Radar

It’s time to replace Itchen’s old Raytheon RL9 - which has done well enough but lacks features common to most newer models. It’s on the usual swing-out companionway mount, very handy. We’ve been looking at Raymarine’s C80 (or E80) and the Furuno 1724 or new cutting edge (bleeding edge?) Navnet 3D FMD8. Anyone have experience with any of these?

Here’s my question. I’d like to remount the 18" or 19" radome lower on the mast, as seen on some other BCC’s, below the staysail stay mast tang, to get the weight and windage lower down and improve close-target reception. What height above the lower spreader has worked well for anyone out there? The mast is currently in the boatyard shed so it is hard to eyeball or measure clearances but before I start drilling holes I will probably do a mock-up on the ground, with the staysail stay pulled out to an imaginary cranse iron.

Another complication is the spinnaker pole track, which now is long enough to interfere with a lower radome mount. This is only a factor it the pole is housed against the mast and is over a certain length. So here is another question – what is the minimum pole length that anyone has used and found reasonable adequate. Not being into racing anymore, I don’t feel the need to get every last inch (or foot) of pole extension. We use it as a whisker pole only, as we have an asymmetrical spinnaker, and that seldom used.

Any potential or actual “gottcha’s” I should watch out for?
Scott

Scptt:

I published the following on the forum sometime ago. In summary, on small boats, the size of the radar array is not as important as the size of the display screen. Depending on range, large screens show clear separation of targets, whereas this is not always the case with small screens. Range is dependent on the height of the target and the arrray above the water. If the array is mounted at 20’ and the mean height of the ship is 50 ft. above the horizon, the line of sight range is 16nm. If the array in the above case is 10 ft above the water, the range is 12nm.

Thoughts on Radar Array and display size:

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 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.

Rod
BCC IDUNA

IDUNA Wrote:

Scptt:

I published the following on the forum sometime
ago. In summary, on small boats, the size of the
radar array is not as important as the size of the
display screen. Depending on range, large screens
show clear separation of targets,

Yes, I had seen your earlier posting and of course agree. No arguing with trigonometry plus experience.

My thoughts are that offshore I am usually interested in big fierce fast freighters before they get “too close”, so we usually have the old RL9 looking out at the 6 to 8 mile range with an occasional switch to max range which practically speaking seems to be around 10 to 12 miles. At those distances I doubt that radome height matters much for freighters and tugs. I find the RL9 to require a lot of manual fussing with gain and rain and sea clutter adjustments to get reasonably clear target returns in rough conditions. Naval warships and submarines off the coast of North Carolina always seem to know all about us when they are still over 25 miles away – wonder what they are using?!

Thoughts on Radar Array and display size:

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.

Agreed. 18"- 19" seems optimum for a BCC. 24" would be nice but too big and out of scale for a 28 footer.

In terms of resolution, let’s discuss the distance
two targets (ships) must be separated before two
dots are resolved on the display screen.

Even if all
conditions are not met, one will see at least one
target on the display screen.

At a few miles distance one target seems enough – by the time they get closer they will resolve into however many are actually out there. Tug and tow can be tricky under bad conditions unless our RL9 is adjusted “just-so”.

Radar resolution comparisons do not provide the
complete answer. The real limiting factor on a
sailboat is the screen display area.

i.e it becomes
more difficult to distinguish one target from
another target as the radar range is increased
when the display screen is 6" (13cm) or 7" (16cm).

Yes. For our BCC compamionway swing-out mount either the Raymarine C/E80 or new Furuno seem the best compromise twixt size and availability. Have not looked at other brands.

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.
<SNIP -I agree>

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.

Locally here in Belfast, Raymarine and Furuno are both popular - may be some price/feature trade-offs at work.

All considered, I will probably get either a Raymarine C80 or dig deep, very deep into my pockets and go for the new Furuno FMD8 with 19" HD radome. The Furuno 1724 does not accommodate their ARPA circuit board and the next size up has a 10" screen, perhaps too big for our companionway mount.

Why upgrade our still-working RL9?

