Radar Arch Project

While it is fun doing your own boat improvement projects, sometimes you just have to let an expert do most of the work. Such was the case when I had a radar arch installed on my boat. My input into the project was primarily focused on pre-planning tasks rather than the actual construction.

The options for retrofitting an existing boat with an aftermarket radar arch are few. You can sometimes find OEM style arches from salvage yards, but properly fitting the arch is beyond the skill level of most boat owners, especially considering that often, the boat's canvas connects to the arch.

The alternative for most of us is the tubular style arch, which usually does not interfere with the canvas. There are alternatives here as well. Some manufacturers offer pre-manufactured generic kits with some of the major components welded, that you have to cut and bolt-together yourself, somewhat like a swing-set. Other manufacturers offer custom-built single piece welded arches designed specifically for your boat.

For my arch, I selected a custom arch builder; Towers Unlimited; of Zeeland, Mi. In my marina, there were at least three boats that had arches made by Towers Unlimited, so they are quite popular and have a good reputation. If you are wanting to go this route, you'll have to do some legwork and find a local fabricator.

I worked with Jim from Towers Unlimited in specifying the radar arch I wanted. Turns out that he had already done an arch for my model boat, so it was simply a matter of making another arch from the notes he already had.



Towers Unlimited maintains a web-cam at their shop so that you can view Jim building your arch (www.towersunlimited.com).

Jim personally installs each arch that Towers Unlimited makes. Unless you have a trailerable boat and can get it to his shop in Zeeland, he will install your arch on-site.

If your boat is slipped at a marina, you need to be aware that many private marinas have rules or restrictions regarding third party contractors working on your boat. Conversely, many municipal marinas have more liberal policies in this regard. In either case, you need to determine if the arch installation is allowed at your marina.

Installation Day

Approximately 6 weeks after ordering my arch, it was ready for installation. Due to 3rd party contractor restrictions at my marina, I rented a transient slip at the local municipal marina the day that Jim installed the arch.

One consideration to the installation is the level of the boat. The horizontal beam of a typical radar antenna is quite narrow, around 25 degrees or so. Therefore, maintaining the proper azimuth (the radar beam's horizontal angle) is important to proper operation.

Jim will install the arch so that the radar mounting surface is parallel to the waterline. However, my boat's gas tanks are in the stern area, so it tends to sag a bit under a full load of fuel. To ensure I had compensated for the worst-case scenario, I put a full load of fuel into the boat prior to the arch installation.

The entire installation took less than two hours, and you could sure tell that Jim has done this before, because the whole process went smoothly. My son and I were Jim's assistants that day, and one of our first assignments was to carry the arch from his pickup to the boat. I was really amazed that the arch only weighed around 50lbs.


I was very impressed with the perfect fit of the arch. I had at first wanted Jim to move the arch aft a bit for aesthetic reasons. However, Jim pointed out that on my boat, doing so would restrict the ability to board the boat from the side and walk aft. Had I performed the installation myself, I would have surely made this mistake. Its this experience that Jim brings with him that makes the selection of a custom arch worthwhile.

Here Jim is drilling 1.5" dia access holes into the side of the boat to allow antenna cabling to be run inside of the down tubes. There is a cable run inside of each forward tube, port and starboard.


The arch installation is completed at this point. I really do like the look of the arch, and with all of the other tubular metal fixtures on the boat it accentuates the overall look of the boat.

After I tested the tower under way, I discovered it did flex a bit from side-to-side, which put stress on the sides of the fiberglass flybridge. Therefore, I beefed up the inside area of the mounts by sandwiching a 1/2" piece of plywood the interior surface, along with heavy-duty aluminum straps to the base of the flybridge as shown below. This improved the situation to my satisfaction. While I did this in hindsight, care should be taken to ensure there is a sufficient area to mount the tower.

