They have two docks in St Ignace (where we stayed) with one at each end of the business district. While we were there, they had ferries leaving about every 35 minutes from 7:30A to 8P or so depending on the day. They loaded very quickly wasted little time anywhere. They claim about 17 minutes to cross. The fun part is the rooster-tail spray of water from the hydro-jets which I am sure is pure marketing versus functional necessity.
This has little to do with the ferry itself but I recommend you go over early to avoid some of the crowds that arise on the island later in the day by which I mean noon or so. The Arnold line was using slower but much cheaper crossings at 2-hour intervals when we were there. We decided we would rather pay more for the schedule flexibility especially to be able to return quickly once our batteries ran down on the island... and were glad we did.

We just went to the Island and were recommended to use Starline...so glad we did. They grabbed our luggage out of our car and we didn't even see it again until we got to the island where the hotel staff was there to get it. The Starline staff even drove my husband and son back to the ferry on their golf cart so they didn't have to walk! Super great company! Will use them again for sure!

We went on a particularly cold day, complete with the bug attack mentioned in other reviews. We sat outside on the way there to view the Mackinac bridge, but nearly froze and had dead bugs swirling around our ankles with the draft. That said, the crew did an excellent job keeping the ride smooth on return. We watched another ferry douse their outside passengers with a particularly rough trip out of port. However, our captain eased passengers over the high waves and gently exited port. We all squished inside to ensure a warm return and were very glad we did.

We bought tickets and they did not inform us that 2 of their 3 ferries were broken. So we had to wait 1.5 hours until we could catch the next ferry. That was disappointing and I would have appreciated a heads up from the company. On the other hand, they DO allow dogs on the boat. The ride was quick and smooth.