SAULT STE. MARIE—In hard hats and safety vests, the two women shimmy up aluminum boarding ladders onto a converted World War II tanker. The rumble of the ship’s 8,040-horsepower engine makes conversation near-impossible on this blue sky morning. But public health workers Charity Zimmerman, a nurse, and Jill Schaefer, a clerk, know the drill on these steel giants that travel through the Soo Locks, the iconic commercial squeeze point between lakes Huron and Superior.
