Appointment time slot availability

I run a charter boat there is only one boat. If I set up a 2 1/2 cruise and a 4 hour cruise and I show my start time as 8am the available time slots are based on the length of the cruise/service.

I was looking for it to show 8am, 9am, 10am, 11am etc till 7pm no matter what service they select. Then if they wanted to take a 4hr trip and start at 11am they could. Right now if I select 4hr service my times I can pick from are 9am, 1pm,5pm and 9pm