I am US citizen, flying into Shanghai direct from US, departing 40 hours later by cruise ship.

Ellen, to be eligible for the 72 hour free transit policy, you have to fly into and out from China. So you cannot make use of this policy. However, you can check whether you are qualified for the 24-hour direct transit policy. The 24-hour period starts from 00:00 after your arrival, so you can actually stay longer than 24 hours. If this policy is not available to you either, you have to apply for a transit permit in advance.