One of the nice things about e-books, aside from the fact that they weigh nothing and don’t mildew, is that they can cover specific areas of interest efficiently and inexpensively, without a lot of stuffing.

After an exchange of Twitter greetings with C.J. Medway, author of the e-book called iPad Route Planning Using Navionics and Marine Tides Planner, we downloaded a copy from the iTunes app store for a look-see.

The e-book offers a step-by-step guide to the Navionics and Marine Tide Planner apps, and a mental exercise in the use of current.

In a nutshell, this is a methodical, old-school, step-by-step piloting lesson of the kind we used to go through before the days of LORAN (much less GPS), when an exercise might include, for example, a multi-leg coastwise voyage: You plotted magnetic courses from buoy to buoy with parallel rules. You measured distances with dividers. You worked out and adjusted for the effects of current by drawing vector triangles with the aid of tide charts. You verified your position with compass bearings on fixed objects, and with the math of dead-reckoning.

Now, obviously, all that has changed. GPS gives a constantly updated position, and the electronics in GPS receivers, coupled with other electronics, give us so many trigonometric and arithmetic solutions that we can find and line up a huge array of solutions to our navigation problems.

One thing that’s missing, though, or hard to come by, is how to work with tidal current along a route, both to maximize its positive effects and minimize its negative ones. This is what Medway, a sailor and retired IT professional in the UK, focuses on in his e-book – specifically how to use the Navionics iPad app in coordination with the Marine Tides Planner app. He walks the reader through a short voyage along the south coast of England from Portland to Christchurch, explaining how to measure distances, lay out courses, set and name waypoints, establish routes, determine when to begin a voyage to make the best use of current, and how to analyze the effect of current at different points along the route. Tidal current changes in both direction and speed over time, and this is what makes the exercise interesting. (In the US we say set and drift; in the UK the terms are set and rate.)

Medway uses a four-knot average speed through the water (convenient because the boat would cover one nautical mile every 15 minutes). Readers in faster boats could plot using other speeds – six knots is a mile every 10 minutes; 10 knots is a mile every six minutes, and so on.

For most people, especially powerboaters who simply use speed and fuel to fight the problems of current, this e-book will not be a practical on-the-water guide (although with today’s fuel prices only a One-Percenter would set off against a foul tide if a fair one would serve). It’s really more of a voyage-planning tool that uses the modern technology to help you understand and conquer the age-old problem of being in the right spot at the right time to make the best use of current along your route.

It also serves as a button-by-button guide to both the Navionics and the Tucabo Marine Tides Planner software.

Medway has two other titles in the app store: called iPad Route Planning Using Marine Imray Charts and Marine Tides Planner and Marine Imray Charts Commands. All of these titles run only on Apple iPads running iBooks 2 or later, and iOS 5 or later.