  1. Much too bright at night even at the lowest setting, and that hurts our night vision, #1 on my priorities at night. Newer ones have red backlighing “night mode”
  2. No MARPA/ARPA.
  3. Very fussy fiddling with adjustments needed under bad conditions and when switching between ranges.
  4. Newer circuitry, better resolution in all newer radars.
  5. No more clusters of dry marker dots on the screen, the newer ones all acquire, mark and track targets!
  6. Chart overlay - not necessary, but nice.

Why lower the mast mount?
I used the identical RL9 model on a Victoria 34 we used to cruise and race a lot and the radome was mounted on a taffrail pole just a few feet over our heads. Had much better target resolution at short range (say, under 100 yds) and could often pick up pot buoys and seagulls under calm conditions, which the current above the BCC second spreader position fails to do.

Other opinions welcome . . .
Scott

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.

Rod
BCC IDUNA

I have a Furuno 1724 on my BCC and a Raymarine C80 on my powerboat. I find the Raymarine much easier to use. The Furuno seems to require a lot of adjusting to get a good image. It may be partially a function of my inexperience.

On Sun, Apr 13, 2008 at 3:40 PM, BCC Forums bccforums@samlmorse.com wrote:
Thanks, that is very interesting – I believe that Practical Sailor
found in a recent test that the Raymarine model did better than the
Furuno on the Auto setting but that the Furuno did somewhat better
after fussing with the manual adjustments. I’ll see if I can find the
article and which specific models they were comparing.
Scott

I recently purchased a GARMIN 3210 BUNDLE & GARMIN GMR 18 dome for my 28’BCC along with a QUESTUS SELF LEVELING backstay mount. I will be installing all of the components in May. I’ll report back on how the installation went as well as how it performs.

My intent was to keep the windage and weight as low as possible and keep the radar dome as level as possible.

If anyone has any experience with the GARMAIN or QUESTUS please add a post.

Thanks!
Rick Peterson
BCC JUNESSA

Currently in Gig Harbor, WA

Rick: Hi!

I have no experience with Garmin or Questus - but I’d like to pick up your notion of mounting the radome as low as possible on your backstay.

For one, mounting the radome as HIGH as possible will give it the greater range, ie simple height of eye.

For two, you must ensure that the main beam of the radar does not sweep your cockpit and deck. The main beam will disperse 20 degrees above and below the horizontal. A little geometry will show you the simple lowest limit of mounting on your backstay.

Opthalmologists report an epidemic of cataracts among working and cruising sailors who have radar beams sweeping their decks. And typically the cataracts from such radars are found on the back of the lens of the eye (whereas cataracts from sunlight exposure (ie from not wearing sunglasses and a cap/hat) are found on the front of the lens. You can imagine how a backstay (or pole) mounted radar has a beam that penetrates the back of the head and first strikes the back of the eye lenses of anyone sitting in the cockpit or working on deck, no?

Cheers

Bil

One thing i’ve used in the past on the schooners for night time radar use is a gel from a stage light - purple gel worked well to dim the screen a bit. we’d just tape it up there on the screen when needed. Cheap solution for overly bright displays. Elizabeth has a RL9 - so I’ll see how it goes!

I’ve always heard that radars are often mounted too high…something about losing your close range vizibility, but have no data to back it up. I barely ever use my radar on anything more than 3 mile range. Seems like as low as possible without frying yer brains is the way to go.

-B

Bil
Thanks very much for the info on aft and low mounted radars causing cataracts. We attended a 2 hr course on radar today and he talked about such mounted radar causing cancer and DNA damage. He was very interested in the cataracts angle, which he hadn’t heard about. Do you have a reference that I could email to him? I mounted our radar just above the upper spreaders 10 years ago in NZ and have had no complaints. I did have to chop a foot or so off the spinnaker pole and its track. No problem with the staysail.

We’re presently in Olympia, WA, working on the resteping of the mast. Dockwise from Brisbane to Ensenada, Mexico, and Dudley trucking from San Diego to Olympia all went well, with no damage. This Spring has been cooler than usual here in the Pacific NW, but we’re gradually getting used to cooler temps after 15 yrs of no winters. We’re looking forward to BC cruising this summer.

