Basic GPS/VHF NMEA System

Project date: 2002

I consider this a single project - although it covers the installation of a GPS Chartplotter and VHF DSC Receiver - interconnected via NMEA (National Marine Electronics Association) bus. A NMEA bus is an interconnection/communication standard that describes how electronic components exchange data.

The GPS Chartplotter is a Standard Horizon model CP-150, and the VHF DSC radio is a Standard Horizon Intrepid LE. DSC is Digital Selective Calling, and is a system that digitally transmits a database number and GPS coordinates of your boat. This is very useful in emergency situations - if you declare a MayDay, the radio broadcasts your GPS position.

The Radio receives the GPS information from the GPS Chartplotter via the NMEA bus. The VHF radio also displays the current GPS Latitude and Longitide on its display - confirmation that it is receiving NMEA data. In a similar manner, if you receive an emergency broadcast from another boater having a similar system, the GPS will plot the coordinates of the received transmission on the Chartplotter.

While just about all modern Chartplotters and DSC capable VHF radios have a NMEA bus, I chose the Standard Horizon brand for both units for their asthetic value.

My boat originally did not come with either VHF radio or GPS system from the factory. The previous owner installed a VHF radio, but it was not a DSC capable unit. Therefore, part of the installation is removal of the original radio, and repair of the holes drilled into the helm.

The Four Winns dash has room for both a VHF radio and Chartplotter. If ordered from the factory, this is the location that would have been used. Unfortunately, the only DSC capable radio that would fit into the right side of the dash was a really expensive Raytheon unit. Therefore, I had to mount the radio on the left side of the dash, since it had more room.



I also do not like the factory position of the GPS in the dash. It is simply too hard to read from that location. To improve this situation, I decided to mount the GPS at a higher location - in the right-most cup holder. By using the cup holder, I could install the mount without drilling any holes in the dash. I might want to relocate stuff, so the fewer holes the better. For the same reason, I purchased a spare dash from Four Winns for $75 in case I ever want to replace the radio.


The Project Location


Here is how the dash looked when I purchased the boat. You can see how the VHF radio was installed on the kick panel.

When I remove the existing radio, there will be 11 holes that I need to do something with. Since the boat is 1 1/2 hours away, I can only get there on weekends, so the project will take several weeks to complete. In the mean time, I will be thinking of how to take care of the holes. I have thoughts of making a chart pocket out of vinyl or Teak, or maybe putting an inspection plate here. However, this area is a compound curve, so getting something to fit right might be difficult.



Part 1 - The GPS Chartplotter Installation


The first thing I did is obtain some Corian. I purchased a scrap piece that came from a sink cutout from a cabinet shop. It cost me $20 for a 2ft x 3ft piece. Green was the only color the shop had - fortunately, the boat trim is green, so it worked out.

Corian is actually a plastic, and I talked with several woodworkers that have used this material for advice on how to work with it. It is easily shaped using carbide tipped woodworking tools. Since woodworking is one of my hobbies, it gave me a chance to work a new medium.

DuPont has recognized the artistic applications of Corian, and you can purchase small quantities for woodworking and other art projects at:

Art Specialties (1-800-724-4008)   

You just won't be able to buy it wide enough to install a kitchen countertop, as you can only buy the material up to 15" wide here. Of course, eBay always has odd quantities and colors available.

Here I am making the baseplate that will cover the cup holder. Both the GPS unit and it's external antenna will be mounted on the Corian.


Here I trued the ends and rounded the corners of the Corian. The trick to working Corian is to take it slow.

Next, I routed the top surface with a 45deg carbide router bit. I then marked all the holes for the antenna and GPS mount, and tapped the holes.

The last step was to sand the entire piece with 600 grit, 1,000 grit, and finally 2,000 grit wet-or-dry sandpaper. The piece was highly polished after this step.

I then placed a foam rubber pad on the bottom of the Corian to keep from marring the boat dash.


I created a Tee-Bar out of oak that allowed me to secure the Corian to the underside of the cupholder. Next, I mounted the antenna and GPS mount by screws into the tapped holes. Be careful not to overtighten the screws as the Corian is only plastic.

The unit is now pre-wired and ready for installation.



Here is a drawing of how the whole thing fits together - and through the cup holder.



Installation of the GPS unit consisted of removing the cup holder, sliding the Tee-Bar into the cup holder hole, positioning the Corian for the best angle, then tightening the Tee-Bar screws. This is what I was looking for - a very good viewing angle.

You cannot see the antenna, but it is behind the GPS unit - yet it is not under the windshield. This allows an unrestricted signal from the GPS satellites. All of the gauges on the dash are visible as well.

The Corian is highly polished, fits flush on the dash top, and did not require drilling any holes in the fiberglass dash. While I do give up a cup-holder, I just don't know if I could have done a better installation.

Gosh, now I am down to having 5 cup-holders left in the cockpit!



Part 2 - The VHF Radio Installation


Now on to the VHF radio. After removing the dash plate, I made a test template out of cardboard and test fit the radio. This is just about an essential step. The flush mount kit had mounting ears that made the radio too large for the right side of the dash. The ears were just about too large for this side of the dash as well. I only had a inch or so of "wiggle room", so the template let me determine the proper location.

You might notice that I screwed the template to the dash with two screws along the bottom. These are the standard dash mounting screws, and they served as alignment marks for transferring the template to the dash insert.


I taped the dash insert to allow for a easy method of transfering the template alighment marks.

The dash ended up being made out of aluminim, with a facing of plastic wood grain. A plastic/aluminum laminate of sorts. I used a Dremel moto-tool with an abrasive cut-off wheel to cut the dash out. This became quite a labor-intensive step.

