Present invention relates to a hull (1) having a fixed keel (3), coupled under said hull, characterized in that said keel (3) comprises a plurality of elements (6',...,6n) telescopically coupled each other, so as to permit varying its length, and a ballast (4) at the end of said keel; and in that it provides lengthening-shortening means (7) for adjustment of length of said keel (3).