QUESTION: Why is the thickness of a keel important for performance even if its a NACA (predecessor of National Aeronautics and Space Administration) shaped foil?
ANSWER: It’s important to not only use a NACA section, but the proper one for best in-the-water performance. Having a proper thickness ratio is as important as the section shape. A proper ratio allow the keel to develop lift for efficient windward performance. 12% is an optimum number for moderate displacement cruising hulls. Ask for this figure when comparing keels.

NACA Foil Keel Sections

Width / Length = Thickness Ratio (%)

 Moderate Displacement Cruisers require optimum Thickness Ratio of 12%. Anything smaller means the keel will tend to stall when going to windward. Hence tacking angles are forced into the 100º - 110º range.

Optimum NACA 12% Thickness Keel Section: High Lift for Windward Sailing and Low Drag for Offwind Sailing. Tacking Angles of 85º - 90º
Narrow NACA Foil: Tends to stall out. Forcing yacht to fall off when sailing to windard. VMG drops substantially with Tacking Angles of 100º - 110º