Building a PC plotter

Those that know me know I’m not a fan of chartplotters. I’ve always thought that faffing around with a plotter and some clunky waypoint function is a slower and less flexible way to plan a passage than is a quick round with a notepad and chart, and that the time saved before departure is more purposefully spent in the nearest pub. Yes they are convenient underway, but an old school GPS with a waypoint on your chart’s compass rose is a pretty quick way to get a fix too. However, for the last year or so we’ve had a little old netbook available on the boat to just draw the boat’s position on a PC charting package, and I’ve been very impressed with the marine PC onboard Inseyandra, which makes some pretty good navigation tools available underway. Having taken up RORC racing I’ve also become a fan of GRIB files as part of the weather planning. So I have become a convert, if not to chartplotters, to the idea that electronic nav has some features that are worth having. So I’ve taken the plunge and installed a fixed navigation PC onboard Karisma.

As PCs are getting quite small and cheap now, lots of people are looking at building onboard computer-based nav systems, but there are no real guidelines to follow regarding the choice of system- computer, screen, and power supply. So, although I want this blog to be about sailing and not tech-y, I thought I’d document what I’ve done in case it’s of use to others.

The starting point

If you’re going to go all the way to an onboard PC system, then you want to get the best out of it. PC nav software can do a lot more than the average dedicated plotter but it will be at its best if you can get all the boat’s instrument data in. The little laptop plotter had only ever been connected to the boat’s GPS and a USB-powered AIS (Automatic Identification System) ‘black box’. For this installation I decided the new PC needed to get all the boat’s data, so that is:

  • GPS data on NMEA0183;
  • AIS data. for power reasons I wanted to change from a USB powered box to one with its own power supply, so this meant upgrading to an AIS box which also transmitted on NMEA0183;
  • Wind (direction and speed) and log (depth and water speed) data. This is provided by two Simrad IS20 instruments and therefore lives inside a properietary NMEA2000 ‘Simnet’ network. Which, of course, is not compatible with NMEA0183.

The other driving concern was the type of power supply that the system should run on. Obviously most PCs and their peripherals expect to see a 240v supply. You can create a 240v supply on board with an inverter- and a computer will need a relatively expensive bit of kit called a ‘pure sine wave’ inverter, going on for £200 on its own. In any case there can be power losses associated with stepping 12v DC up to 240v AC, and for simplicity, I decided to keep the power at 12v.

Getting the data

The first thing to do was to decide how to handle two NMEA0183 data sources as well as a N2k source and get them all into a single COM port on the computer. it was pretty obvious that the N2k data was going to have to be converted to 0183, and Simrad make a device intended to do this called an ‘AT10 universal converter‘. This is a very simple black box with two N2k simnet connections in/out, an inbuilt cable with 0183 tx and rx wires, and powered by the simnet bus. It was a simple plug and play fit in the bridgedeck/garage behind the cockpit instruments. I connected it to the boat’s electronics locker via a couple of unused twin-core cables helpfully run between the garage and locker by Hanse when the boat was built.

Now there were three NMEA0183 tx feeds terminating in the electronics locker (GPS, AIS and AT10) with one 0183 rx (AT10). The tx feeds needed to be multiplexed to the computer’s USB port and- ideally- also fed back in NMEA form to the AT10 so GPS data could be fed to the simnet network (this isn’t strictly essential, but I wanted the wind instrument to display true wind as well as apparent, so it needed to see the GPS course and speed over ground sentences. See also note (1) about AIS data).

The solution was to fit a ‘miniplex-lite‘ USB multiplexer by a company called Shipmodul. I’d never heard of this range of multiplexers before but it is a smashing bit of kit. The shipmodul has three NMEA0183 inputs, two at 4800 baud and one cycling at up to 38400 for high-speed (i.e. AIS) input. It has a single NMEA output which transmits the multiplexed data (2), as well as a USB connection to the computer. I connected up the GPS, AIS and AT10 tx to the inputs and the AT10 rx to the output, and fitted the unit in the electronics locker alongside the AIS box.

Digital Yacht AIS100 (left) and the Shipmodul Miniplex-lite (right) installed in the electronics locker. In the bottom of the locker is the Amperor 12v DC/DC regulator which provides a spike-free constant power supply to the PC and monitor.
Digital Yacht AIS100 (left) and the Shipmodul Miniplex-lite (right) installed in the electronics locker. In the bottom of the locker is the Amperor 12v DC/DC regulator which provides a spike-free constant power supply to the PC and monitor.

Now that the NMEA0183 and N2k data was multiplexed into a single USB port, complete with the 0183 data being fed back into the N2k network, it was time to choose the PC itself.

PC choices

I spent a lot of time researching the appropriate PC. The first thing I looked at, naturally enough, was the dedicated ‘marine PC’ market.

