[Andy]

Create a new line over each section of the bitmap and position these along the hull until you have a 'vertex skeleton'.

In vertex mode, you can select two of the lines and use the convex function. That will form a shape around the vertices. Switch to Surface select mode (press 's' and 'v' to switch modes, it's a bit quicker) then select and delete any extra surfaces - mostly from the front, back and top. The keel is concave so you'll need to do these sections in more than one go.

Actually it might be better to select the vertices in horizontal slabs. You may be able to make the majority of the shape in one go - apart from the concave section.