Radar Reflector Requirement

[Ergo]

The following is a revision to the radar reflector requirement:

Equipment Requirements 4.19: A PASSIVE RADAR REFLECTOR PROPERLY MOUNTED AT LEAST 13 FEET ABOVE THE WATER. IF A RADAR REFLECTOR IS OCTAHEDRAL IT MUST HAVE A MINIMUM DIAGONAL MEASUREMENT OF 12 INCHES. IF THE REFLECTOR IS NOT OCTAHEDRAL, IT MUST HAVE A DOCUMENTED AVERAGE RADAR CROSS SECTION (RCS) OF NOT LESS THAN 1.5 METERS SQUARED OVER 20 DEGREES OF HEEL.

This revision is unofficial until Synthia comes back from Kauai and it is posted on the race web site but she has authorized this change.

Bill Merrick

[jbarthelmass]

If I understand this right, once the official word comes down, then the Mobri S-2 would now comply with the rules, as its published RCS is 2.0 sq meters?

[tiger beetle]

If I understand this right, once the official word comes down, then the Mobri S-2 would now comply with the rules, as its published RCS is 2.0 sq meters?

Hi John -

yes, at 2 square meters RCS, as documented by the manufacturer, the Mobri S-2 would meet the new rule as stated by Bill on behalf of Synthia.

I would caution you, however, that the Mobri tube-style reflectors have scored the worst in at least two radar reflector tests. Here is one done in 1995 with Stan Honey:
http://www.ussailing.org/safety/Studies/radar_reflector_3.htm#The%20Results

And again in 2007 by Qinitiq, which states the 4" tube reflector performed very poorly (refer to table on page 18 of their study).
http://www.ybw.com/pbo/pdfs/radar_reflectors.pdf

I would recommend reading both studies (I learned a lot about radar reflectors!) and then make a decision.

- rob

[Alchera]

If I understand this right, once the official word comes down, then the Mobri S-2 would now comply with the rules, as its published RCS is 2.0 sq meters?

Actually, no, it doesn't comply according to the test results. The requirement specifies an *average* RCS no less than 1.5 meters squared *over 20 degrees of heel*. The published number from the manufacturer is probably a maximum RCS over 0 degrees of heel. You can't use the published number from the manufacturer to meet the requirement unless the published figures specifically state that it is an average over 20 degrees of heel. Or, you can find an independent test which supports your position. The 2nd reference from Tiger Beetle seems to be a good one.

However, I doubt if you will find any data that will confirm that the Mobri meets the requirement. If you look at the test results for the Plastimo 4" tube, which is a similar design and yet twice as big, it does not meet the requirement. In fact, the Davis barely meets the requirement when you use the average return and also factor in the 20 degrees of heel.

- Mark

[tiger beetle]

Actually, no, it doesn't comply according to the test results. The requirement specifies an *average* RCS no less than 1.5 meters squared *over 20 degrees of heel*. The published number from the manufacturer is probably a maximum return over 0 degrees of heel.
- Mark

Oops - I missesd the over 20 degrees of heel part of the new rule - thanks Mark!

John - I retract my earlier statement. Based on reading the two studies, I would not accept a single Mobri 4" radar reflector to meet the rule. Based on average RCS from the Qinetiq study, and Stan's point that radar reflectors are additive, you could festoon the rigging with a whole bunch of them and probably pass. I would guess you're looking at installing 15 4" diameter reflectors if the average RCS at 10 degree heel is 0.10 square meters.

Mark - when the rule states "AVERAGE RADAR CROSS SECTION (RCS) OF NOT LESS THAN 1.5 METERS SQUARED OVER 20 DEGREES OF HEEL" - how is that being calculated? Are we looking for that mininum average RCS at any point heel angle from 0-20 that is >= 1.5 square meters, are we averaging the measured average RCS determined at various heel angles, or some other method?

Based on the Qinetiq study, there are four reflectors that would meet the new requirements if they were the single reflector on the boat:

Plastimo 16" octahedral
Davis Echomaster (12") octahedral
Tri-Lens Large & Standard

The Tri-Lens seems to be the best of the lot. Too bad they are large and expensive. On the plus side, they have a smooth plastic case which might alleviate Warrior's chafe problem should he elect to not carry the navigational buoy for the race.

- rob

[jbarthelmass]

Thanks to all for the clarification. I'd already bought a new Davis Echomaster, but it'll be a PIA to install, as I worry about chafe also. The Mobri was very tempting........

I'm looking at pole mounting the Davis at the stern (13' above the water, of course), and will now continue in that direction.

[Alchera]

Mark - when the rule states "AVERAGE RADAR CROSS SECTION (RCS) OF NOT LESS THAN 1.5 METERS SQUARED OVER 20 DEGREES OF HEEL" - how is that being calculated? Are we looking for that mininum average RCS at any point heel angle from 0-20 that is >= 1.5 square meters, are we averaging the measured average RCS determined at various heel angles, or some other method?


Well, the statement of the requirement is ambiguous, isn't it? I know what *I* was thinking when we discussed this - I was thinking that the minimum average RCS at any given heel angle should not be less than 1.5 square meters. However, as it is written (or possibly *will be* written), it allows for some angles of heel to be less than 1.5 so long as the average over all angles measured is greater than 1.5. That's clearly a less stringent requirement. Oops.

- Mark

[seabird51]

Well, the statement of the requirement is ambiguous, isn't it? I know what *I* was thinking when we discussed this - I was thinking that the minimum average RCS at any given heel angle should not be less than 1.5 square meters. However, as it is written (or possibly *will be* written), it allows for some angles of heel to be less than 1.5 so long as the average over all angles measured is greater than 1.5. That's clearly a less stringent requirement. Oops.

- Mark
There has to be some benefit for having a large aluminum mast. When I have had others with radar on, on the vhf there is no difference with my return echo with and without the Mobri or Davis Echomaster deployed. Are we splitting hairs here.
Hello to all and my check is in the mail.
Lou

[haulback]

probably not Lou, otherwise those of us who are quite happy as we are now configured (and with smaller mast sections) would be able to be remain as we are.

Shipping paper towels again this year???? Why not some of the ice cream, or frozen steaks??

I am sure they are heavier than 4 rolls of paper towels.......

Looking fwd to examining your transom from an ever-increasing distance again this year.

Jim/Haulback

[Jeff Lebesch]

What about the SeaMe active radar reflector, at about 8 square meters average cross sectional area at 20 degree heel? The performance is considerably better than any of the passive units, but the rule does state "passive."

[Alchera]

What about the SeaMe active radar reflector, at about 8 square meters average cross sectional area at 20 degree heel? The performance is considerably better than any of the passive units, but the rule does state "passive."

I believe the idea behind requiring a passive reflector was that it still continues to work just fine even if the power fails on the boat for whatever reasons, and also there's no temptation to 'turn it off' to save electrons. Nothing prevents a competitor from supplimenting the passive radar reflector with an active radar transponder like the SeaMe.

- Mark

[Warriors Wish]

FYI, Warriors Wish will use a Davis Echomaster hung from the windward spreader to avoid chafe. Just one more thing I have to do when I gybe. Thank You for all of the info posted to this site.