If anything falls foul of the old ‘put the word marine in front and multiply the price by ten’ trap, then it is the marine PC market. Prices here are astonishing- £700 upwards for entry-level black boxes. Their only marine-specific qualifications generally seem to be an IPX-something or other water proof case (waterproof electronics are not really necessary in the saloon- the PC needs be no more waterproof than my Roberts radio is); an internal 12v regulator to ensure smooth power supply, and a handful of actual NMEA-friendly RS232 serial ports (i.e., the sort of port you’d connect a mouse to- in 1989). This wasn’t even wanted thanks to the amazing 21st-century multiplexer, so I was able to discard this market pretty quickly. What I was looking for was a PC which:

  • Ran off an external 12v DC power supply, rather than containing an internal transformer;
  • Came pre-loaded with Windows 7 or above;
  • Had a VGA output, either alongside or instead of HDMI, to keep my monitor options open;
  • Would match the tech specs needed by our charting software, Meridian’s SeaTrak;
  • Was easily secured to the boat, which ideally meant VESA mountable;
  • Was cheap, or at least, less than a couple of hundred notes.

This is the market for the new world of ‘mini PCs’, small black boxes with about the footprint of a CD case, aimed at turning home TVs into smart devices or for mobile business presentations, that sort of thing. To keep them small they all have external power supplies. After a lot of googling I came across the absolutely marvellous sumvision cyclone mini-pc, which is tiny, runs off an external 12v DC, has 2gb of memory, 32 gb of solid state storage, a 1.8 ghz processor, comes pre-loaded with a completely legal copy of windows 8.1, has VGA and HDMI monitor ports, 3 usb ports, a 75mm VESA mount, and is less than £130 notes on Amazon. Sorted. A wireless mouse and keyboard combo, also from sumvision, completed the package.


Finding a monitor was the most difficult piece of the puzzle. The first thing I started looking for was a dedicated 12v PC monitor. This doesn’t really exist. What is out there in terms of dedicated 12v monitors can loosely be divided into CCTV or car reversing monitors (7 inch or less, too small, too poor a spec, not VESA mountable) and the caravan TV market. Now caravan TVs are popular in boats, can easily be VESA mounted, and a lot of people are beginning to use them as dual-purpose computer monitors. Most have VGA inputs. However, they have some drawbacks that I didn’t really want.

Firstly, they are quite expensive, starting at about £130 for the no-brand Chinese end, and up to several hundred pounds for the quality Avtex brand. Secondly, they are too big- none are really now sold under 19 inches in letterbox format, and the ideal size for Karisma’s chart table would be 15 to 17 inches, in the ‘old fashioned’ 4:3 format. Finally, almost all of them have integrated DVD players, something I absolutely didn’t want or need, and which makes them very bulky at the chart table. If I couldn’t find a better option a caravan TV would have served, but I kept looking.

I had a look at the current PC monitor market, hoping to find something with a 12v power supply. Of course most monitors have an internal transformer, so their power supply is 240v AC. Those that I could find with external power supplies used laptop-like transformers, needing 14-19v DC. As I really didn’t want to have to step up the voltage from 12v in any way, these, sadly, were all out. In any case, the current PC monitor market suffers from the same drawback as the caravan TV market, in that cheaper technology has led to ever-bigger screens, and little can be found below 19 inches in size.

Intensive googling and ebay-ing, however, did turf up an number of leads. It seems that around 10-12 years ago, a number of PC monitors were indeed built with 12v DC external power supplies. Mostly these were OEM type monitors supplied with business PC’s, and they were now going very cheaply (sub £30) on ebay. However, getting good descriptions of the monitors, assuring that they could be VESA mounted, and that they were in good condition, was very difficult- manuals for most of them were particularly hard to find. I bought a couple of monitors on spec for proof of concept- one turned out to have some screen burn, the other not to be VESA mountable. Neither was acceptable.

Eventually, however, I came up trumps with a monitor called the AOC LM720. This is a 17-inch TFT with a 4:3 ratio, powered by an external 12v DC supply, and with a 75 mm VESA mount. Best of all, it seems to have sold in the tens or hundreds of thousands, so it is easy to find lots of them on ebay at any given time. It has a hardwired VGA cable of a couple of metres’ length, and you can even download a manual. I paid £26.99 for a perfect example and the monitor problem was solved.