One word of caution here. The cut-off wheel throws fine aluminum "dust" all over the place - so tape up all electrical connections and switches so you don't have electrical problems later.


I installed a terminal block for the power circuit and NMEA bus connections. Both GPS and VHF radio terminate on this block. If I ever need to install a remote speaker for the VHF radio, it appears on this block as well. I mounted this block in the right dashboard hole area, but left room for future gauges or accessories to be installed in the right dash area.

Notice the small mounting area I would have had to contend with if I installed the radio here.

Wiring was pretty straightforward as far as NMEA 0183 wiring goes. Note however, that this example is only one of the many different wiring schemes used by marine electronics manufacturers:

Please refer to my white paper NMEA 0183 Primer for an in-depth look at NMEA-0183 connections.


Here is the final installation of the radio with the dash re-installed. Since I am left handed, it is actually an advantage for me to have the radio here. The right side of the dash is now available for future gauges - but that is another project.



Part 3 - Repair of the Mounting Holes


Please note that I am not an expert at Fiberglass Repair - but this technique worked with pretty good results...

I decided I would repair the holes left over from the previous radio installation.

First, I enlarged the holes by drilling them with a countersink bit. This elongated surface area makes for a stronger repair.

I then mixed some polyester resin, then added some West System #410 faring compound to make a putty. Although intended for epoxy resin, it worked fine with the polyester. Epoxy is not compatibile with gel coat, so I had to use polyester resin.


If you do, the gel coat will not cure over epoxy. You must use polyester resin. Using the polyester "putty", I filled the holes up to the existing gel coat layer, leaving an 1/8in "valley" for the new gel coat.


The next step was to mix some exact color gel coat I purchased from the Four Winns dealer. Four Winns calls the dash color "Mica".

However, you can purchase factory color-matched gel coat for many boat brands from Spectrum Color, which is essentially the same item I used from the dealer.

This gel coat had the consistancy of clay, so it filled the holes really nice. After the gel coat cured, I sanded the area smooth. I had to apply a second coat to get everything smooth.

I finished sanded the area with 2,000 grit wet-or-dry sandpaper, used wet. This will leave the area dull from the sanding. After smooth, I used Flitz brand Fiberglass polishing compound to restore the gel coat shine, then finished it off with a coat of Teflon wax.

The holes are slightly noticeable, since the gel coat repair not quite the exact color. However, I believe that over time, it will fade into the exact same color.



Bill of Materials
GPS Chartplotter Standard Horizon CP-150
C-MAP-NT Map Cartridge Lake Michigan
VHF DSC Radio Standard Horizon Intrepid LE
VHF Flush Mount Kit Standard Horizon
Spare Dash Panel Four Winns V268
Electric Components Terminal Blocks and Connectors
Four Winns Gel Coat Repair Kit Four Winns Mica
Corian Kitchen Shop
Misc Materials  


Sources: Components were purchased at West Marine, Boat US, Defender, Consumer Marine Electronics, Menards, my local Four Winns dealer, and Radio Shack.


Summary. Although an expensive project, it is essential to have a chartplotter and VHF radio on a modern cruiser. The cost was offset somewhat by selling my old VHF radio on eBay. I am not 100% happy with the radio location, but there is no alternative in-dash location possible. The only option would be to replace the radio with a smaller one. I am more than 100% happy with the GPS installation and gel coat repair.




I have come to the realization that the GPS antenna location is marginal. Several times this year, the GPS unit would not lock on any satellites. While this was happening, I was able to move the antenna outside of the windshield and it worked. I have been thinking about how to redo the boat VHF antenna location anyway. When Four Winns built the boat, they glassed in two mounting pads, designed for mounting an antenna mount for both a GPS and VHF antenna. Unfortunately, the original installer of the antenna did not know about this, and missed the mounting pads. Consequently, the antenna mount is located in a non-reinforced area of the deck, and the stress of the 8ft VHF antenna flexing could cause the gelcoat to crack. Fortunately, the previous installation used a nylon antenna mount, so it takes most of the stress, but I plan on upgrading to a metal mount, which wiould transfer the stress to the boat.

Unfortunately, I could not find a metal antenna mount having the same exact bolt pattern as the nylon mount used by the original installation - even the same brand metal version of the nylon mount. This delayed the project until I determined the GPS antenna had to be moved. I found the West Marine Zamac antenna mounts had nearly the same bolt pattern, at least close enough that the old holes would not show outside of the mount's base. The first step was to remove the old mount, clean up the sealant (fortunately, 5200 was not used), then mask off the hole areas and fill them with epoxy.

After the epoxy dried, I redrilled the holes in the correct locations. The epoxy also ensures no water can get into the coring.

I installed the new antenna mount using a liberal amount of 3M sealant (the non-adhesive type), which, along with the epoxy, should result in a trouble free seal. I used stainless hardware, including 1in dia fender washers. The fender washers will help spread out any stress from the antenna mount.

I used enough sealant that it gushed under the fender washers on the inside of the boat. One problem I had with the GPS antenna is Standard Horizon installed the connector at the factory. Unfortunately, this requires a 3/8in hole, so I could not fit the antenna lead through the antenna mount. The connector looks to be hard to find, so I really did not want to cut it off. The solution was to purchase a Shakesphere 12in fiberglass extender, which allows the cable and more importantly, the connector, to pass through the side. I was not sure how the GPS antenna would look sitting on top of this "stub", but I think it came out OK. And, it will probably improve reception as well.

In a similar manner, I installed the VHF antenna in the proper location (where the boat builder added a thicker area). Even though there was a 3/4in plywood stiffener here, I still used fender washers.




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