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I have been using my fishfinder to locate structures on the bottom such as holes and ledges. My question is, how far back behind the boat is the stucture that the screen is showing? When I anchor I always wonder if the hole I am seeing on the screen is right behind the boat, or 20 or 30 feet behind the boat. So even though I've found a ledge or hole, I don't really know if my lure or bait is really over it. I assume that most brands and models would show about the same distance behind the boat -- mine is a Raymarine L-470, and the owners manual doesn't say anything about how far behind the boat that the screen image is.
 

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The image on a fish finder depends on your transducer. The transducer should be pointed straight down, for most types. There are a few that show images to the front or sides, but they are rare. The view is cone shaped. It can be anywhere from 8º to 42º and obviously, the deeper the water, the wider the coverage of the bottom.
 

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Transducers look down in a cone shape. Find the angle of your transducer and you can determine how much bottom you are looking at as long as you know the depth. It is not looking so much behind the boat as below, in a cone shape.
 

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I'm not an expert but your transducer is reading in the direction it is pointed. When anchored, your looking directly under it. When under power, your speed determines how far back your seeing. My finder actually shows the distance. The slower you go, the closer to the boat you are seeing. There is a mathmatical calculation but I don't know what it is. Some one else can probably tell you.

Chris
 

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Unless of course you have a Fishin Buddy....120ft behind the boat...works great back trolling plugs in smaller rivers...lets you see what you are backing into....plus you can turn it to look towards the bank on either side of the boat...helps pinpoint the fish. It can be used with a bottom screen at the same time so you can track bottom fish as well as surface or suspended fish. Made by Bottom Line. Russ
 

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Figure you have a beam angle most are around 21 degrees at mid sensativity. As the hole or structure enters your screen your just entering your beam area. In 30' of water with a 21 degree cone angle you see 23'of the bottom. Here is a link to a right triangle solver so you can play with cone angles and bottom coverage.

Here is also some great information from Lowrance on how fish finders work, cone angles, fish arches and so on. This page is about cone angles.

Lowrance info (good stuff)

Angle solver

[ 07-16-2003, 07:27 PM: Message edited by: fish_on ]
 
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