DMI
DMI SPEED TRANSDUCER TO RAYMARINE DISPLAY ADAPTER


The Cal came with the 4" round DMI instruments on a 6" spacing, speed and wind position and strength. On my Santana525, I had installed Raytheon ST50 system, wind, speed, close-hauled, speed change, and depth. This allowed for sailing to wind maximizing hull speed and VMG, Velocity Make Good. I would adjust for VMG and when hull speed would start to fall, bear off a little, hull speed would raise and VMG would start to fall...... It would help define the width of the slot.  You can use the same tactic going down wind.  I've had great luck with the Raymarine instruments and only have had to send back a ST30 speed display. I discovered that it would quit working when the temp was below 40 f. I also discovered that I also started to quit working at the same temperature. It’s hard to sail when you can't feel your hands.  The Raymarine service center replaced the board at no charge which surprised me; it had been out of warranty for about 6 months.
Back to the Cal....  I had installed a Raymarine 630 Chart plotter for winter sailing, the water level drops 50-60 ft on the annual tide thanks to the TVA. We have mud flats that appear and yes you can get stuck on them. There are two types of sailors, those that have been stuck aground, and those that will be.... A display option on the 630 was the window mode that can be setup to display the seatalk data from the instruments.  The DMI instruments are pre NMEA and could not interface. I wanted to install ST60 instruments that could communicate.  When I bought the boat I installed the ST60 masthead connector block and cable for the wind instrument and ran the cable back to the bulkhead. I had the only boat at the dock with two masthead sensors, one turning clockwide and the other counter-clockwise. The mast head instrument worked and I was waiting to haul the boat to change out the Speed transducer that was not comparible with the ST60 speed display.  I liked the  DMI transducer with its wider paddle wheel being  more responsive at low speeds.  Thus the idea of trying to combine the two systems.

circuit board
The Raymarine had a Hall Effect 3 wire system and a 0 to 12 volt output, the DMI is a magnetic reluctance unit with 2 wire small signal output.  I designed an  analog circuit and had a PCB made to adapt the two and tried several modifications without success.  I could apply a signal to the BNC input and the LED would blink and the Raymarine display would start to display speed. I simplified the circuit as much as possible but could not get the speed transducer to interface with the front end of the circuit. After many attempts to design a compatible front end, I took the DMI apart and traced the circuit to discover the magical required interface.  The  transducer required an excitation current  that I had failed to supply. I modified the board again and this time it worked.
Not a pretty site but functional.  The PCB turned out to be a good breadboard for the required  modifications. I have the final PCB version designed if I ever need a replacement. One of the interesting finds was that the display needs a 10K resistor across a couple of the terminals to activate the display. The Raymarine speed transducer has a 10k thermister so it can sense and display water temp. One easy option would be to replace the 10k resister with a thermister. Instead of water temp, it would display cabin temp.



2nd design

My installation options were to make an adapter plate and mount the ST60 3 1/2" required holes  into the larger 4" DMI holes or to use the surface mount bezels which required just larger square holes 4.3" x 4.5" than the 4" round ones.  I did CAD models to see what it would look like. If I used the 1/2" or 3/4" starboard the instruments would protrude further from the bulkhead and there was the possibility that the BNC connector  and cable would not clear the mounting hole. I had soldered the connector at a 30 degree angle to help the clearance. if needed.

cad drawing


 I debated for weeks and made a template out of 1/2" starboard to see what the surface mounts would be like.  I did not like the original ST60's top mounted in the round holes. It looked just like an attempt to make it work.
I took the top surface mounting gray bezels off and put on the larger square flush mouunt ones.  They just snap in and out, but have additional mounting screws for the surface mounts. The instrument housings were thicker than 1/2" so if I wanted to use the surface mount with an adapter plate it would have to be around 3/4" thick.  I decided to router the larger square holes in the bulkhead and direct mount the instruments without an adapter board.

cad image


The starboard template with the square surface mounting holes became my router template and I used a ball bearing  1/4"   by 1" long following bit to router the bulkhead approximately 3/4" deep.  The Cal was designed with a balsa cored bulkhead approxamately 3/4" thick with an interior trim wall made of 1/2" teak plywood. overall about 1 1/2" thick. I left the interior plywood alone with the existing 4" round holes required for the original DMI installation. I can add an inteior trim piece later.
  It was easy to align the template over the holes for best fit and tape it in place. I did discover that it could make quite a mess, balsa and a fiberglass went everywhere.  I did a dry fit prior to epoxy the exposed balsa in the new routered holes.   The countersinking of the displays with the surface mounts allowed the instruments to sit deeper in the bulkhead and the BNC connector on the adapter board allowed the cable to attach easily. I was able to mount it almost flush to the PCB board.

st60 flush mount







Should I ever want to reinstall the DMI instruments, I would have to make an 1/2" adaper board with 4" holes on 6" centers. The DMI instruments are long enough  to allow for this and still have enough length for the installation ring on the inside to secure them.
You can tell from the inside photo how little overlap the top mount would have been. I really hate to put holes in a boat, or make small ones larger, but this time the results were worth it.


st60 flush mount





















The instruments can communicate with the 630  chartplotter so I can monitor the instruments while at the nav table. I'l add photos of the interior trim when I get that far.    7-27-09 E.R.