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Old 16-01-2019, 13:49   #31
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Re: REEFS, & THE LIKE CRASH AVOIDANCE, DETECTION!!

Used to be mast steps and a sharp-eyed crew. Now we have FLS (which you need to practise with, just like radar) and proper charting that acknowledges WHERE the data supplied by Capt. Cook might be a predictable amount off. We also have, with boat-launched drones, a way to see trouble ahead (in the right conditions). There's even the idea of sending your tender ahead with a glass-bottomed box or one of those hand-held depth finder. I would suggest use one or more of these as needed.
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Old 16-01-2019, 13:53   #32
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Re: REEFS, & THE LIKE CRASH AVOIDANCE, DETECTION!!

FLIR isnít going to work, FLIR of course is a heat detector, everything in the water that isnít giving off a substantial amount of its own heat, wonít be seen as if there is any temp difference, it will be within the minimum resolvable temperature.
FLIR may be good for finding a Man overboard, but not a reef or coral head.
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Old 16-01-2019, 14:31   #33
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Re: REEFS, & THE LIKE CRASH AVOIDANCE, DETECTION!!

I note that submarines, with literally billions of dollars expended on equipment and underwater surveys for their charts, still occasionally run into underwater seamounts. Perfect solutions are hard to come by.
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Old 16-01-2019, 15:22   #34
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Re: REEFS, & THE LIKE CRASH AVOIDANCE, DETECTION!!

Beat me to it, my wife and I split time in the spreaders negotiating passes and maneuvering inside the reef. Only way to avoid coral heads and shallow water.
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Old 16-01-2019, 15:22   #35
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Re: REEFS, & THE LIKE CRASH AVOIDANCE, DETECTION!!

I prefer visual from the spreaders and last steps make this easy
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Old 16-01-2019, 15:27   #36
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Re: REEFS, & THE LIKE CRASH AVOIDANCE, DETECTION!!

Exiting a small atoll, one dawn, when the Polarised Sunnies werenít cutting it, we sent my sister ahead on a paddle board. She was spot on!
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Old 16-01-2019, 15:42   #37
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Re: REEFS, & THE LIKE CRASH AVOIDANCE, DETECTION!!

There is no real protection from semi-submerged shipping containers. I hit one off Freeport harbor in 2004 that was not visible above the water. Reefs are usually well marked on most all major GPS chartplotter maps especially along general navigation route. Out of the travel lanes the barrier reefs are generally marked as broad areas such as "uncharted or not surveyed".
I have been traveling south Florida and the Bahamas for 5 decades starting with paper charts, loran A, then loran C, then GPS. It still amazes me that my Garmin GPS chartplotters and Open CPN charts have the detail that they do. They regularly show a marked isolated rock and sure enough look over and "there it is". Sometimes both set of digital charts show a structure but if not at least one of them does. I like two sets of data to use when possible in touch waters.
On night crossings from FL to Bahamas I stay to the well marked and traveled areas across the bank and only travel reef or even broken reef areas in "suitable daylight". Suitable daylight may not always be determined by a clock. Going East in the morning or West in the late afternoon may not qualify as "suitable daylight" in some waters.

Always plan your travels with things like this as ONE of the deciding factors in your safety plan.
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Old 16-01-2019, 15:48   #38
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Re: REEFS, & THE LIKE CRASH AVOIDANCE, DETECTION!!

I like my forward looking sounder, some places have not clear water, saved my daggerboards several times, but yes, speed = walking speed.

Satellite imaging with chart overlay is excellent to avoid reefs and uncharted obstacles like 20 year old container ports navionics refuses to chart.

Wherever I sail, i download the pictures and create overlays on the chart, chart alone does not help between coral stock

Look for openCPN.org (free program, pc & android) and for ge2kap (Google Earth 2 kap-file, from mr. Higgings). There is a huge section here on cruisersforum too

OpenCPN can display radaroverlays too as well as ENC charts. In US waters all charts are free anyway. CM93 charts, little outdated, but often used by the cruising community, can be displayed too
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Old 16-01-2019, 16:19   #39
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Re: REEFS, & THE LIKE CRASH AVOIDANCE, DETECTION!!

I have spent a fair amount of time researching this. This is what I plan.

This is my solution.... https://www.navionics.com.au/sonarchart-live Hop in the tender and go and map the anchorage or follow the tender up the river or through the bar or into an atol. It is a pita but it is a 100% solution where all others are less than.

