The mules maintain pressure on the cables, keeping the ship centred in the lock. Most fascinating is how they attach the cables. Two men in a tiny rowboat approach the ship to catch the line which is then attached to the steel cable on the mule. Rowboats are used for safety reasons. Timing is critical, as the rowers must approach the often monstrous ships, connect the lines, then move away quickly.
Motorboats can break down, putting the men in danger.
Rowboats, on the other hand, don't break down. Our ship was only 174 feet long, so we entered the lock along with a mid-size freighter and two tugboats. As soon as we were in place, the 700-ton gates swung closed and water flowed in, raising us almost 30 feet in less than 10 minutes. The water comes from man-made Gatun Lake, formed by damming the Chagres River. Because the lake is 85 feet above sea level, we went through three locks to raise us from the Pacific Ocean to the lake, then three more locks to bring us back to sea level at the Atlantic Ocean.