Modern sailboats have either keels or centerboards that protrude considerably down into the water. These wide wing-shaped boards keep the boat from sliding across the top of the water as the wind would want to push them. When used with a rudder, the two boards together act as levers against the pressure of the wind.
In large boats the keel is rigid and part of the structure of the boat. In smaller boats the centerboard pivots so that it can flip up in shallow water. But in order to accomplish this, a hole must be cut in the bottom of the boat.
This is a nervous moment-- cutting a hole in the bottom of an otherwise sound boat. But we are adventurous sailors, and we do not shrink in the face of tasks which may cause moderate anxiety in persons of lesser fortitude. No. We soldier on and bear up to the moment. And then when all is done we ask, "What the hell have I done???"
And then we carry on and construct the centerboard, and centerboard case. With thickened epoxy and C-clamps borrowed from neighbors we install the centerboard, adding more thickened epoxy as we go, and when all is done we look at it with skepticism, wondering if there might be some small area that is not completely sealed; some small area that will spring a leak that is at first hardly noticeable; but that small leak will enlarge itself imperceptibly over time until that time when we are twenty miles out into the middle of the Great Salt Lake, far beyond the range of emergency radio communication. We watch the boat slowly filling with water. We dab at it, first with a sponge, then with a five gallon bucket, but the ingress of water has become a flood, faster than a five gallon bucket can tackle the task. And now we begin to imagine search planes circling overhead, but not spotting this little speck of a boat.
And so we add more thickened epoxy to every seam of the centerboard case until our self-doubts ease away. Wasn't that fun?