I will also permanently mount a camera to the mast top spreader and keep satellite images of the areas. Maybe a drone. This should reduce the amount of times I have to put the tender in.

I looked at forward facing sonar and decided against it, even the manufacturers best attempts at trying to make it look good, didn't.

I'm sure it is a valuable tool in skilled hands, it was too valuable for me and I didn't think I'd spend enough time getting to understand it fully to get the full benefit.
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Old 16-01-2019, 17:09   #40
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Re: REEFS, & THE LIKE CRASH AVOIDANCE, DETECTION!!

Does anyone use a drone for examining reef areas? I know they don't have long battery life times, but they can provide a vertical view.

Polarized glasses remove reflected light which has been polarized during the reflection process. Commonly called glare. However, no matter how good the glasses are, the effectiveness depends absolutely on the angle of reflection. (see Brewster Angle) But better than nothing. Many good eyes, good knowledge, very slow speed, check out area with dingy, follow a deeper draught boat with local knowledge. Don't be an idiot.

However, care is the word. There are bold sailors, there are old sailors, but there are few old, bold sailors.

incidentally, recently an acquaintance removed their forward looking transducer. It was on the bow stem about a metre down. It had been impacted by an object and was damaged and nearly ripped off the hull. He also said it was often misleading and tricky to interpret.
Many years ago I ventured into a very small bay on the Cobourg peninsular (near Blue Mud Bay). This bay had coral columns all over the place but I had been in with a dingy earlier and had put down a marker buoy.
At the right tide I entered the bay and put down a pick. We put on our anchor light and after checking further we went to sleep.This bay is right next to the cruising boat path and to our sheer horror a yacht was motoring flat out into the bay (we could only see his nav. lights). He did not respond to radio calls. Did a fast circuit around us and dropped an anchor. In the morning when the tide was low, we were surrounded by these ugly rock columns. The visiting boat pulled anchor and motored out at speed. We just had no idea how he had survived so far.
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Old 16-01-2019, 17:48   #41
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Re: REEFS, & THE LIKE CRASH AVOIDANCE, DETECTION!!

Our boat used to have interphase forward sonar but apparently it was next to useless, possibly because we do 5+ knots just in gear.
Head unit died and it was never replaced though we still have the box behind dash and house brick sized transducers on the hull.

Now we just have an old but very loud numbers only sounder and a koden colour fishfinder but of course neither will show up the 100ft to 5 ft reef edge.

Height of wheelhouse gives us a superior view approaching shallows.
Anything murky and we anchor out and scout in the dinghy which has a chart plotter/sounder
Opencpn and kap overlays are invaluable as well.
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Old 16-01-2019, 17:50   #42
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Re: REEFS, & THE LIKE CRASH AVOIDANCE, DETECTION!!

In Fiji, where the charts are unreliable at best, I followed these rules:

No sailing unless the sun was overhead or behind, but still up in the sky.

Polarized glasses, for everyone involved, are a must.

In close quarters, someone must be at the bow or on the mast, or in a dinghy ahead (with a handheld depth gauge and portable VHF).

Speed is your enemy.

Even with these precautions, I came close few times.
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Old 16-01-2019, 20:01   #43
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Re: REEFS, & THE LIKE CRASH AVOIDANCE, DETECTION!!

For poking around anchorages at slow speed we really like our forward looking sonar. It is very effective at slow speed - that's the speed we travel when trying to find an anchorage spot in a tricky, poorly charted area. It is no use at 6 knots other than as a backup to our standard depth sounder. Its an Echopilot.
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Old 28-01-2019, 11:27   #44
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Re: REEFS, & THE LIKE CRASH AVOIDANCE, DETECTION!!

+1 for using Navionics Sonar Chart paired with a good bow watch and good sun up high.

I was amazed at the detail available everywhere in the Bahamas or Great Lakes on Navionics.
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Old 28-01-2019, 11:40   #45
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Re: REEFS, & THE LIKE CRASH AVOIDANCE, DETECTION!!

Quote:
Originally Posted by Svan View Post
+1 for using Navionics Sonar Chart paired with a good bow watch and good sun up high.

I was amazed at the detail available everywhere in the Bahamas or Great Lakes on Navionics.
Pretty lines on a screen don't necessarily mean reality in real life
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