Dan

Bil
Thanks for the info on cataracts due to low-mounted radars. The instructor at a radar class I took recently had never heard of this angle, but was very interested. He did mention DNA damage and cancer due to this. If you have a reference, I’ll send it to him.

Sorry we never met up in Queensland. We’re now in Olympia, WA, after a Dudley trucking from San Diego. The trucking and the Dockwise trip to Ensenada both went smoothly.
Dan

Radar exposure can definitely cause cataracts, but there does not seem to be a consensus about small boat marine radar in normal use. Google radar cataracts.

Regarding height: Using a Garmin 18 radar as an example, the minimum range is 20 meters and the beam width is 25 degrees (half above, half below horizontal). So a height of 4.3 meters (14 ft) would give that minimum range.

Dan: Hi!

I’ve seen authoritative work on cataracts caused by radar from several sources. The earliest was from US medicos who had noted the ‘epidemic’ amongst motor boaters in SoCal and FLA.

But the easiest references to which I can point are Australian - the Aus radio authorities made a big deal back in 2001-02 about ships stations (and other transceivers) checking whether they complied with safety specifications on exposure to RF. One of the associated documents on radar, which mentions cataracts, is at http://www.arpansa.gov.au/pubs/rps/rps3_manage.pdf

Posterior subcapsular cataracts are a marker of radar-induced cataracts, but the situation is complex because some sun-induced cataracts are posterior too, due to cell migration around to the back of the lens.

I noted that you’d made Olympia when I saw Shaula’s marker move on this site’s ‘Who’s Where’ feature. Glad to hear the Dockwise trip and the trucking went well.

As you saw, Zygote sat in Gladstone Marina for one month, while we let the bizarre weather of 2007-early 2008 work itself out. One cruiser reportedly sat at anchor at Pelican Bay (inside Inskip Point) for 6 weeks waiting for fair conditions to cross the Wide Bay Bar, before turning tail back to Bundaberg.

We worked on the basis that the monsoonal trough activity had to quieten after the March equinox. We picked a more than perfect weather window and crossed Hervey Bay with light but favorable winds, had a fast tide-assisted run down the Great Sandy Strait, and crossed the Wide Bay Bar (in the company of several boats returning from the Brisbane-Gladstone Yacht Race) with less than 0.5 m of swell, only to be becalmed after 30 mins of sailing. We motor-sailed all the way from Double Island Point to Moreton Bay - glassy conditions! The racing yachts, with delivery crews on board and longer LWL than us, all motored at 6 knots, so we soon had the sea to ourselves.

In Scarborough Marina, we were met by the Galatea Appreciation Society. Seems every second live-aboard and one quarter of the weekend sailors had met Tom and Jill and admired BCC28-117 Galatea. So our washing and standing down of Z was punctuated with chats about Tom & Jill on G (G is on the hard in Trinidad, after the long passage from S Africa, while T & J are with family in Kansas), Lyle Hess, BCCs, Lin & Larry etc.

Cheers

Bil

The vertical bean width of most small marine radar units is either 25 or 30 degree,depending on the manufacturer, i.e. one half of the beam is below the array’s horizontal plane and one half above.

How high must one mount the array on the mast before the radar beam is above a crew person’s head if they were standing on the bow.

Let’s assume,

             we want the beam to be 10 ft above the bow,

             vertical beam width is 30 degrees (1/2 beam width is 15 degrees),

             "a" = height of array in feet above the deck,

             "b" = horizontal distance of array from bow,

              Tangent a = a/b where "a" is opposite the angle and "b" is adjacent to the angle,

              http://id.mind.net/~zona/mmts/trigonometryRealms/introduction/rightTriangle/trigRightTriangle.html

Therefore,

              (b X tan 15) + 10 ft = a (height of array from deck)

If the horizontal distance of the array from the bow is 15 ft and the vertical beam width is 30 degrees,

Then,

              (15 ft X tan 15) + 10 ft = 14.05 ft

Tan 15 degrees = 0.27

Tan 12.5 degrees = 0.22

The array must be mounted 14 ft above the deck for the vertical beam to clear the bow by a vertical height of 10’- mast mount.

