You can make it to Leverick from Trellis with no problem in late July. You will have a good 2 hours of daylight left and easy trip over. I would just make a reservation on a slip just to secure a spot. If you figure a mooring ball at Leverick is $30 and you run the generator all night you are looking at a cost of around $68. A slip with power will cost you $70 and save on the fuel.