[Capt. Marc]

Hi,

Considering a duel frequecy fishfinder 50/200khz. The 50 KHZ has a cone angle of 36 degs and the 200 has a 12 deg.

If my geometry if right. Tan (cone angle) X the depth of water = the area of the circle under your boat the fishfinder will see on the bottom.

Lets say I am fishing 300' water.

50 KHZ would pick up fish in a circle 218 Dia (big area)

200 KHZ 12 deg cone would be 64'

So if I only care about what is just under the boat or within a short cast when fishing deep water, why do people say the 50 khz is better in this case.

It appears that the best would be a narrow cone angle and a low frequency for truely deep water fishing.

I now have a 200 khz deg cone transducer and at the same depth it is showing a 42' dia circle which is all I need to see.

?

Capt. Marc

 

[Agawan]

Frequency Differences

The cone angles are one thing. The other is what the transducer will "see". Basically the longer frequencies ie, 28 and 50 will be very good at larger objects or VERY compact schools where as the the shorter frequencies, >200 will show bait very well but may miss larger single fish. Then again "tuning" your depth finder to eliminate noise and clutter withh the proper gain is most important.

Also the longer frequencies are better for deeper water, >300 feet. The dual units are nice especially if you can run a split screen. It is amazing to see what shows up on one frequenciy yet not the other. The by how it shows up you can determine the relative size of whats lurking down below!

Dave

(This post edited by Agawan on 04/10/2003)

 

[617110]

good answer agawan

Marc,

using different frequencies is a trade off. 50khz is a higher power with less sensitivity and the 200khz is less power more sensitivity.

good question...I am kind of curious if you math is legit though!

 

[Agawan]

soh-cah-toa

Noted that as well

28 KHtz - 152' radius @300'
50 KHtz - 97' radius @300'
200 KHtz - 31' radius @300'

Also note that lower frequecies emit an oblong pulse.

Dave

 

[617110]

Actually, if we want to get into the geometry

soh cah toa is only half of it.

divide the angle that the transducer put out by half or 2, this gives you half a triangle. Solve like Marc said using [Tanget (angle in degrees)* adjacent side which is water depth] that will give you the radius of the circle under the boat.

To find the area of the cicle the formula is (A)rea = 3.14 * radius squared.

If the angle Marc said for 200khz is 12 degrees, 12/2 = 6.

(tan6)(50ft) = 5.26

(A) = 3.14 * (5.26 * 5.26) = 86.87

OK everybody; there will be an exam on this tommorow.

This was assuming fishing in 50' of water.

(This post edited by somoan on 04/10/2003)

 

[Agawan]

Duuuuuuuuuuuuh

try using 300'...........hey wait.....your answer is the same as mine.........geeeeeeeee

dave

 

[617110]

Dave

I didn't say you had it wrong. I was just trying to explain it to others who might read this thread and want to figure it out for themselves.

Hey by the way, what is the cone angle for the 28 khz?

brian

 

[Agawan]

I was just goofing off with you.

28 khtz has a beam angle of 20 X 27, quoted in airmar transducer lititature. Beam angle is 1/2 of cone.

Dave

 

[617110]

no problem

I didn't think anything was wrong...just wanted to clarify.

Anyway, I am having trouble understanding those #'s 20 x 27.

Length x Width?

 

[Capt. Marc]

Hi,

Yes checked my math.

50 khz 36 deg cone at 300' is a circle with a diameter of 195'

Now on to using a duel freq. I guess you use them on split screen for locating fish. Drive around looking at the 50khz for anything and use the 200khz until they are under the boat (in both screens).

I guess a chartplotter will work well if the scale on the GPS is up and as soon as you see something on the 50khz, you save the spot and start doing circles around that point until you locate the fish.

Is this how it is done?

Thanks for any help.

Capt. Marc

 

[Capt. Marc]

All this came up because I am thinking about the Lowrance XDF88 or XDF98.

Still trying to figure out what is the difference between the two. Same pics, Power (peak to peak & RMS), options. etc except for the extra $$ for the 98.

Capt. Marc

 

[617110]

Duel Frequency

I think you have it right. Sounds like you have a plan.

Good luck with your decision I do not know much about Lowrance. I am sure if you asked you will get many more reponses than you can handle. People have lote of opinions. You know what they say about opinions right?

I'll give you mine....go with a color scope!
Brian

 

[Capt. Marc]

somoan,

Color is the way to go when I leave the Center C. behind and move into a cabin and can justify the extra $$.

Capt. Marc

 

[617110]

Lowrance FF

Marc,

I just happened to get a West Marine circular in the mail today.

In case you did not know, the X88DF is 188W of power to 1700 feet.

The X98DF is 375W of power to 2500 feet.

Brian

 

[Capt. Marc]

Somoan,

I called lowrance on the west Marine RMS info and they said it was wrong adn refered me to their web site www.lowrance.com. Both units have the same wpoer 500w rms and 4000ptp

X98DF
Unique Standard Features:
True 500 watts RMS transmit power (4,000 watts peak-to-peak)

X88DF
Unique Standard Features:
Solid 500 watts RMS power (4,000 watts peak-to-peak)

What they said was that the 98 has a better reciever over the 88 and the 88 has incandesent lighting while the 98 has florecesent. Othere then that not much difference. I really wounder how much different the reciever is from the 88 to the 98.

Capt. Marc