If something is simple, you're more likely to do it, which could be a premise behind the log book app WaveTrax. After a quick set up where you make your account user name and password, you can then enter boat and crew info which can be selected for particular journeys. At the start of a new journey, you simply select the boat and crew (optional), pick a starting and ending point, input the weather conditions and then go. There are four buttons on the "Live Journey Tracking" screen where you can switch between Sailing, Motoring, and Motorsailing. And that's it. The app keeps track of your trip, and at the end, you can see at a glance how much time you spent sailing or how many engine hours you racked up. Trips are cataloged, and you can share them, and add notes and photos to them. Sync them to the website where you can also view and edit the trips and even make PDFs of them if you want to print them.
WaveTrax is available at the iTunes App Store for $12.99. Check it out!