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Old 07-26-2017, 02:38 AM   #1
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Visibility over the horizon

We are currently on a passage from Victoria, BC to San Francisco, and I'm trying to kill time on the graveyard shift.

We are passing occasional fishing boats coming in the opposite direction, and it's interesting to see what detects their approach first.

AIS picks them up first at about 13-14 miles. My antenna is about 35' above the water line.

But at just about the same time, say 12 miles, I can start to see their lights. They are lit up very brightly, and at first it's just a glow in the otherwise pitch black night Then I see a steady light at about 10 miles.

Last is the radar, which doesn't start to show a good return until about 8 miles. It's a very good radar, 12k open array, and about 30' off the water.

This all makes sense given the height of everything. But it's also a good illustration of what range is important on a radar. People often get in a twit over whether a radar has a max range of 24, 48, or 96 miles. Unless you are Captain Cook looking for new lands over the horizon, it just doesn't matter since you aren't going to pick up anything closer than 8-12 miles anyway.
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Old 07-26-2017, 04:11 AM   #2
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Those are interesting observations!

Like you, we use our AIS extensivly.

Radar gets set to 6 mile range when in open waters, and gets adjusted downwards from there, based on the geography and the ammount of traffic around us.
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Old 07-26-2017, 04:41 AM   #3
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I use 6 miles as well in typical open water. But now in open open water I've got it up to 12 miles.
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Old 07-26-2017, 04:42 AM   #4
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Visibility over the horizon

Oh, I picked up a more distant AIS target at about 22 miles. Turns out it was a 500' freighter, so antenna presumably much higher than the fishing boats.
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Old 07-26-2017, 05:43 AM   #5
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I did learn the expensive way (I replaced a customers entire ,brand new 25kW system, thinking it was defective) years ago that X-band radar targets are also affected by the weather ,as strange as it seems. There've been days where I've popped out of my creek on the Chesapeake Bay and couldn't see the ships channel markers 6 miles away. The very next day I could clearly see the Eastern Shore 15 miles on the other side of the Bay, ALL of the ships channel markers and even the Norfolk shoreline & Chesapeake Bay bridge tunnel 40 miles away! This on multiple boats with 4 kW open & dome radars no more than 12 feet off of the water. I normally keep mine on 1/2-3/4 mile range anyway ,for navigating.
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Old 07-26-2017, 06:51 AM   #6
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Auto controls...nice.

Auto controls..... not always so nice.

In the old days you needed a full time opetator just to kerp tweaking the radar for distant and near targets.

Now we have auto controls, but usually still not tweaked for the best picture. Usually very good, but sometimes a little tweaking brings out what you are looking for.
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Old 07-26-2017, 08:15 AM   #7
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The only time we use 24, 36 mile range is when we are monitoring squalls down here. Not that we change course to avoid that much, but nice to know how big etc. Completely agree with the 6-8 mile to aquire target, then come in from there if necessary.
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Old 07-26-2017, 08:19 AM   #8
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It's all about you antenna and the target. On the Great lakes, I've been routinely picking up the ore carriers on AIS at 50 to 75 miles on the open lakes. On the connecting waterways, 20 to 30 miles seems about the norm. While AIS is on the VHF band, it's not entirely line of sight.

Can usually pick up the ore carriers at 12 miles or more on radar, but we're still more than a half hour apart in a worst case scenario. Usually run split screen with 6 and 1 mile ranges on the open lakes and half that in the connecting waterways.

Ore carriers are big targets. This is a thousand footer (1,013') passing me in the St Clair shipping channel. The https://en.m.wikipedia.org/wiki/MV_Paul_R._Tregurtha is the largest ship operating on the Great Lakes Complex.

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More interesting when they're coming at you in the single lane areas.

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Old 07-26-2017, 08:35 AM   #9
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Here's the link for Furuno's Radar Horizon Calculator. As an example, if the unit is 25' off the water and a target is 50' off the water, the return effective visibility range is 15 miles.


Radar Horizon Calculator
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Old 07-26-2017, 08:59 AM   #10
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I agree. I often wonder why a guy on a 25-30 ft. Open Fishing Boat (Rampage, Grady White, etc) opts for a 4 ft. Open Array that is sitting on his hardtop 10 -15 feet above the water.
They are not going to get the distance they think they are going to get.

