DIY Navigation System of the Gods!

July 12, 2011

Convivia is one heck of a sailing boat. I love her dearly, but she is sadly lacking in two areas (no binnacle, no nav station) that made my navigation/chart plotter system decision somewhat harder than the typical, 1. Buy Raymarine, 2. Install Raymarine, 3. Enjoy Raymarine,  process. Add to that the fact that I am a dyed in the wool hacker with a natural distrust of the closed and proprietary, and you have a recipe for innovation.

I started down the software chart plotter path thinking that I would adopt one of the open source solutions. While there are many good choices, none worked particularly well on my Mac (I know, proprietary/closed, <grimace/>). I finally settled on MacEnc for my laptop and had pretty good success with it and a USB puck GPS. The US charts (raster and ENC from NOAA) were free and great, but the international charts were somewhere in the neighborhood of $300 per region. I mentally prepared to buy them all.

Then I discovered iNavX (by the same people) for the iPhone. Shortly thereafter the iPad came out and iNavX was released for the iPad. iNavX on the iPad turned out to be the perfect chart plotter. It’s merits include:

  1. Portable (really, easily, more on that later)
  2. Intuitive interface. The top end hardware chart plotters are just now getting touch screen interfaces, and none of them (to my knowledge) are as easy to use or intuitive as iNavX on the iPad.
  3. Inexpensive charts. The average cost for a Navionics Gold region is ~$30 – $90. For that I get all of the US & Bahamas; all of Mexico; all of the South Pacific; or New Zealand. Compare that to any other system and lift your jaw off the floor.
  4. Redundancy. We have two iPads. iNavX runs on both (with one license). The charts can be downloaded on to up to 2 devices (with one license).
  5. Easy to stick in the oven during electrical storms.
  6. Multi-use.Convivia is small for a43’r. Everything has to serve multiple uses or space isn’t well used. OuriPadsare used daily for:
    1. Facebooking
    2. e-mail
    3. e-books
    4. reference materials (all of my manuals in pdf, HAM/SSB information, etc)
    5. kid’s movies (most importantly)
    6. clinometer (I kid you not, it works well)
The biggest downside to the iPad is that it is not weather proof and it can overheat in direct and sustained heat. I will follow-up from the tropics to let you know if that turns out to be a problem, but I think it will not be.

So with the hardest decision out of the way I set to work on addressing the rest of the system. I decided that I wanted to have my windspeed/direction, SOW, and Depth available in iNavX too. The best way that I found to handle this was by installing the Brookhouse iMux ST. The iMux comes in two main flavors, a SeaTalk and non-SeaTalk. I have older instruments and decided the ST was the best option. I also got the high baud rate AIS option, in case I decided to add that later. I also wanted a better (faster, slightly more accurate) GPS, so I ordered a compatible PS2 puck GPS.


Installation  of the iPad was dead simple. I put some velcro on the back of the iPad (2 strips of 1″x 6″ works well)  and a strip anywhere on the boat that I wanted to mount it (bulkheads over the v-berth, inside the cabin trunk, in the cockpit under the dodger (on the outside of the bulkhead). This allows me to easily move the iPad out of the weather when it’s foul, and into the cockpit when it’s fair. (Incidentally, I have the same software on the iPhone and use that in foul weather, tucking it into my pocket when I’m not using it.)

For the iMux integration, I cut one of the sea-talk terminals off, exposing the wires. I connected the exposed wires according to the colors listed on the iMux (Silver, Ret, yellow, from top to bottom) and plugged the GPS into the PS2 port. When I turned the instruments on, the iMux sprang to life. Then I went into my iPad, set the network to Brookhouse_iMux (changing this voids the warranty :S) and configured the WiFi network like so:


(click to enlarge)

Then I opened up iNavX and went into the Instruments screen and tapped on Setup, then TCP/IP. I set the values as follows:


When everything was right I saw that green text (in the black window) scrolling by at a rapid pace. That’s all the NMEA datagrams, and it means your iPad is getting instrument data.


AIS (Automatic Identification System) is an elective (for pleasure craft) ship tracking system employed by over 40,000 ships worldwide. What makes it particularly useful is that (in the US at least) it is required for vessels over 300 Gross Tons. This means that in pea-soup fog a vessel like ours, equipped with AIS, can “see” a tanker coming from 30+ nautical miles away. AIS displays that vessel’s COG, Bearing, Range, Name, Location and  more. The name in this case can be critical when hailing large vessels for more information. We waffled on whether or not to invest in AIS but ultimately decided that, at ~$300 the Standard Horizon Matrix AIS+ GX2150, was a bargain. This sweet little device combines VHF, AIS, and DSC (Which can also be used for positioning) in one integrated package.

