Keelboats are any sailboat with a fixed counterweight fin underneath, which provides stability and lateral resistance… These tend to be bigger boats with more stability and more sail area! Our large keelboats, the 23 foot Sonar and 18 foot Ideal are our fastest and most powerful boats on the river. Sonars also allow larger groups to sail, and both boats provide stable teaching platforms for learning spinnakers, or other advanced techniques and concepts.