New fishfinder - dual or single frequency?

[Janos]

Well Martha, the much loved club rib, needs a new fish-finder. We can get a bog standard one for around £80 or £90, and a fancy dual-frequency with colour screen for around £300 to £400.

My question is: Are the fancier models worth it? And can anyone give any good recommendations.

Cheers,
Janos

[Nigel Hewitt]

My question is: Are the fancier models worth it? And can anyone give any good recommendations.I thought the trick was that higher frequencies are more directional so better in deeper water. The low frequency is a wide scan system to say 'fish in our vicinity'.

I think the two apertures are about 40 degrees and 15 degrees and if you're trying to drop a shot on something you want narrow.

Or are you going fishing?

[Janos]

I thought the trick was that higher frequencies are more directional so better in deeper water. The low frequency is a wide scan system to say 'fish in our vicinity'.

I think the two apertures are about 40 degrees and 15 degrees and if you're trying to drop a shot on something you want narrow.

Or are you going fishing?

I thought that the dual frequency models (different to twin beam) allowed you to know the composition of the bottom?

Janos

[David G]

I think it will depend on what the single frequency actually is.
I believe that units use either 50kHz or 200kHz and my understanding [1] is that the higher the frequency the better the detail but the less penetration you will achieve.
Therefore a 200kHz will give better resolution of the seabed at shallower depths than a single 50kHz unit.
You may also wish to look at the beam angle to see what sort of coverage you can expect at a particular depth. The narrow the beam the better the differentiation of seabed items.
I think that seabed composition (ie rock or mud etc) is determined by strength of echo received. The unit knows the depth by the time it has taken for a transmitted pulse to be received back after it has bounced off the bottom, it then knows how much of its signal strength has been absorbed or reflected by the surface it hit (mud a lot, rock less).

As a slight aside I notice that the newer Garmin Chartplotters (450s, 550s, 555s) which are combined GPS/Fishfinder units, have a built in detailed map so you dont have to buy the cards, but will accept a card which gives you 3D info on the seabed....looks really impressive, but of how much use in practise I don't know.

HTH a little.

David

[1]9 years in medical diagnostic ultrasound service support ;)

[David G]

Have a look at the information here:

http://www.fishfindercity.com/fitiandin.html

Regards,

David.

[GarethJ]

Hi Janos, I went through this deliberation early last summer when my Lowrance X85 unit packed up. The Lowrance was a black and white, single frequency unit runing a transom mounted tranducer. I asked around but didn't really get any useful feedback as to what you would actually need to find wrecks up to 80m or so. I eventually replaced it with a Koden unit designed for commercial fishing. This is high power, dual frequency, colour, through hull transducer, and has many fish finding features that are common on commercial fishing units such as bottom track, bottom zoom etc. As I run a 32ft stern trawler as a dive boat, it worked out ok. However, after comparing the two, my take on this is -

Transducer power is crucial. If you want to sound 50m wrecks that are flattened, then don't buy a unit with less power than a Lowrance X85. Many times this unit easily out performed other sounders on RIBS in the rather murky West Wales water off Pembrokeshire and never failed to identify small targets.

Colour does not really help in identifying wrecks or fish. The monochrome works just as good.

Dual frequency is helpful at times. I can switch the Koden into two screen views showing high and low frequency simultaneously. This sometimes helps pick up difficult targets, but is certainly not essential.

Bottom zoom is a must.

So a black and white, single frequency unit will do, assuming power at least as good as X85.

Hope this is of some help,
regards,
G