If the array if mounted on a pole near the stern and is 10 ft above the deck, the vertical beam will sweep the plane of the deck 37 ft away from the array. Assuming the array is 28 feet from the bow and the array has a 30 degree vertical beam angle, the beam will be about 2.5 ft above the deck at the bow.

The results of the above calculations suggest the safest mounting location for an array with a vertical beam width of 30 degrees is at least 14 ft above the deck for a mast mounted array. This assuming the horizontal distance between the array and bow is 15 ft. Let’s add a 20% safety factor to this height to ensure the beam sweeps at least 10 ft above the bow. The minimum safe height to mount the array on the mast is 17 ft above the deck for the example discussed.

These calculations are for example purposes only and are not for a Bristol Channel Cutter. When I bicycle to the boat this morning, I will measure the distance from the mast to the bow and to the cranse iron.

Warning: It is recommended you do the calculations for your boat or use a scale drawing to geometrically determine the safest mounting location for the array on your boat. The vertical beam width is listed in the manufacturer’s specifications.

Fair Winds,

Rod
BCC IDUNA

Calculations for the for the Bristol Channel Cutter IDUNA:

Distance from front of mast to front of gammon iron = ~10.8 ft

Distance from front of gammon iron to back of cranse iron = ~6.5 ft.

Let’s assume a vertical beam width of 30 degrees for the mast mounted array.

Then for the beam to clear the bow by a height of 10 ft:

(Height of array above deck at the mast) = {10.8 ft X Tan (15 degrees) } + 10 ft

Height of array above deck at the mast = ~13 ft minimum.

Let’s assume the same array is mounted 20 ft above the deck. What height will the beam clear the deck at the bow.

H = (tan 15) X { [20/(tan 15)} - 6.5] }

H = 18.25 ft

The deck at the bow will be 18.25 ft below the radar beam for the parameters listed for IDUNA

This same information my be derived geometrical with a ruler and protrator and a right triangle drawn to the scale of your boat based on the location of the radar array.

Warning: These calculations are specific to the Canadian built Bristol Channel Cutter IDUNA and do not applied to other Bristol Channel Cutters or other boats.

Fair Winds,

Rod

Hi,

I was going through some photos and found one that illustrates the position of Galatea’s radar mount, which has worked well for us. As you can see it is well below the staysail halyard block, and just above the end of the spinnaker pole track. The staysail does touch when we tack, but when we retired the sail after 25 000 nm, there was no noticeable wear on the sail from crossing the dome. HOWEVER, probably less than 1 500 of those miles was to windward, so your mileage may vary.

We have a 12’ whisker telescoping whisker pole, which in retrospect, could have been a non-telescoping pole, as we have used it. Even for poling out the drifter, 12’ seems to be enough. (I did find the extra length useful last month as I was removing the bowsprit, as the marina we were at had very high concrete docks, so we moored stern to and used the dink and the “crane” to pull the sprit.)

We have a Raymarine SL72, now 5 years old, and the scanner was in for service when I took that picture. It needed new bearings, as the old ones had become corroded. Darn salt air! We were fortunate to find someone willing to help us take it apart, since a Raymarine tech had been unable to find the problem. There was only a slight catch in the rotation, but that was enough for the system to stop the rotation and give an error message. We found the bearings at a local SKF bearing shop, which is fortunate because the new scanners are not backwards compatible, and we would have been forced to replace the whole system.

Hope this helps.

Tom
Galatea
#117

Here’s the pic.

Tom:

Based on the picture of the mounting location of the array and the length of the whisker pole, I estimate the mount is about 16 ft above deck. From my calculations from IDUNA, see above, The bottom of the radar beam, is well above a crew member standing at the bow and at least 11 ft above the deck at the transom.

Rod
BCC IDUNA