Power supply

Finally, I needed something to deliver a regulated 12v DC power supply to the monitor and PC. You can’t just wire them to the boat’s power supply, because the voltage is spiky- engine and shore power can drive the voltage up over 13v, whilst with depleted batteries it might drop to 11v. Delicate computer electronics will at best stop working and at worst be damaged by this. What was needed was a DC/DC regulator- a device that constantly delivers 12v despite variation in the input voltage- for a 12v power supply. Lots of these exist but they often are rather odd looking bits of electronica designed to be fitted inside something or attached to a circuit board. Not an option. What was wanted was a ‘black box’ power supply, and eventually I found it in the Amperor 12v stabiliser, a 12v DC/DC regulator with a cigar lighter type plug on one end and- excellently- two DC jack outputs on the other. The Amperor can supply multiple devices provided the total draw doesn’t exceed 6 amps (this was fine, as I expected the screen to draw 2a max and the PC 1a max).

The Amperor needed a connection to the boat’s power supply, so I simply wired an in-line cigar lighter socket from Maplin to a spare switch on the boat’s electronics panel. The amperor’s cigar lighter jack is internally fused so there was no need for an inline fuse. The power jack output tips turned out to be 2.1 x 5.5 mm; both the Sumvision PC and the AOC monitor required 2.5 x 5.5 mm jacks, so a pair of converter tips were also sourced from Maplin.

Up and running

The final stage was to mount and connect the PC and monitor. I enlarged the hole which allows electronics to pass out of the locker to the chart table in order to get the power cable from the Amperor, as well as the USB cable from the shipmodul, out into the wild. Then, using a pair of slimline VESA mounts from Amazon, I mounted the PC on top of the port-side lockers and coupled it to USB, power and the monitor’s VGA cable.

The sumvision mini-PC securely VESA mounted on top of the port lockers in a sheltered, dry position.
The sumvision mini-PC securely VESA mounted on top of the port lockers in a sheltered, dry position.

The VESA mount for the monitor was bolted through the chart table bulkhead to ensure it remains secure in a seaway, and the monitor secured in place. With a couple of cable ties to tidy up, the job was done.

Final installation. The monitor is displaying Imray raster charts in Meridian's SeaTrak as well as repeater instrument dials in NavMonPC. Note that the Yeoman plotter and Imray paper charts are still easily available on the chart table.
Final installation. The monitor is displaying Imray raster charts in Meridian’s SeaTrak as well as repeater instrument dials in NavMonPC. Note that the Yeoman plotter and Imray paper charts are still easily available on the chart table.

First impressions

We’ve taken the setup out for a first sea trial and been very impressed. In power terms, the computer is responsible for only around 2 amps of draw on the house battery (which, helpfully, is new). With both the cockpit and chart table GPS units, the AIS, and the simnet instruments up and running, the total load is about 3.5 amps. This means that 12 hours’ daylight sailing could comfortably be achieved on the battery’s 120 amp-hours without depleting it below 50% (depleting house batteries more than this is not generally good practice). In any case, one of my next jobs is to fit a 20w solar panel which, on a sunny day, will offset this load when underway with about 1.5 amps of trickle charge.

The nav software setup is simple: the shipmodul multiplexer was autodetected and instantly installed over the internet by the computer, without the need for a driver disk. The computer assigned it to the data port COM3 and this port is read by NavMonPC, an excellent bit of freeware which provides NMEA data management and comprehensive repeater instruments. The data is then passed to Meridian software’s SeaTrak package, which provides a PC plotting solution and is the only thing on the market (either PC or plotter based) which allows you to use electronic Imray charts. We use paper Imray charts, so this makes sense, and these are always available on the nav table, mounted on a Yeoman plotter beneath a clear, working overlay. This means that the navigator can work either wholly electronically on the PC, partly electornically using the Yeoman, or wholly on paper, and always be using exactly the same charts, with a consistent representation of the sea bed and coast.

In terms of weather data, the PC runs ExpeditionLT, a freeware GRIB viewer which I use to visualise data from PredictWind and Theyr. This allows realtime visualization of the forecasted wind and sea state underway, and having the computer thus lifts the weather forecast from a pre-planning tool to something that can be responsively reassessed whilst sailing.

Overall Jen and I are really pleased with this system and looking forward to getting the best out of it for our sailing this summer. There’s only one drawback- I shall now have to admit to being a convert to electronic navigation!

Costs and suppliers

Getting it right led down a few dead ends (particularly the two unsuitable TFT screens) and I have not included costs for consumables like crimps and wire, or postage, so the estimate below is not the actuals spent but basically the cost to do it again, correctly.

The AT10 was only needed because Karisma has a propreitary Simnet instrument network. Most boats wouldn’t need this and it’s not strictly speaking part of the PC setup so I have kept it off the cost estimate. It was obtained from Cactus at £76.96. Similarly the AIS box can’t be considered part of the computer itself. It was also obtained from Cactus at £134.95.

