For making a 39 mile trip in a 25 foot sailboat, I would probably plan to average around 4 to 4.5 miles per hour. This should give you decent guestimate of the overall time for planning (looks like between 8 to 10 hours). Not sure how big your mother is, but in my experience sailing that area I tend to motor a lot more than I plan (unfortunately, I end up motoring 60% when traveling to a specific destination).