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Gary Hendrickson (Garyh)

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Posted on Friday, March 01, 2002 - 10:04 pm:

CLASS RESEARCH: Keel Fairing
Submitted by Gary Hendrickson (#094), Class Rules Committee

Besides the normal wear & tear on keels – nicks, gouges, rounding of the bottom edges, especially the front knuckle – most keels have efficiency-robbing imperfections that may not be easy to spot. We have observed minor dips & rolls in the side surface on a number of keels. These slow the boat by consuming energy to pump the water as it moves over the keel. Two keels in the Lake Michigan fleet exhibited another similar fault: a shallow concaveness on the starboard side 12” – 14” wide, extending through about 2/3 of the height of the keel.

In addition, later keels poured with the sand-molding process showed a tendency to exceed the design width dimensions by as much as 10% - 12%.

Checking and fairing the keel is considered to be an absolutely essential project, and we’ve seen two basic approaches used:

- Fair the keel to the G & S keel design dimensions using accurate section templates. This approach can entail shaving up to 300# of lead from the keel.

- Fair the surface of the keel as closely as practicable to the intended shape, filling in where needed, while removing little or no lead.

A few of the fastest boats in the Lake Michigan fleet have been faired using the no-shortcuts first approach, e.g. Paradox, Scooter. Two other very fast boats have utilized the 2nd approach, e.g. Spectra, Kato. G & S designer Scott Graham states he’s uncomfortable removing too much lead from the keel and favors the 2nd approach. He feels if you’re going to have to put in a terrific amount of effort to trim & fair your keel, you’d be better off starting over with a new keel made right.

Whichever approach is used, a good fairing job requires a good technique for finding the high and low spots on the surface of the keel, and also a long-board sanding tool.

Checking fairness in the horizontal plane requires a smoothly flexible batten. Checking fairness vertically requires a stiff straight-edge 4 feet long or so, and guide-lines marked on the keel as follows.

1. At three different heights measuring up from the bottom of the keel, say, 8”, 24” and 40”, draw horizontal lines. Measure the length of the keel chord, front to back, along each line.

2. Calculate what the distance is at 20%, 40%, 60% and 80% along each chord line and mark these positions on the keel at all 3 heights. (HINT: These % distances are conveniently listed in table form in a separate post about Keel Shape Dimensions)

3. Next, draw a straight line down through the 20%, 40%, 60%, and 80% marks. These 4 lines will not be parallel to each other. The 20% line will be almost parallel to the keel’s leading edge, and the 80% line will be almost vertical and parallel to the keel’s trailing edge.

The design of the keel is strictly linearly proportional. That means that when a straight-edge is laid on the keel along these 4 lines, the keel surface should be flat with no bumps, hollows or curves.

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