Radar Scanner

Here in the UK the English Channel is very busy and gets a bit foggy. I want to install radar on the Puffin. The problem is the scanner. A Raymarine 18" Scanner is going to look a bit odd on the mast. Below where the staysail stay attaches it will catch the staysail and above the staysail it could look enormous. I am considering a scanstrut pole attached possibly to the boomkin and gallows. Anyone any better ideas and what have others with radar done?

We installed a Furuno 1623 unit on our BCC and friends have one on their Falmouth Cutter 22. Because the radom array is 15" (33.6cm), the unit looks “just right” for the boat size. The trade-off between a 15" (33.5cm) and an 18" (40.3cm) array is a decrease in resolution.

In terms of resolution, let’s discuss the distance two targets (ships) must be separated before two dots are resolved on the display screen. For our purpose, I will assume the range to the targets is 4 miles (6.7km) and the radar beam intercepts both targets at the same time, i.e. both ships are in the same plan as the signal at the same time.

For an 18" array and a target range of 4 miles, the two ships must be greater than 642 yd. (~593m) apart before the two ships are resolved into two targets displayed on the screen, whereas for a 15" array the two ships must be greater than 766 yd (~707m) apart. If the distance between the two is less than 642 yd for the 18" array and less than 766 yd. for the 15" array, only one target is shown on the display screen. This is under ideal condition. In terms of a percentage, the 18" array has about 16% better resolution than a 15" array at a range of 4 miles.

The probability of two ships being that close together and in the same plane at the same time as the radar beam signal cell is low. Even if all conditions are not met, one will see at least one target on the display screen.

Radar resolution comparisons do not provide the complete answer. The real limiting factor on a sailboat is the screen display area. For our purposes, let’s assume the display unit has about a 6" (13cm) screen. At a range of 4 miles (6.5km), the display uses about 3" (6.7cm) all around the center point, to show the 4 mile {about 7,000 yd (~6,500m)}. If the two ships are 1,000 yd (923m) apart, the ships are displayed as two dots separated by a space representing 1/7th of the 3" portion of the sceen, i.e. the two dots on screen will be about 0.4" (1cm) apart. If the two ships are 800 yd (738cm) apart, the two dots on screen are 0.3" (0.7cm) apart. If the radar range is set at 8 miles, the distance between the two dots on the screen is halved for the above, i.e it becomes more difficult to distingish one target from another target as the radar range is increased when the display screen is 6" (13cm) or 7" (16cm).

The above discussion, became very real when I worked part time on tug boats after I retired from chemistry/engineering. Besides using large radar arrays, tugs employ large display screens.

While I am on this long rambling spell, let’s talk about range. If the radom is place 20 ft above the waterline and we assume the mean height of the ship is 50’ (15m) above the water, then the earliest detection distance under ideal conditions is 16 miles regardless of the whether the radar unit has a 16 mile range (27km) or a 72 mile (120km) range.

In summary, the limiting factor for small radar units is not the resolution or range of the unit but the size of the screen. Ideally, I would like to have a radar unit with a 15" (37cm) array and a 12" (27cm) display screen. Unfortunately, the sales/marketing wizards have other ideas.

You may wonder why I selected a Furuno unit. I selected Furuno because it is the manufacture that one sees almost universally in the commercial marine industry.

If you are interested, “radar range resolution calculation” are available via the Internet.

In closing, there is one other factor which is probably just as important as the equipment and that variable is the skill and experience of the operator.


Hi Peter,

My FC has a Furuno radar antenna mounted below the staysail stay mast tang. It does interfere sometimes with the staysail, gybing in light air. Nevertheless, the sail does need an antichafing patch with this antenna mounting.

I have also seen a FC (name withheld to protect the innocent) with radar antenna mounted on a pole slipped over the backstay. To keep the antenna from interfering with the main sail, it hung off the backside of the pole. It appeared that the pole was attached to the backstay by “set screws”. Otherwise, the weight of the antenna would rotate the whole unit 180 degrees putting the antenna right into the path of the mainsail’s leech when tacking or gybing. I would also worry about problems with the backstay in this arrangement caused by the “set screws”.

A vertical mounting pole attached to the bumkin and stabilized by brackets also attached to the bumkin might be your best bet. Be sure the antenna is on a gimble so that it is always level. Otherwise it might be looking up into the air in some directions and into the sea surface in the opposite direction.

Good luck.

ron walton
editor: FC News

Hi Folks,
I actually went for Raymarine in the end, particularly because I had used it before and it enabled me to link everything including the autopilot together. I abandoned Standard Horizon and replaced the lot. Actually that’s not true, I replaced everything except the log impeller (The boat was in the water) and you get no prizes for guessing which bit of my lovely new system doesn’t bloody work.
I was petrified with size of the Radome (18") and was sure it would look silly. We fitted it just above the crossstrees and it looks fine. (Photos follow)
Where has this season gone?