A sailboat is tender when it is unbalanced, heeling excessively in breezy conditions and requiring adjustment by crew to sheets and trim. In keel boats, insufficent keel weight, lead or otherwise, will cause the boat to heel. Conversely, too much sail area in proportion to ballast, or an unusually high center of balance, will create the same effect.
The reverse applies when a boat is stiff.
True Blue . . .
sold the Nauticat