Shipmodul miniplex-lite (yachting software): £79

Amperor 12v stabilizer (amazon): £34.95

Inline 12v cigar socket (Maplin): £2.99

2x 2.5mm DC jack converters (Maplin): £4.58

Sumvision Cyclone Mini PC (Amazon): £128.99

Sumvision Paradox III keyboard & mouse (Amazon): £10.95

AOC LM720 17-inch TFT (ebay): £26.99

2x 75mm slimline VESA mounts (amazon): £11.96

The only paid-for software is SeaTrak. A single licence with an Imray Chart Pack costs £95.00. PredictWind and Theyr are (expensive) subscription services, but there are free, if very much lower resolution, GRIB sources on the internet, such as those provided by NOAA.

Grand total (hardware and software): £395.41

(for reference, the largest chartplotter Force 4 can sell you for that money has a huge 5-inch screen. So apart from the power and flexibility of the option, think the PC route represents excellent value for money, too).

Some Notes

(1) AIS data and the AT10. The multiplexed NMEA output from the Shipmodul to the AT10 includes not just GPS but also AIS data, and my first thought was that this would be the obvious way to feed AIS data to the simnet network if a cockpit chartplotter were installed in the future. However,when I checked the manual, AIS sentences are not included in the AT10’s tx or rx list. Nothing in the one-page manual mentions its baud rate, but the lack of AIS sentences makes me pretty sure it is tx and rx at 4800 baud only, as AIS would require it to be able operate at 38400 baud. Not to have it ‘listen’ on higher baud rates seems a missed opportunity on Simrad’s part.

(2) Multiplexed NMEA from the Shipmodul. Pre-purchase, I was uncertain how the multiplexer would manage its dedicated NMEA0183 output. Would it be a multiplexed copy of the input signal (ideal) or would it be a talker port for the computer? Would it continue to work without USB power? The manual has little to say about it other than ‘sentences received from the computer are sent to the talker port… to control an autopilot’. However a page on the site, where I purchased it, unequivocally says that the multiplexer does not send data from its inputs to its outputs, and that the navigation software is responsible for what data is transmitted to the tx port. There is no similar statement on Shipmodul’s own site, but lacking anything else this is the basis I was working on until I fitted it.

The unit shipped with an utility called mpxconfig.exe which I think is a configuration package for the range of shipmodul multiplexers. This utility doesn’t work in any way with the miniplex-lite, and I have tried it under windows XP and 8.1. This first made me suspect that the guidance on the web might not be quite right. At first sea trial I turned off and powered down the PC (and thus the shipmodul’s USB power supply) and the wind instrument continued to calculate true wind. Thus means that GPS data was being passed from the shipmodul’s inputs to its output despite USB power down and no nav software present. Therefore it seems that, contrary to some of the blurb, the unit does indeed simply pass-through the multiplexed data.


5 thoughts on “Building a PC plotter

  1. Great article, so great in fact I’m inspired to copy your set-up completely. Given around a year has passed since you published the article, is there anything you would change or any new tech you think would make it even better? I have a much smaller boat than yours (Jeanneau Sunway) and intend to use the system for navigation when sailing and as a general PC for emails and perhaps the odd movie in port. Great blog.

    Liked by 1 person

  2. Hi Dan, thanks for the comment! The system is still much the same as it was a year ago, except for a new monitor. Sadly, the AOC screen died of old age during the winter, so I replaced it with a brand-new 17-inch CCTV monitor form SAC Electronics via Amazon:

    This bumps the cost up a bit and being a CCTV monitor it doesn’t quite produce the same resolution as a dedicated PC monitor, but it’s brand new and should last for years. It was a direct plug and play replacement- it requires 12v DC via the exact same sized jack and accepts the VESA mount also. you can also patch in audio via a 3.5mm stereo jack connected to the PC’s headphone output.

    Re. the shipmodul passing through multiplexed data, I was wrong about that on the original blog post. It is in fact controlled by the computer. You can set up NavMonPC to echo out any or all of the received data, so no problem there, but it does mean that anytihg you connect to the output of the multiplexer will only receive data when the computer is turned on (and NavMonPC, which distributes the NMEA data within the PC, is running). This might not be a problem with a more expensive multiplexer but more expensive multiplexers are- more expensive.

    Over the winter I also fitted a cockpit chartplotter which, whilst it doesn’t directly impact the system, meant I wanted to use Navionics charts on the PC alongside the SeaTrak raster. To do this I installed a second plotting program called PC Plotter:

    This means that we have both Navionics vector and Imray raster cartography runing at the same time. However I had an issue with this software working together with NavMonPC, which is a bit too complicated to go into in a comment but was solved eventually by buying a purpose-made serial splitter program. I don’t believe you would encounter this problem with any other package (ie you can run the software setup described in the article if you are not planning to use PC Plotter) but I thought it best to mention it as I don’t know what software you will prefer.

    You are more than welcome to email me if you want any help with the hardware or software choices. You can find my email on the ‘Crew’ page. Cheers, Huw


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