Thread: 400hz vs. 800hz si

800kHz will have more detail with less range ...for stuff close to the boat ...

455kHz will have more range with less detail ...for stuff farther from the boat ...



Rickie

I don't know about SI, and I would defer that to "rnvinc" for his take.

But, when it comes to DI ... as you can see in the picture above ... detail is much better in 800kHz, especially as it defines the fish (white marks) suspended above this dropoff.

I've heard that the 455kHz signal is better in deeper water ... so that may translate to using it when your SI is set for 1/2 to full distance ... and using the 800kHz when your distance settings are less than 1/2 maximum distance. You'll likely have to tweak your sensitivity settings for each kHz setting, as well, so you don't wash out the images or lose them against the background.

Originally Posted by rnvinc

800kHz will have more detail with less range ...for stuff close to the boat ...

455kHz will have more range with less detail ...for stuff farther from the boat ...



Rickie

Is there a "magic" distance for the 800 say 50'? Or depth?

Originally Posted by Tristan Bladon

Is there a "magic" distance for the 800 say 50'? Or depth?

Environmental variables will play a large part in range capability of each frequency ...

A hard gravel bottom in deeper water may reflect more sound than a mud bottom in shallower water ...
----

Deciding which frequency to use when should be determined by the task at hand ...:

If one wants to see detail of stuff close to the boat = use 800kHz ...

If one wants to find new stuff farther away from the boat = use 455kHz - then drop a waypoint on it and reposition the boat closer to scan it again with 800kHz to see the detail of it ...

Rickie

I understand in Di the difference in 455 and 800 because it changes the size of the cone but in si which is supposedly (as I understand it but I'm still learning) is a thin slice like beam that you are determining how far to the left or right it shoots how it makes such a difference since your not changing the size of a cone.

I think I will try the 800hz on my bow unit. I like the 455hz. best so far on my console unit for finding stuff so far. I have not been to satisfied with the bow unit so far as I troll at much slower speed. Maybe the 800hz will give me better and clearer picture. I am running a 899ci HD SI. Question is what do most of you all use to get clear pictures on your bow sonars while slow trolling?

Originally Posted by scrat

I think I will try the 800hz on my bow unit. I like the 455hz. best so far on my console unit for finding stuff so far. I have not been to satisfied with the bow unit so far as I troll at much slower speed. Maybe the 800hz will give me better and clearer picture. I am running a 899ci HD SI. Question is what do most of you all use to get clear pictures on your bow sonars while slow trolling?

I almost always use 800 on my bow unit for di.

Originally Posted by ccollins

I understand in Di the difference in 455 and 800 because it changes the size of the cone but in si which is supposedly (as I understand it but I'm still learning) is a thin slice like beam that you are determining how far to the left or right it shoots how it makes such a difference since your not changing the size of a cone.

This is how I understand it ...
In DI the single beam is directed downwards (only), and the difference between the two kHz settings is the angle spread (45deg & 75deg). When using SI, the beam is projected from both sides of the transducer (and also down), so the 455kHz spread is nearly 180deg (greater distance coverage), while the 800kHz spread is over 100deg (better defined images within the beam).

And I believe the actual degrees spread is actually slightly different, by a few degrees, depending on which brand of depthfinder you're using.

But, as always, I defer to Rickie for clarification and/or correction.