Installing the GX2150

Installing the GX2150 was as straightforward as it comes. I already had the antenna wired and terminated from the previous VHF, so all I had to do was cut and butt splice the power cord (you’ll need a crimper, wire cutters and the appropriate butt joint or step-down joints). The harder part was installing the AIS, DSC-In, and DSC-Out leads to the iMux. Here, a picture is worth 1,000 words:



(click to enlarge)


The Top 3 terminals are the aforementioned SeaTalk terminals. The from the top it’s Grey, Green-jumper, Brown, Green-jumper, empty, empty, Green+Green-jumper, Blue. I included this picture because this wiring was by far the hardest part of the whole project. The GX2150 only provides a single ground, so you have to create your own ground bus. You could do this a number of different ways, but this was easy and seemed elegant.


Once it was finally wired up I had to program the VHF with the MMSI number. You have obtain your MMSI from the FCC by going to their website, and registering for a FRN (your FCC “account”), a SA (Ships Recreational License) and (optionally, if you carry an SSB) an RR (Restricted Radiotelephone). In the application for the SA you will be asked if your ship carries AIS (yes), DSC (yes), EPIRB, etc. Selecting the DSC option, promts the FCC to generate a MMSI number. I needed to program that into my radio for any of the DCS/AIS functionality to work. Once I had the MMSI number configured, I hit the AIS button on the front of the radio and was pleased to see my Lat/Long and a bunch of other helpful information on screen. Then I switched over to iNavX, started the TCP/IP service, and selected Show AIS = All from the Chart -> Setup screen. After hitting Save I marveled at all of the boats that popped up on my charts.



(click to enlarge)

Now I have a full featured chart plotter that is fully integrated into my ship’s instruments, AIS, and DSC systems. My charts cost a fraction of the going rate, and I can take it anywhere, and use it for miriad applications. Can you play Angry Birds HD on your chart plotter?

I would be really interested to hear what experience others have had with off the shelf, custom, or home brewed navigation systems.  Have you tried what I’ve done? How has it worked for you?


I realized this morning that I left out one of the most important details of this whole exercise, the cost. Here is a rough estimate of the system cost. Remember that two of the most expensive components have MULtiple uses.

Standard Horizon GX2150 $300
iPad $500
iNavX $50
US Navionics Gold Charts $50
Brookhouse iMux ST $350
GPS $35
Total: $1,285


  1. Comment by Rob

    Rob July 13, 2011 at 6:44 am

    “I am a dyed in the wool hacker with a natural distrust of the closed and proprietary” – I love reading your stuff – always makes me 🙂

  2. Comment by Cidnie Carroll via Facebook

    Cidnie Carroll via Facebook July 13, 2011 at 11:34 am

    No t pretentious, funny! I’m reading along and taking notes…

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  5. Comment by Lauri

    Lauri March 20, 2013 at 2:35 pm

    Hi, started reading your recent ipad article that referenced this imux article. Just noticed there is nothing for the “as follows” in this statement: “Then I opened up iNavX and went into the Instruments screen and tapped on Setup, then TCP/IP. I set the values as follows:”

  6. Comment by Erik

    Erik April 29, 2013 at 3:28 pm


    This sounds great. Behan on s/v Totem referred me to your site. We’re looking to purchase our first boat this fall and start cruising as a family. As a happy iPad owner, I’m looking forward to trying out your system. Thanks for all the details!


    • Comment by Tucker Bradford

      Tucker Bradford April 29, 2013 at 3:30 pm

      Erik, so glad you found it helpful. Don’t forget to let me know if I left something out. I’m happy to answer questions and can use them to help other people in the future!

      • Comment by Erik

        Erik December 21, 2013 at 11:00 pm


        An update: many months later, we are now getting ready to close on our boat. And as luck would have it, the chart plotter is shot and needs to be replaced. I’m completely new to this (except for reading your post and trying to get my head around the lingo), but I want to try and set up the same system you describe. I just bought iNavX for my iPad and am looking at Navionics. I’m sure this is my naiveté shining through, but it looks like I also need to buy the Navionics App as well (not a chart plotter version), corret? And then iNavX will pull in the charts I buy from Navionics? Just wanting to check so I don’t waste $55. And I’m happy to chat off-line if you prefer: erik [at] erikorton [dot] com. Thanks, Tucker.


  7. Comment by flip

    flip June 26, 2013 at 5:59 pm

    Great articles! We are refitting a 77 westsail 43 and we stuck at buy Raymarine, until we read your article and found your blog. I have a question, what about radar? Does that interface with your iNavX system? What are you doing for radar?
    Big thanks

    • Comment by Tucker Bradford

      Tucker Bradford July 7, 2013 at 1:21 am

      Flip, I don’t believe iNavX supports radar. We don’t use radar on Convivia. iNavX does do a fantastic job of plotting AIS targets, and I’m seeing more and more ships using AIS. Technically all comercial ships are required to use it. That said, we did see several commercial ships in Mexico and French Poly (Tahiti in particular) that didn’t. If I were to draw a number out of thin air I would say 9/10 of container ships (and tankers) used AIS and 0% of fishing boats did.

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