Why does our canoe need extra flotation via bulkheads?

Our canoe’s concrete mix is designed to be less dense than water, in order to ensure that it will float (although other factors also influence whether our canoe floats). This year’s mix is less dense than water, but only just. So as an added precaution, we add foam and air pockets (encased in concrete) to the canoe in the bulkheads (at the bow and stern). This way we have some added flotation factor just in case something goes wrong with our mix design!