First, the Widgeon is more of a planning hull than displacement hull. But, you are correct that on any boat with a ballast keel removing the mast WILL make her roll quicker due to a change in the Metacentric height, the mast tends to dampen out the roll by it's moment of inertia, balancing out the keel. On a lightweight centerboard (unballasted) sailboat there is not enough weight to the CB to cause that drastic change in the metacentric height and with no fixed ballast deep in the boat, the mast is not needed to dampen the roll, and in fact the boat will be more stable without the mast since the center of gravity has been lowered, even though the mast may not weigh much (about 20# on a Widgeon) remember that as a boat rolls or heels, the center of gravity of the mast will tend to move out over the side potentially increasing the capsizing force. There is not enough weight in an unballasted CB to prevent this (it may slow down the capsize, but will not prevent it!), the stabilizing force in a small sailboat is provided by the crew's weight sitting near or on the side opposite to the side that the boat is heeling towards. An unballasted keel or centerboard in the down position will serve to potentially "dampen" the roll of the boat since it creates resistance to the rolling. This is why large ships and yachts have shallow "bilge-keels" out near the chines to help reduce uncomfortable rolling. When designing a skeg, or keel for a power vessel (or rowing/paddle boat) the depth and total area of the skeg or keel must be calculated to be sufficient to provide directional stability (and potential roll resistance) yet small enough to not add excessive drag. Lowering the CB too far when under power instead of sail creates drag and really does not make the boat any more stable and in fact could make her less stable when turning as the boat may "trip" on the lowered CB and be more likely to roll right over.
Small boats also depend on "form stability" more than larger boats, that is to say, the shape of the hull section is designed so that the center of buoyancy will move towards the Low-side" as the boat heels keeping the center of gravity towards the high-side tending to right the boat. That is why even a hull said to be "round-bottom" will NEVER have truly round, continuous shape, there will (or at least SHOULD) always be a flattening of the curve or even a slight "V" to the bottom to create that movement of the CoB that keeps the CoG always in a position to tight the boat (up to a certain point obviously). a true "Round" bottom boat, if the hull section is indeed a true 180 deg arc, will never reach a point where the CoB moves to keep the CoG in position to right the boat (unless ballast is fitted) A rounded hull like that will always be in equilibrium and have no resistance to roll. (Gee, I finally had a chance to use what I learned while studying my old WESTLAWN Institute of Yacht Design lessons! <GRIN!>)
