This Dumb, But?

[JOHNS]

Sitting In Front Of Boat In 10 Ft Of Water, Have Dual Beam (20/60 Degree) Humminbird And Showing Fish.

Only Second Time I Have Used It.

How Do I Know In What Direction And How Far From The Boat They Are?

Also Is There Any Way To Tell If They Are Crappie.

Thanks

Johns

[Humminbird_Greg]

The sonar beams from the transducer on your 'Bird are conical in shape. This means that there is no directional information that can be gained from them other than that the fish is inside of the narrower 20 degree sonar beam and/or wider 60 degree sonar beam.

If there are being shown within the narrower 20 degree beam than they should be within a 0.35 X depth circle directly under your transducer (at the depth they are shown). Example: if a fish is shown at 10 feet deep than it is within approximately a 3.5 foot diameter circle under your transducer at 10 feet deep. Half that diameter to get the radius (ah the heady days of high school geometry...) which tells you that fish can be up to 1.75 feet away from a point that is 10 feet directly below your transducer.

The wider 60 degree beam uses a 1.0 depth multiplier to get the size of the sonar beam at a particular water depth. In the example above that fish at 10 foot could be within a 10 circle or up to 5 feet away from a point that is 10 feet directly below your transducer.


Okay, the purists out there will shoot holes in what I just stated and they would be correct. A depth sounder does not show the depth of an object but instead the distance from the transducer to that object. If something is shown 10 feet deep it is really 10 feet away from the transducer. If the transducer is is mounted on your boat so that it is 1.5 feet below the surface of the water; the object shown at 10 feet could actually be 11.5 feet deep as measured from the surface of the water (not the transducer). You also need to take into account that the distance measurement from the transducer really shows the distance in an arc and not a straight line. The wider a transducer's beam is the greater the arc. A fish shown at a 10 foot distance in a 20 degree transducer beam in reality could be anywhere from 9.85 to 10 feet deep (assuming that the transducer is on the surface of the water and not mounted below it). The same fish shown at a 10 foot distance with a 60 degree transducer beam could be anywhere from 8.82 to 10 feet deep (again assuming that the transducer is mounted on the surface of the water and not below it).

[willfish4food]

Now I am really confused.

[CatFan]

Now I am really confused.

I'm going to say the same thing Greg did, I'm just going to use some different words to see if it's clearer.

Imagine you are shining a flashlight straight down. The beam of light gets larger the farther from the flashlight you go. The area the beam covers will look like an upside down snowcone cup. The deeper the water, the larger the cone. Anything that is anywhere within this cone will show as a return on the display, and the only thing you know is how far it is from the transducer.

The transducer cone angle is a measure of how quickly the beam gets wider as it gets further from the transducer.
For a 20 degree transducer, your cone's diameter on the bottom will be .35 times the water depth, so at 10ft depth, the cone will be 3.5ft diameter at the bottom or at 20ft of depth the cone will be 7ft diameter at the bottom.

For the 60 degree transducer, the beam is quite a bit wider. The beam is about 1.1 times the water depth, so in 10ft of water, the beam will cover an 11 ft circle on the bottom. In 20ft of water, the beam will cover a 22ft circle.

This cone shape means that if you are in 20 ft of water and see a fish at 10ft depth on the 20 degree transducer, it is within 1-3/4 ft of directly below the transducer, but you have no way of knowing which direction.

Apologies to Greg, but Lowrance has an excellent tutorial on their site at http://www.lowrance.com/support/Tips...utorial_01.asp
that is very informative.

[WarrenMN]

Greg and Cat are right but one thing about the cone "every one" should remember is that it only represents the half power point or you might call the sweet spot. It can and does read things out side that cone but how far out depends on how big and how reflective the object is. A large northern will easily read outside the cone. Some thing metallic or objects shaped right can read quite a ways out side the cone. If you've ever gone by one of the continuous piers, not multiple pilings, you'll get a false reading way higher up on the pier off to the side than it actually is. I wouldn't be surprised if it returend a depth read about the same horizontal distance it is from you, or higher.