I used standard exterior-grade plywood for the backer board as the mounting area was out of the weather. However I encapsulated it with two coats of epoxy. When mounting the backer board I mixed a good amount of West System epoxy with #404 high-density gap-filling compound and buttered the inside surface of the plywood. This provided sufficient bedding for the backer board that I didn't have to use fiberglass tape to secure it to the hull sides. Finally, the brackets transferred the load from the flybridge's sidewalls to the base, which really helped stiffen the arch.

Antenna Installation.

While I will provide details on the installation of the electronics in other articles, It is appropriate to show the antenna mounting techniques here. The radar arch has 7 hard points for mounting antennas and other accessories. This should be sufficient for all of the goodies I should ever need to install on the boat.


VHF Antenna Fundamentals.

The first antenna that was installed is a RayMarine 125 GPS receiver. This is really a receiver built into the antenna, rather than simply an antenna. This provides a higher degree of reliability I think, because GPS operates at extremely high frequencies, and this technique minimizes signal losses that might otherwise occur in the cabling between the antenna and display unit.


When installing the component, standard stainless hardware was used, but with the addition of shims to keep the antenna vertical. It might only be for aesthetics, but to me, it makes all the difference in the world between a great looking project and one that is sub-standard. The antenna mount is a Shakespeare 4710 flange mount and 418 shim kit. The shim kit comes in two pieces, and allows rotating each disk independantly for an infinite number of angles.

Finally, I used feed through grommets where the wiring entered the arch down tube. Grommets are available from many marine and electronics supply stores. My favorite is Mouser Electronics (www.mouser.com).




The radar arch included mounting pads on each side for a VHF antenna, which was installed next. I used a Shakespeare 4187 mount, 414 shim kit, and 4700-1 stainless one-foot extension tube. Due to electromagnetic radiation hazard considerations, I felt its important to get the antenna up an away from any occupants nearby, and the extension tube helps in this regard.


Electromagnetic Radiation Issues on a Recreational Boat.


Prior to installing the radar antenna, you need to consider the effects of the boat underway. Many boats, especially of the inboard motoryacht style, experience significant bow rise under way. The typical radar antenna has a beam of about 25 degrees or so. When the boat experiences bow rise, there is a risk of the antenna beam being angled back, with the result of it being too high. You may then miss "seeing" that close-in boat or jet ski on radar.


Should you not compensate for this bow rise, you will end up getting a nice radar image of birds and clouds whenever you are at cruising speed. To accurately determine what angle I needed for my radar antenna, I temporarily attached an angle gauge to the radar arch, then ran the boat at cruising speed to see how much bow rise I had.

I found that with my boat, I had a 10 degree difference between the boat at rest and the boat at cruising. Since the radar arch was perfectly flat at rest, I installed a shim of 5 degrees under the radar antenna. This angle provides the best compromise should I wish to cruise at planing or displacement speeds.

One last issue prior to attaching the radar antenna is its interaction with the GPS antenna. Installation manuals will typically tell you to ensure the GPS antenna is either below or above the radar antenna. If at the same height, the high power of the radar antenna may interfere with GPS reception. Due to the narrow beam of the radar antenna's transmitted signal, locating the GPS antenna above or below the radar antenna will suffice. This is usually solved by the use of a radar mast, which elevates the radar antenna above the GPS antenna.

There are several different brands of radar mounts available. I used a 6" high rear-raked mount by Scanstrut. You can typically find mounts either forward or reverse raked, as well as different heights, and even pre-drilled for different brand radomes. Typical accessories you can buy may include shims, light masts, and hard points for mounting GPS antennas above the radome.


Cabling the Radome and GPS is pretty straightforward, with only one cable required for each antenna. You may want to review my paper on pulling wiring through a boat for tips as to how to pull wiring through the radar arch tubes.

The last step is to wire the anchor light. Check with USCG regulations for your boat at www.uscgboating.org as to the permissible locations for relocating the steaming and or anchoring light.

I still have 4 hard points on the arch for additional antennas and other appendages. However, I will attach items to them as this project progresses.

In conclusion, even though much of this project was done by 3rd party, I think you will agree that there is still plenty to consider and do before the arch can be considered functional.


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