Why my new hot water heater performs worse than my old one?
You or the plumber must have missed something because if you had 38 gallons of hot water at the start of the shower, you would not run out of hot water during the first shower. I would re-check that the dip tube is actually hooked up to the cold water inlet and that the thermostat actually switches the power to the bottom heat coil when the top element is satisfied. If only the top element is working then only the top part of the tank will get hot giving you a short supply of hot water. Either of these 2 items could yield the problem you are having. I would check the tank thermostat first since this is the easiest to service and is more likely the problem.