One other thing people say I get a kick out of is, you could see the fish come up to the bait. Times what happens is the fish is on the same level as the bait and it cruises over to check it out. What appears as "coming up" is cause by, lets say the crappie is at 10 foot, when its close enough to the sonar to show, its on the out side of the cone. That slant distance from the crappie to transducer is longer than than the straight down distance to that same level of 10 feet where your bait was. So it makes it look like the crappie is swimming up when its just swimming over at the same level.
Thats not saying they don't come up, but you should think it more of some kind of combination. Dropping your bait down to the level you thought the fish started from could be a big mistake, putting the bait below it.
Doc goes into this in his understanding sonar video, but thats some thing all the walleye guys know him from, his DVD's and fishing pro tournaments. I wanted to add to that you might have seen Bruce's articles in all most every national fishing magazine or from the Lowrance classes he's know for.
WarrenMN

[Team9nine]

One other thing people say I get a kick out of is, you could see the fish come up to the bait. Times what happens is the fish is on the same level as the bait and it cruises over to check it out. What appears as "coming up" is cause by, lets say the crappie is at 10 foot, when its close enough to the sonar to show, its on the out side of the cone. That slant distance from the crappie to transducer is longer than than the straight down distance to that same level of 10 feet where your bait was. So it makes it look like the crappie is swimming up when its just swimming over at the same level.
Thats not saying they don't come up, but you should think it more of some kind of combination. Dropping your bait down to the level you thought the fish started from could be a big mistake, putting the bait below it.

BINGO! :D :D I've tried explaining this to some of the guys over on the bass websites too, that graph all these suspended spotted bass and have arching "spaghetti" all over the screen.

[WarrenMN]

If you've never heard of Doc or me

http://www.usatoday.com/tech/news/te...-walleye_x.htm

or Doc's site

www.hightechfishing.com

WarrenMN

[Team9nine]

Good to finally meet you Warren, albeit virtually :D I've conversed with Doc a few times on other sites...have the tapes, studied the man and his methods

Between you 2, Catfan and HGreg plus Canepole, this sites becoming a pretty good place for some high tech knowledge :p

-T9

[PanMan_VA]

Greg and Cat are right but one thing about the cone "every one" should remember is that it only represents the half power point or you might call the sweet spot. It can and does read things out side that cone but how far out depends on how big and how reflective the object is. A large northern will easily read outside the cone. Some thing metallic or objects shaped right can read quite a ways out side the cone. If you've ever gone by one of the continuous piers, not multiple pilings, you'll get a false reading way higher up on the pier off to the side than it actually is. I wouldn't be surprised if it returend a depth read about the same horizontal distance it is from you, or higher.

One other thing people say I get a kick out of is, you could see the fish come up to the bait. Times what happens is the fish is on the same level as the bait and it cruises over to check it out. What appears as "coming up" is cause by, lets say the crappie is at 10 foot, when its close enough to the sonar to show, its on the out side of the cone. That slant distance from the crappie to transducer is longer than than the straight down distance to that same level of 10 feet where your bait was. So it makes it look like the crappie is swimming up when its just swimming over at the same level.
Thats not saying they don't come up, but you should think it more of some kind of combination. Dropping your bait down to the level you thought the fish started from could be a big mistake, putting the bait below it.
Doc goes into this in his understanding sonar video, but thats some thing all the walleye guys know him from, his DVD's and fishing pro tournaments. I wanted to add to that you might have seen Bruce's articles in all most every national fishing magazine or from the Lowrance classes he's know for.
WarrenMN

I don't want to start an argument, but different manufactures specify cone angles at different reductions in full power level (i.e. -3dB for some and -10dB for others). The -10dB case is less than 1/2 power.

In order to compare cone angles, they must be calculated at the same power level. I'm not going to attempt to discuss Peak vs. RMS power levels.

[WarrenMN]

PanMan PA, your right too, its just been for a long time the -3bd or half power point has been an accepted standard. Unfortunately there are different ways to measure and compare.
One thing too that Bruce talks about that throws a monkey wrench into it all. You can change the cone width yourself on any sonar by simply turning the gain up and down. Then we joke about the transducer I was using for mapping with my Simrad before I went to the Humminbird 997 this summer. Its about the size and shape of one of those old irons they put on the wood stove to iron clothes. It's "average" cone is 3X5 degree for 200 khz and I think it weights 9 lbs with out the bracket. Bruce asked me one day how well it shows fish. I told him the cone width is so small and I usually keep it the narrowest it can be by turning down the gain that to see a fish it almost has to come up and kiss the transducer. I can see a pimple on the bottom of the lake but I usually didn't see fish. I think its a foot diameter for ever 10 or some thing like that, and thats if I don't turn the gain down to shrink it more. More like a pencil than a cone.
As for coming here, its nice to get around and fun to meet new people. The DrDepth question drew me in but when Humminbird Greg speaks I always stop to listen, he knows his stuff. He just had another good idea the other day. Try using Marine double back tape to temporarily mount a side scan transducer different places on the transom before you screw down to find the best spot and thats likely a good idea with any transducer.
WarrenMN