02 June 2009

Tying it all together - Overview

{Updated: June 16, 2009}

Enough has been done now to show what the entire electrical and navigation system on Merrimac II is going to look like. The goal was to have a system that's easy to use but not clutter up the entire boat with displays at various places. Not that you'd guess this from the image shown here at right -- you'd better click on it to get a readable image if you want to see it in more detail!

The cabin/saloon will not have any displays, buttons or nav systems except for a radio and a TV (hidden in a cabinet) and a charging cradle for the iPod touch remote.

The doghouse will have a PC display and a chartplotter and nothing else; all complexity will be operable from the PC and the remote (iPod Touch).


Let's start from basics: power supply. The electrons are going to come aboard as follows:
  1. Solar panels. 6 x Sunware SPR-90. 90Wp per panel. These should keep her happy while we're not there -- and even for a large part when are! I estimate that they can supply up to 2 kWh per day.
  2. High power Mastervolt 24V/75A alternator on the engine coupled with an Alpha charge regulator. I've purposely decided on this model, as it charges at higher amps when the engine is running slowly than it's larger brethren.
  3. A Mastervolt Chargemaster 24V/30A charger.
This will be sunk into 6 x 225 Ah Mastervolt AGM batteries, connected as 3 x 2 to provide 24 V @ 675 Ah. This means we can use about 330 Ah == 8 kWh before we need to top up.

On the consumer side we'll have a Mastervolt DC/AC converter that generates 230V, and a DC/DC converter that provides 12V for the electronics. There will be no AC-AC path from the shore connection to anything on board. In this way we don't need a isolation transformer.

The AC side is used by the following:
  1. Hydraulic power pump for the lifting keel.
  2. Washing machine (Miele, with hot fill capability so it can use warm engine water.)
  3. Dishwasher.
  4. TV.
The 12 V bits are used for anything electronic that needs/wants this. This keeps the supply clean, even when we use heavy electrical engines like the anchor winch. All motors etc are to be kept off this circuit, obviously.

The 24 V bits are for everything else: lighting, fridge, freezer, winches, autopilot.


The electronics are going to be running on three networks:
  1. Ethernet
  2. NMEA 2000
  3. Masterbus
The Ethernet will interface the Wago Linux PLC, the "integrator" Linux computer that provides WiFi and cellular access, a navigation computer running Microsoft Windows, a HDS8 chartplotter and a Broadband Radar radome.

The NMEA 2000 data will carry all navigation data. The components on this network are: Simrad IS20 instruments, autopilot and AIS class B transceiver; Lowrance HDS8 chartplotter/fishfinder/radar.

The Masterbus will couple all Mastervolt chargers/converters to each other. It integrates to the Microsoft Windows PC using a Mastervolt USB interface and possibly to the Linux system usign a Modbus interface.

What's not yet entirely clear is what will be used to tie the NMEA 2000 and Masterbus to the PCs, and what the PCs will be like. I'm investigating whether I can interface to these buses using my own software and off the shelf or custom CAN bus interfaces, as both of them are actually CAN. The fallback scenario is to use a Simrad AT10 interface for the Nobeltec software, a CANUSB for snooping the NMEA 2000 data and a Mastervolt Modbus interface for snooping the Mastervolt data.

For the Linux PC I'm contemplating various systems, but at the moment it looks as if I might as well use two (almost) identical systems such as the fit-PC2 for both the "always on" server and the Windows navigation system. An alternative is to make the server something that is ARM or MIPS based, as that should use less power. Sourcing an ARM based computer that is frugal with power (e.g. 2-3 W in total), has Ethernet, Wireless and USB or even better CAN directly on board turns out to be harder than I thought. There are a good many suppliers, but most systems fail on either being able to support an uptodate Linux BSP, or have the wrong peripherals, or are very expensive, or have a lifetime that is too short. If that is the case I might just as well use an expendable x86 computer that has plenty of USB ports and low power.

Navigation data

There will be four displays in the cockpit: IS20 Wind, Graphic, Multi and an AP24 pilot control. This gives ample data at disposal. There will be two depth sensors: one on the hull and one at the bottom of the keel. This is not just for redundancy, but also to keep from having to perform mental arithmetic as the keel depth can be varied from 0,80 m (2.6 ft) to 3,15 m (10 ft). This is important as our home waters are very shallow. The sensors work at different frequencies so they should not interfere with each other. The HDS can switch its depth sensor off when it's not needed.

The doghouse will have two screens: a Lowrance HDS8 chartplotter that also shows depth-over-time (aka a Fishfinder) and Radar. Next to that will be a computer display running Nobeltec Admiral and various other software. It will also run MSCAN Meteo (for receiving weather and NAVTEX data) connected to an ICOM PCR1500 remote controlled SSB receiver. The doghouse will also contain a Simrad RS82 VHF as well as an ICOM portable VHF and a Simrad WR20 remote. The remote allows you to control all Simrad equipment remotely as well as use it as a Bluetooth "headset" for a cellphone.


  1. I have tested a bunch of PCs for this purpose, and my current pick is the Fit-PC slim. It draws only 5W of power, and is a full PC. It requires a USB expander, and perhaps a small WRT54G hub to expand the Ethernet ports, but as a core and low power internal server, it is pretty good. And it is fanless - which means you can seal it to make it water proof!

  2. Devon, I would instead go for a beagleboard or perhaps even a pandaboard.
    Then we are talking very low power (below 2W
    for pandaboard and even less for the beagleboard)
    I've used a Fit-PC slim with a 3G modem in my boat, then the consumption is about 6W (then constant connected).
    Living in Sweden, that means it is ok for summer
    but in autumn and spring it is actually a bit much to be connected all time.
    With less than 2W (I'm using pandaboard) I hope
    to get by all season :-)


  3. Per,

    See CANboat design part III choosing the server hardware for my final decision process for the always-on server.