My Mill Creek 13 tracks fantastic.  I only imagine the 16.5 is even better.  Can't imagine needing any assistance from a keel.

I have built both the 13 and 16.5 and they don't need a keel , it would only cause the boat to bottom earlier in shallow waters.

I have the 16.5 and it does get pushed around alot in the wind. It tracks well in calm water, but it was essential to add a rudder for windy conditions. I think you would be better off adding a rudder than a keel if you find it to be difficult to control. My advice... finish it first & add on later if you need to. A keel should be easy to retrofit.

If you are considering adding a keel to improve directional stability, please read my comments under "Which Rudder for Sailing a Triple" here on teh forum.  The comments about lift to drag and aspect ratio all apply to this.  A keel will certainly add drag and yet will give you precious little improvement in directional stability.  If this is your issue, install a high aspect ratio skeg or rudder.

Paul G

If your primary concern is weathercocking in a beam sea or with a wind on your beam, you might want to consider adding a retractable skeg.  CLC offers both a kit and plans, and they're pretty easy to install, even after the boat is finished if you have a rear hatch.  I have them in my Ches. 14 and LT-17, although I'm not planning to install one in the Mill Creek 15 I'm building.  The nice thing about a retractable skeg is that it adds negligible drag when retracted, and not that much even when deployed.  Also, when retracted it doesn't add anything to your draft.

  -- Jim C