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> True Depth of Your Cannonball, Chart
sdcaller
Posted: Aug 15, 2018 - 08:45 pm


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Earlier this week, I got wondering about the above.

When you see a fish "hook" on the screen of your sonar , that is the true depth of the fish.

When your cannonball shows at a depth, that is the distance from your transducer.

As an example, if your cannonball is showing on the screen at 75 feet (or the counter on your downrigger shows 75 feet of wire out) and the angle on your downrigger wire is 25 degrees, the ball is only down 68 feet.

Go to the link below for a chart.

http://thescientificfisherman.com/ There is a search future at top right. Search "cannonball depth".

Also, if you do a Google search for Chitown Angler, the also have a downrigger " blow back" chart .

I printed off both charts and plan to put them in a plastic sleeve and keep them handy in the boat.

SDC

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rolly608
Posted: Aug 15, 2018 - 10:10 pm


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I have the same charts laminated in my boat as well as one that gives reference as to what colour shows its actual colour to certain depths. Wouldn`t be without them

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Dinky
Posted: Aug 16, 2018 - 08:58 am


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Interesting Ralph, but we really have not been fishing more than 80 down for salmon and the chart shows only 8 ft discrepancy ......I'm sure a salmon would chase that 8 ft to eat my hunk of metal....No?

I have always wondered the actual depth of my cannon ball........but I sometimes wonder the readings on my sonar.......I've let out 70 ft of steel line on rigger and on my sonar most times it shows pretty darn close to 70 ft......although there is an obvious blowback on the cable.....I guess the guys that have the fish hawk know for sure their true depths.....

Great topic

Thanks
Greg

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Fisherman
Posted: Aug 16, 2018 - 09:52 am


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QUOTE (Dinky @ Aug 16, 2018 - 08:58 am)
I have always wondered the actual depth of my cannon ball........but I sometimes wonder the readings on my sonar.......I've let out 70 ft of steel line on rigger and on my sonar most times it shows pretty darn close to 70 ft......although there is an obvious blowback on the cable.....I guess the guys that have the fish hawk know for sure their true depths.....

Great topic

Thanks
Greg

The way I understand it, the transducer puts out a signal that's curved, not flat, sort of like a flattened "U" , that's why you see your cannon ball at 70 feet on both the F/F and line counter on the rigger. If the F/F put out a flat signal, then yes it would account for the blowback difference. If I could only figure out how to draw and post it you would understand.

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Sneaky Pete
Posted: Aug 17, 2018 - 10:32 am


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Thanks SDCaller!
The physics/geometry are clear but I never did the math. The chart provides a quick and easy reference... printed and in the boat.

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Longshank
Posted: Aug 17, 2018 - 03:45 pm


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great topic for sure and so many variables.....charts help for sure.

things to take into account

speed
cannonball weight
cable wegiht/thickness
line weight/diameter
lead length
what you are running...eg, mag spoon/flasher fly

if you run your stuff down 50 to 60 feet, the blowback is not that severe, but once you go past 80. 90 0r 110 it is quite a difference. I have never been able to state with absolute certainty how deep my stuff is with 120 feet out on the rigger cable

again, a most interesting topic

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ldub
Posted: Aug 17, 2018 - 06:03 pm


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This is very interesting for sure. When people complain about how boring downrigging and trolling is, they certainly aren't putting much thought into it.

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Dinky
Posted: Aug 18, 2018 - 07:36 am


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Ya boring until .....zzzzzzzzzzzzzzzzzz...........reel starts screaming !....lol

And then second reels starts....while the first one hasn't stopped.........

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Grandpa Jim
Posted: Aug 21, 2018 - 01:16 pm


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A very interesting and pertinent topic. I noticed that the chart referred to states that the calculations are based on straight line measurements. This isn't really valid for rigger weight positions or depths as the cable does not descend in a straight line - the shape formed due to drag on the cable, weight, lures, etc., is parabolic. The shape and curve of the parabola or ellipse changes significantly with speed. I'm not suggesting at all that the information is not in some ways beneficial but don't take the deeper calculations as being overly accurate.
As always, Longshank is right on with his comments concerning blowback when downrigging. The other problem I have personally with the charts is estimating the angle from vertical that the cable shows. Maybe I'm just not very good at measuring angles visually but this really isn't an easy task on a lumpy day.
My curiosity often gets the better of me and I have made a number of attempts to "calculate" the true depth at which my weights are running at various speeds as well as a variety of shapes and carefully measured accurate weights. A conventional cannonball stamped 12 pounds may be considerably less heavy when measured on an accurate digital scale. My totally unscientific experimentation produced some interesting results. As with Werner, I don't worry much at all about blowback in the top fifty or sixty feet of water. The deeper weights obviously produce much more drag from the cable and attached lines. My methodology is very rudimentary but I was confident that the results were more accurate than some of the scientific calculations. I simply ran my boat at 2.3 mph speed over ground and slowly let out sufficient line to reach and maintain contact with the bottom. I repeatedly recorded the same results following the track displayed on my graph. I required at least 115 of cable to make bottom contact at a recorded bottom depth of 100 feet. The amount of cable required to contact bottom increased very dramatically as I repeated the tests in water deeper than 100 feet.
I have two dipsey rigs I use, one with wire and one with braid. The rigs are identical including the depth counter reels and the size and setting on the dipseys. Naturally, the expectation was that the wire rig would run deeper than the braid, given the same length of line out on each. I was very surprised to find that the braid hit bottom before the wire rig although the difference wasn't very significant. I tried this several times with the braid running slightly deeper than the wire on each and every occasion.
The only conclusion I could make was that the wire is thicker (hence more drag) than the braid. The point I guess I'm trying to make is that personally checking performance of your gear on your specific boat would seemingly provide a more accurate picture and record of the "question" being tested than might be found in charts. In the case of the rigger weights, knowing precisely the depth at which these are running just might enable you to put more fish in the boat at day's end.

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Longshank
Posted: Aug 22, 2018 - 09:12 am


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Jim. a great response and eloquent as always

so interested in your trials with braid vs wire dipseys. I have had lots of issues with accurate depths ever since the newer charts appeared when you purchase a dipsey diver as they are geared for 20 pound mono.....wow!

so, have been doing what you have, running various amount of line out at 2, 2,2 and 2.5 mph with braid...50/10 and wire. I compare all this to the charts and run the lines shallower until I hit bottom and makes notes. been shocked more than a few times as to how far off the charts are.

A guidline , yes, accurate , no, so experimenting is always a good thing,

Always learning old friend

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