Though I wonder about clarity and detail.
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Old 07-26-2017, 09:12 AM   #11
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I believe they are using their radars to look for birds. Diving birds are on bait fish and where there is bait fish....
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Old 07-26-2017, 09:13 AM   #12
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Edit: What he said. Here's a picture of porpoises with a school of yellowfin tuna driving up the bait fish.
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Old 07-26-2017, 09:57 AM   #13
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I use a compact 24nm radar that is about 15' above the WL. Pretty useless looking for anything on the surface beyond 6-12nm. I too tend to leave it on 6 or 3nm offshore.

24nm range has one use for me: Storms. Since they are well above the horizon, I get a very good image and plan accordingly.

I think the open arrays on CC's are more about bling than function.
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Old 07-26-2017, 10:33 AM   #14
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Quote:
Originally Posted by twistedtree View Post
This all makes sense given the height of everything. But it's also a good illustration of what range is important on a radar. People often get in a twit over whether a radar has a max range of 24, 48, or 96 miles. Unless you are Captain Cook looking for new lands over the horizon, it just doesn't matter since you aren't going to pick up anything closer than 8-12 miles anyway.
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Originally Posted by Sealife View Post
The only time we use 24, 36 mile range is when we are monitoring squalls down here. Not that we change course to avoid that much, but nice to know how big etc. Completely agree with the 6-8 mile to aquire target, then come in from there if necessary.

Yeah, about the only time we use longer radar ranges is when we're looking for advance weather info. (And even then, our various phone apps are pretty good at that.)

We can see birds, but we don't usually fish in situations where birds matter... so we usually don't go out of our way to watch for 'em...

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Old 07-26-2017, 10:57 AM   #15
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Interesting!
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Old 07-26-2017, 11:06 AM   #16
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Originally Posted by psneeld View Post
Auto controls...nice.

Auto controls..... not always so nice.

In the old days you needed a full time opetator just to kerp tweaking the radar for distant and near targets.

Now we have auto controls, but usually still not tweaked for the best picture. Usually very good, but sometimes a little tweaking brings out what you are looking for.
Wifey B: Isn't that the way even televisions are? The Auto picture is like very good, but if I mess a little with these settings I can make it better.

Auto controls on radars, sonar, thermal, everything get you very close but you must know how to adjust. Biggest place I see people don't know is autopilots. Most have great sensitivity adjustment available, but people often don't know how to adjust to conditions.

Isn't it the same thing we talk about all the time that all the electronics are super wonderful but we can't just turn the job over to them. They are tools for us to use. Just like our eyes and ears are. All tools. And we need to practice and learn to use them.
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Old 07-26-2017, 01:27 PM   #17
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I agree. I often wonder why a guy on a 25-30 ft. Open Fishing Boat (Rampage, Grady White, etc) opts for a 4 ft. Open Array that is sitting on his hardtop 10 -15 feet above the water.
They are not going to get the distance they think they are going to get.

Though I wonder about clarity and detail.
I've sat just offshore BImini with a bunch of other small boats waiting for a thunderstorm to clear 35 miles away. For this passage, you need to commit to a 45-60nm transit in the thunderstorm season, and you might be in a 20' open boat. AND, only one of the pack needs radar. Cloud tops at 35,000 feet. Plug that into your range calculator. That particular day, I had XM weather, and the animation made it even more useful than radar for squalls.
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Old 07-26-2017, 07:52 PM   #18
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Larger diameter antenna equals better target separation. Is that a boat next to the seabuoy or just the seabuoy? If it made no difference, everybody would have a 12" radome. Have a 15+ year old 4' open array Furuno Navnet and a 10+ year old 2' dome tied to the other Navnet. Night and day difference between the two.

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Old 07-26-2017, 09:00 PM   #19
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Visibility over the horizon

I have a circa 1998 4' Furuno open array. I primarily use the 0.5, 0.75, 1.5 and 3 mile settings. I'm looking for immediate collision risks. Most commercial vessels of concern show up on AIS, long before they are shown on radar and I check the AIS data and forward course vector to determine when possible collision might occur. The biggest collision concern I have is with fast sport fishing boats in the fog. Most operators either don't have radar or are not paying attention.

I'm find that radar has become most useful for logs. We spotted several today long before we could see them in the drizzle and that was on the 1.5 mile setting.

I also use the 0.125 and 0.25 range settings when entering tight inlets and for anchoring.

Jim
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Old 07-26-2017, 09:36 PM   #20
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Unless your height of eye is 96 ft....

We used to look straight across to the flight deck of a CVN from a TAO...

When you can read the name tapes on a uniform they are way too close!!!
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