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Using gps/sonar units to position over a wreck/reef

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Using gps/sonar units to position over a wreck/reef

Old 02-07-2020, 08:10 PM
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Default Using gps/sonar units to position over a wreck/reef

I've been researching this to death, and have found some good info, but nothing answering my specific questions.

I want to fish near shore (< 10 miles) South Carolina artificial reefs and wrecks for sheepshead, grouper, snapper and whatever else is biting, and in season, and delicious to eat. I have a 19' boat, with a Raymarine unit (Element 7HV - just purchased) that shows downvision, side vision, and 3d, plus typical fish finder sonar, and the other gps features, tracks etc. I also have a minn kota 80' trolling motor with GPS anchor lock on the bow. The water depth would be anywhere from 40' to 100' most places - Reefs like whitewater, hilton head reef, general Gordon, SAV, Beaufort 45.

When I approach the DNR provided coordinates, they are usually not exact. Plus, there are sometimes 15 or more actual choices and descriptions and locations with coordinates of what comprises the area known as the reef. I get that I have to look around a bit, and perhaps do a grid search over the area to find and get a feel for what I am looking for - whether a barge, a sunken boat, bridge debris etc.


1) when I pass something I want to fish, or see fish on the finder, by the time I mark that, or throw a marker buoy (as some have suggested) - isn't what I wanted to fish now some undetermined distance behind me? If I have the sonar set to 100% speed how can I calculate how far behind me what I saw really "was" - since it sure ain't under me by the time I see it.

2) is the answer to rewind the recorded sonar, look for what I saw and liked, mark a waypoint on THAT, and then head back to that new marked spot until the gps (zoomed in as far as possible) puts my boat icon right on top of it? is Gps accurate enough for that? I find that with gps, my boat is very difficult to get RIGHT on top of a waypoint - it would be very easy to be off by several feet. and, If I don't move much the boat becomes a black dot on the screen. Also, the gps doesn't reflect the way the boat is headed, or pointed - it's like starting a new route of navigation in a car - until you move, the satellite doesn't know which way you're facing, and your left turn should have been a right..

any other general tips for me using the equipment I have - I understand that to be 5' off can mean no bite.

lastly 3) if I catch toadfish, small black sea bass or something - is that truly a sign I'm actually in the right place? or potentially the right place? I know how finicky sheephead (for example) are when i'm fishing major bridge pilings inshore, and have a visual of the structure, and KNOW they're down there, but this is an entirely different and more daunting scenario.

appreciate the help!!

Last edited by Sailfish Greg; 02-07-2020 at 08:19 PM.
Old 02-07-2020, 08:52 PM
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Find the structure go back drift over it the mark it

When we fish a new area we search around get lay on the drift and do a longer drift and Mark with the GPS the spots.

Eventually if fish the area enough you will get enough marks.

Then you can differentiate the better ones with a number for the mark or change the symbol
Old 02-07-2020, 09:29 PM
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Change the GPS to "north up" and sound slowly once you locate the wreck, GPS is not exact, it's common to be off a bit.
Old 02-07-2020, 10:18 PM
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GPS on "north up" is the way you should have your plotter set up. . Also set your track plot speed to make marks as fast as possible; mine is set at one second I think. Also zoom way in on the plotter.

If your sonar screen is able to be scrolled back you can make a waypoint on the ledge or structure so that the mark is exact.

GPS is accurate to about 12 feet or less so drive the boat back to the mark and watch your sonar for the structure to appear. You might even make additional waypoints as you show structure on your sonar. This might help plot out the length of the ledge.

Stop the boat at the mark and then let it drift with the current to develop your drift course. This might take two or three times to figure out.

If you're anchoring, which you should if you want to catch grouper, you should go about 120 feet ahead of your waypoint on your drift course to drop the hook.

If you are lucky, your anchor will hold and then you can let out enough line to get back to your mark and hopefully the sonar will show the ledge again.

Turning your steering wheel might help you adjust the boat left or right in the current to get closer to your mark.

If all this sounds complicated that's because it is. It takes a lot of practice to get good at this skill.

So to answer your questions:

1) Yes you are actually past the structure when you notice it on the sonar. How fast you are going depends on how far back

2) Yes, use the scroll back function and make the waypoint on the actual structure..

3) Catching small fish means you are close to the right area but haven't dialed in to the good spot yet. The better fish are closer to the structure or ledge.

It could also mean that there are not any large fish where you are at because you are at a nearshore wreck fishing a public number that has been hammered by hundreds of other fisherman with a map and a chart plotter!

Last edited by Cat Tales MHC; 02-07-2020 at 10:29 PM.
Old 02-08-2020, 05:19 AM
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If you toss a marker bouy anywhere within the vicinity of the area you want to explore you now have a somewhat stationary object to use visually as a reference point in addition to the GPS position. Although tossing a marker is an extra step some don't need I found it helps me tremendously and keeps the "where the F did that go?" Stress level to a minimum
Old 02-08-2020, 05:28 AM
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Go slow, place waypoint, retrace.

As far as catching smaller fish goes, sometimes the bigger ones are slightly higher up in the column, waiting. Drop to the bottom, crank it back up a some. Mark the line. If you start getting bigger fish, don't drop below that level.
Old 02-08-2020, 05:41 AM
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Sonar is a cone, depending on depth and the width of the cone from your transducer, when you first see a mark on the screen the object could be ahead of you or off to the sides and never quite get straight under. You’ll want the narrowest cone you can for bottom fishing, and get good at reading when you are first getting the bump in the reading for structure. Idling around you can certainly be right on top of structure as you are seeing it with a narrow cone transducer. Figure out what your transducer cone angle is.

If you are checking it out for the first time, idle across it from a few directions to be sure of the spot, watching your trails on the gps.

Most depth finders I’ve used recently allow you to create a mark on a past location in the sonar by putting the cursor on the sonar mark. So even if you are past it you can still get a mark in place.

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