The larger screen is much easier to read than the screen of a handheld GPS. The box makes the unit self contained and portable.Photographs by the author

The larger screen is much easier to read than the screen of a handheld GPS. The box makes the unit self contained and portable. As with other marine GPS units, you can load it with waypoints, employ the man-overboard functions, and save tracks and routes. This unit also has inputs for engine diagnostics and wind instruments.

I’ve had a number of handheld marine GPS units for years, starting with a Garmin 12 in about 2000, where all you got was course, speed, and latitude/longitude. I currently have a Garmin GPSMAP 78sc. While these small units are great for kayaks and other small boats, I’ve looked enviously at the big screens in chart plotters on larger boats with 12-volt electrical systems but they’re not designed to be used on open boats.

I’ve been watching with interest the adoption of waterproof electronics by kayak anglers. The GPS units they usually use have 4” to 5” screens, are waterproof, and have waterproof cabling to connect to a waterproof box that contains a small 12-volt motorcycle battery. But these units are mounted to kayaks and aren’t meant to be removed from the boats for use on land or on other boats.

I considered using a tablet computer with a waterproof case and backup batteries, but I don’t need one for other uses besides navigation, and haven’t had great luck operating touch screens with a wet or gloved finger.

Lithium batteries now provide large capacities in small packages. Some entrepreneurs, fueled in part by a market created by stand-up paddleboards set up for fishing, are creating small waterproofed lithium batteries. I thought the new batteries would make it possible for me to create a portable GPS unit that I could use on my small sail-and-oar boats or on an outboard skiff.

I needed three pieces: a compact GPS unit with a 4″ to 5″ screen, a 12-volt lithium battery, and a watertight box to put them in. A NOQUA Pro Power 10ah battery is small enough to fit inside a 1050 Pelican box, which was big enough to accommodate the Garmin echoMAP 54cv GPS that I bought on sale last winter. There are other similarly priced GPS units aimed at the fishing market by SI-TEX, Lowrance, and Hummingbird. I’d be able to mount the GPS in the lid of the box and have enough room for the battery inside. Your battery and GPS selection will dictate your box size.

I trimmed some reinforcing ridges from the box with an oscillating tool and small saw blade; then, using the template that came with the GPS, I cut the hole for the unit and drilled holes for the four mounting bolts. I installed the GPS to read from the hinged side so I could angle it up in use. This only works with a horizontal display.

My unit had a wiring harness for both the battery and the included transducer for the fishfinder, and the NOQUA battery had an output cable with waterproof crimp-on marine butt connectors. Wired up, everything worked beautifully.

I’ve used it now for several months. A full charge on the battery lasts 20 hours. The echoMAP 54cv came with a transducer for the CHIRP sonar fishfinder, but I haven’t hooked up it up as I have to make a mount and I suspect that it will drain the battery faster. The box also has space to tuck the charger and the small strut.

A dowel can be cut to length to hold the lid and the display at the best angle for viewing. The components inside the box are all waterproof, so there's no ham in having them exposed to the elements.

The Pelican case has some small holes meant for padlocks but will fit a 1/4” dowel that holds the lid at angle for easy viewing. The components inside the box are all waterproof, so there’s no harm in having them exposed to the elements. A couple of 2’ hanks of parachute cord go into the case for lashing it into place.

I usually operate the unit with the box open, the lid propped up with a dowel to a good angle for viewing, and rely on the waterproofness of the GPS, battery, and waterproof connectors, so I was not concerned with keeping the Pelican box’s own waterproof integrity. The GPS’s mounting gasket allowed some leakage, and Aquaseal solved the problem; I didn’t seal the GPS to the case so I could remove it for servicing if needed. Closed, the box floats.

I’m happy with the result. I don’t have to grab the handheld GPS to see it. I can easily read the 5″ screen from a few feet away. The lithium battery lasts for several days’ use, a lot longer than the alkaline AAs on my handheld. The battery unit can also charge my phone using a USB cable and, if I were away from power for a long time, I could set it up for solar recharging. This stand-alone unit is quite convenient, and I use my new GPS box on many different boats.

Ben Fuller, curator of the Penobscot Marine Museum in Searsport, Maine, has been messing about in small boats for a very long time. He is owned by a dozen or more boats ranging from an International canoe to a faering.

You can share your tips and tricks of the trade with other Small Boats Monthly readers by sending us an email.