Ludington has two exceptional marinas, one public, one private. I have crossed the lake several times, four times in my CD22. With the 22 it has taken between three and six hours to cross depending on wind and waves. Sturgeon Bay to Frankfort would be your very shortest crossing. A great cruise would be to combine cruising Door County/Green Bay, a crossing from Rock Island to Traverse Bay and its several marinas and destinations. That crossing would be 70-75 miles of open water instead of 55 at Ludington and under 50 at Frankfort. Some days you absolutely can't go, so you should save an ample time window. I wrote some about a trip I did last June elsewhere on this site.
Regards,
Mark