Keels on "keelboats" perform two functions, preventing the leeway and keeping the boat upright. To perform the latter role they need to be heavy and they only start to function as the boat heels over. The more the hull heels over the greater the righting moment that a keel provides. But the weight slows the boat down dramatically by pulling the hull down in the water and displacing more water. This extra mass of water has to be pushed aside as the boat progresses forwards thus slowing the boat down. To reduce the weight carried in keels they have been made deeper and deeper in recent years but then boats with deep keels cannot be sailed into shallow water, as trimarans and dinghies can be, and they are more vulnerable to damage. Many recent monohull capsizes have been due to the failure of the connection of the keel to the hull. If the righting moment is provided by ballast within the boat than the keel can be a more modest affair and sometimes runs the length of the boat but it will not prevent leeway so well. Consequently a centreboard is sometimes added in such situations.
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