be attached at the lower aft corner of the keel.

From the computer I plotted full-size patterns of the bulb sections on Mylar film, and the boatyard, Pilot’s Point Marina in Westbrook, CT, used those sections to build a foam plug of the bulb to the exact shape. The bulb had a V-notch in it so that it could slip onto the trailing edge of the keel, and it fit perfectly.

The yard was ready to cut off the tip—all it took was a Sawz-All. Then they held a lottery: Was the volume of the cut tip, in fact, the same volume as the bulb plug? They filled up an old dinghy with water and marked the waterline. Then they placed the cut-off keel tip in the dinghy and marked the new higher waterline. Out came the keel tip and everyone placed their bets: When the bulb plug went in, would the waterline go higher (too much volume), go lower (not enough volume), or stay the same (just the right volume) as the waterline for the cut-off tip? The result: We had just the right volume. One worker walked away with a lot of cash.

Four 3/4” diameter silicon bronze threaded rods fastened the bulb horizontally through the keel and nutted into recesses in the sides of the bulb. The recesses were packed with putty and faired to shape. A corner fillet piece made out of fiberglass created a nice transition between the trailing edge of the keel and the top surface of the bulb. Finally, the surface of the new bulb was filled, faired, and painted.

I call the bulb design a beavertail-swallowtail design, or a BS-bulb for short (go ahead, you can chuckle). This is because of its flat aft end (like a beaver’s tail) that terminates in a swallowtail shape. The shape and position of the bulb minimizes the tip vortex off the bottom end of the keel, thereby improving lift and decreasing drag. After sailing trials, the owner reported that MAGIC sailed faster and pointed higher than she did before, and she had reduced draft to boot. The cost of the design work was $4,000, and the actual modification cost about $8,200 (1994 dollars). Mission accomplished.