A tablet is great to use on board, due to the larger screen size compared to a phone, but most tablets do not include a GPS so you have to connect to the boat's GPS - and of course you also need to connect to the boat's instruments if you want AIS, wind or other data. Connections can be by USB cable, Bluetooth or Wi-Fi, and here we look at them to see what each one has to offer. There are also differences between iPad and Android tablets, which we go on to discuss.
GPS, AIS and other marine electronics devices generally use either NMEA0183 or NMEA2000 to communicate with other equipment. Whilst NMEA2000 is the newer, better protocol for communication between instruments, it is not yet supported by apps (or PC software). Even if your instruments primarily use NMEA2000, there is normally a NMEA0183 port that you can use to send data to the apps on your tablet. Failing this, you will need an NMEA2000 to NMERA0183 converter, with the NMEA2000 side connected to your instruments, and the NMEA0183 side sending data to your tablet using one of the means discussed below.
There are three ways of connecting your tablet to a GPS or other instruments: Wi-Fi, Bluetooth or USB - each has their own set of advantages and drawbacks. Also,your options differ depending on whether you are using an iPad or an Android tablet, as discussed lower down on this page.
USB, like all wired connections, is inherently more reliable than any wireless connection, but the price to pay is that you lose the ability to take the tablet to anywhere on the boat. There is a maximum cable length of 5m (16ft), beyond which you need to use powered USB extenders. Also, the cable can only connect to one tablet or other device at a time.
If you just want GPS data, then you can just use a low cost USB GPS receiver, with the USB cable providing the necessary change in connector size. To use your boat's own instruments, you will need a NMEA0183 to USB converter (or an opto-isolated RS422 to USB converter), which will connect to the NMEA0183 output of your instruments and take the USB cable.
Bluetooth has the advantage of being wireless, so you are free to move around the boat with your tablet. However there are a number of different classes of Bluetooth device, and you need to avoid Class 2 and Class 3 devices as these have a maximum range of 10m (33ft) or less, before taking into account any bodies or bulkheads in the way; any interference from other Bluetooth or Wi-Fi or other devices in the same band; and any loss of signal due to a small receiver antenna.
As with USB, you can use a simple low cost Bluetooth GPS receiver if you just want GPS data, or you can get a NMEA0183 to Bluetooth converter, though there aren't many on the market. Those that are available all seem to be restricted to connecting to a single tablet or other device, so the data can't be shared across the crew's phones and tablets.
There are also restrictions on many iPads, as described below.
Wi-Fi is a more powerful wireless connection than Bluetooth, both in terms of range and also capabilities. It is also fully supported on both iPad and Android devices.
Wi-Fi offers two protocols: TCP and UDP. You need to be aware that some apps support just one or the other, and some hardware devices support one or the other, or using one switches the other off (though our Wi-Fi Gateway supports both simultaneously). UDP is the simpler of the two, with data simply being broadcast, so it can be received by an unlimited number of devices. This means that the app can only receive data, and not send anything back. Also, UDP doesn't guaranteed delivery of the data, and some data will be lost, though as NMEA typically updates the data every second this is seldom a problem. The other protocol, TCP, is bi-directional, and data is guaranteed to arrive. But many devices restrict the number of connected device, often to just one (we support up to 4). There is one niggle with TCP, though. Some apps do not close the TCP connection properly when the user switches to another app, or walks down the dock till out of range, so unless the gateway monitors connections and closes them down when no longer used it can be that all connections are used by "ghost" apps, and none are available until the gateway is restarted.
For vessels with more sophisticated comms and electronics installations, some Wi-Fi gateways (including ours) can be connected in to the boat's existing Wi-Fi network, so users don't have to switch networks between instrument data and other applications.
Because of its flexibility and lack of restrictions, for most people it is the preferred method of connection.
Unfortunately the iPad is quite restrictive in its options. None have a USB port. Also, Apple have disabled support for GPS (and other NMEA data) over Bluetooth on all iPads except those with 3G or 4G mobile connections, except if used with a few GPS receivers that Apple have licenced. So to use Bluetooth you will need to buy a new, relatively expensive GPS. This leaves Wi-Fi as the only realistic way to go.
Being a more open market, Android gives you a wider range of options. Almost all tablets have a USB port, and Bluetooth doesn't block devices like on the iPad. Wi-Fi, of course, is also available.