Purpose: This gadget is designed to function as a specialized GPS for boating, as well as a fish finder.  The sonar function is what distinguishes it from the GPSMAP 431.  It is intended for use with a small craft on inland lakes and rivers.  For boating and fishing in coastal waters, a similar item, the Garmin GPSMAP 441s, is recommended because it comes with marine charts, and an NMEA 2000 connector to allow easy interface between the 441s and other devices commonly found on salt water vessels.

Garmin Boat Navigation

The GPSMAP 431s should be mounted on a boat using the mounting bracket, and isn’t much used for other purposes.  It won’t read street maps, for instance, so you can’t take it off the boat, stick it in your car, and expect it tell you how to get home.  Due to its specialized nature, however, it will provide boating-specific information that an all-purpose hand held GPS would not give. The GPSMAP 431s can also connect with some other electronic items commonly found on freshwater boats, such as an autopilot device, using the older serial NMEA 0183 technology.

Navigation within the U.S.:  A map of U.S. inland lakes and rivers comes with the GPSMAP 431S, but some people prefer the BlueChart g2 Vision card that can be bought to use with it; they find that the unit displays their current location on the map more accurately when they use this add-on feature.  The extra BlueChart g2 Vision card also contains information about the topography of lakes and rivers, and enables auto-guidance.   This allows the unit to plot realistic routes and avoid obstacles, while taking the size of your boat into account, similar to what a GPS unit for your car would do when plotting a driving route that avoids sending you the wrong way down a one way street.  Boaters report using auto-guidance successfully, without any hassle.

Navigation outside the U.S.: If you want any maps of territory outside the United States, you will need to buy these separately.  If you travel to an area you don’t have a map for, you’ll see satellite images on the display instead.  Garmin offers additional BlueCharts for every continent but Antarctica, though not for every country in the world.  Coverage is pretty good in North America, Australia, Africa, and Europe, but not so good in South America and Asia. BlueCharts cost $100 and up, depending in part on the size of the area covered.  Here’s a link to the Garmin website, where you can see for yourself exactly which maps are available (click the “compatible maps” tab):  https://buy.garmin.com/shop/shop.do?cID=150&pID=28752

Recording routes and waypoints:  This device can import previously recorded waypoints.  It is also capable of automatically recording waypoints along with relevant data, such as water temperature and depth, and comments.  It will store up to 100 routes and 3000 waypoints.  It’s also possible to automatically record a track of your vessel’s actual location for future reference. 

Data transfer: The GPSMAP 431S uses SD cards for data transfer; the power/data cable that comes with it is for connection to the power source and NEMA 0831 devices, not to a computer.  New maps can be purchased already on SD cards, or you can download them and put them on an SD card yourself.  This unit can read waypoint, route, and track information off an SD card, as well as write this information to an SD card.  One user noted that if your computer does not have an SD card reader, this can be a nuisance because you will not be able to transfer waypoint and route data between your computer and your GPS, which may necessitate re-entering the data. 

Sonar: “Dual beam” refers to the type of sonar used. Using two beams allows you to have two different cone widths, which allows you to get detailed information about what is directly under your boat using the smaller cone, while the wider cone gives less detail over a larger area.  Not all fish finders have this feature, though it is not unique to this device.   A transducer is included in the box, but Garmin offers other transducers that can be used with this unit if the basic one that they provide doesn’t suit your purposes.

Ease of use: People generally agree that the device is easy to use.  It comes with an 88 page manual that has a good index, but the information you’re likely to need while out in your book has been condensed into a 2-page quick reference guide. 

Display:  The screen brightness is adjustable.  It can be made bright enough that it is easily visible in daylight, or it can be dimmed for lower light conditions.  Users report that this feature works as intended; they like the screen.  The keypad is backlit to make it easy to see in low light.

Batteries: Be sure that you have a power source for this gadget.  Although one of the listed features is a low battery alarm, this is simply a feature to let you know when the unit isn’t getting enough power.  It doesn’t use an internal battery and it doesn’t come with a power source – just a power cable.  The GPS/fish finder needs to get 10-32 Vdc from somewhere; typically a 12 volt battery on the boat will be used to power this and other electrical devices.

Technical support:  Users have an easy time getting a hold of Garmin technical support to handle questions and concerns about this product.  They responded to my E-mail query within two days, answered the telephone within five minutes.  Representatives provided clear, helpful answers to my questions.  The effectiveness of the technical support has received mixed reviews from others.

Overall impressions: In writing this, I looked at every other review of this product that I could find online.  Although some people offered comments about features that could be improved, overall user evaluations of the Garmin GPSMAP 431s ranged from good to excellent.  I was not able to find a negative review anywhere so I created my own in the hope you think the Garmin Dual Beam Combo is as great as I think it is. To buy it at a great price click here Garmin Dual Beam Combo.