Depth Sounders -- Beam Width

[Dockhead]

I am in the process of replacing my failed DT800 depth sounder transducer, and I'm surprised by two things:

1. That it seems to be almost impossible to find what I would have thought was the most common depth sounder transducer in the world -- the 0 degrees plastic retractable through hull NMEA 2000 Airmar DT800.

2. At what a great variety of configurations the DT800 comes in.

They come in different frequencies, different interfaces, and even different beam widths!

What is the significance of the beam width? I never thought about this before. What is a "fan beam" transducer, and is it any good for our purposes? Or is it for fishing?

Hints much appreciated!

[a64pilot]

Fishing thing, wider the beam the more fish finding coverage of course and the shallower the water it needs to work in, it’s just like a focusable flashlight, want to see any distance? Focus it to to tight beam. Fan is exactly what it sounds like.

Higher frequencies will break out better detail and bottom types etc, lower frequencies will work in deeper water. I don’t think I have that backwards.
Most bottom machines are dual frequency and I thought most xducers also. 75 and 200 Hz maybe?

It depends on the bottom machine and what you want to do with it. If you want real detail of the bottom, a chirp machine has to be seen to be believed, and they are so cheap I have a Raymarine Dragonfly for the dinghy.

However those photorealistic shots you see are in dead calm water, the machine doesn’t know the difference between the boat moving up and down in the waves or the bottom shape being really irregular.
Used to Dive old bridge spans in Panama City, the chirp on dead calm water would show them as if it was a picture, any wave action and you couldn’t even tell what they were, a bunch of wavy lines instead of straight girders.

[a64pilot]

My Zeus with its built in bottom machine is I believe dual frequency and works to well past 1000’ in low freq.
It’s a pretty good bottom machine too, surprisingly good.

[a64pilot]

Go here, scroll down and click on best of downvision shots. I can’t get it to embed with an IPad
http://www.raymarine.com.au/view/?id...=155&col=11222

[Dockhead]

OK, but this is just to feed depth data to the network - not for imaging of the bottom. Simple depth transducer. I guess I want a narrow beam for that.

[a64pilot]

Originally Posted by Dockhead

OK, but this is just to feed depth data to the network - not for imaging of the bottom. Simple depth transducer. I guess I want a narrow beam for that.

You want a narrow beam, and unless I’m mistaken run it in low frequency if all you want is depth.
However the Zeus is capable of much more, you ought to play with it. It can tell you bottom type for instance, which is useful for anchoring.
You can tell mud from rock at least.

[Dockhead]

Originally Posted by a64pilot

You want a narrow beam, and unless I’m mistaken run it in low frequency if all you want is depth.
However the Zeus is capable of much more, you ought to play with it. It can tell you bottom type for instance, which is useful for anchoring.
You can tell mud from rock at least.

Can you get simple depth data AND sonar imaging from the same transducer?

[a64pilot]

Originally Posted by Dockhead

Can you get simple depth data AND sonar imaging from the same transducer?

Of course, any depth transducer you can buy to my knowledge will be a good bottom machine, and give depth of course.
I think most are actually the same transducer, just repackaged in different housings and different wiring connectors until you get into the big, high powered ones.
A Chirp transducer has to be long, like 8 or 10’, I think and of course requires a chirp machine.
I went looking for a B&G image, didn’t find any, but mine seems to be as good as a non chirp machine, which would have made it cutting edge 10 yrs ago? It’s better than the paper chart huge bottom machine I had for fishing 20 years ago.
Of course someone who knows what they are doing can glean a lot of data even from a flasher, remember those?
The manual I think explains it pretty well if so remember
Page 55 in the manual is where it starts

[Jammer]

Originally Posted by Dockhead

Can you get simple depth data AND sonar imaging from the same transducer?

The transducer you have now is not just a transducer. It has the sonar electronics in it also, so that it can send depth via NMEA. Those are still available, but falling out of favor.


The world is going back to transducers that are, well, just transducers. The chartplotter (usually) or separate sonar module (less often) typically provide a depth output on NMEA, so yes, indirectly, you do get simple depth data. The catch is that the chartplotter or sonar module, as the case may be, has to be powered up and working.

[Pete7]

Originally Posted by Dockhead

OK, but this is just to feed depth data to the network - not for imaging of the bottom. Simple depth transducer. I guess I want a narrow beam for that.

If you want information on depths deeper than I can dive then you need the narrow beam and low frequency to punch the signal all the way down sometimes through thermoclines and back again to give you the data.

However, if you use a wider beam and a lower frequency you will capture rocks that could just be outside the immediate column of water under the boat. Could be useful giving you the bigger picture if creeping around shores with poor charts. However, to make use of this information you really need to display it at the wheel were it will be most useful. Certainly a wide beam will give a good picture down to 60m and anything beyond that you don't have to worry about. At depths less than say 30m with practise you can identify the bottom types and the difference between rock and sand easily.

Can your Zeus go to a split screen mode showing charts and a fish finder picture? Our little Garmin is very good at this and a boon for creek crawling using 50hz and wide beam cone with 1000w of power.

I think we discussed this recently but I value the depth displayed and the history given by a fish finder display invaluable and a fish finder was the first electronic item I ever bought.

[Dockhead]

Originally Posted by Jammer

The transducer you have now is not just a transducer. It has the sonar electronics in it also, so that it can send depth via NMEA. Those are still available, but falling out of favor.


The world is going back to transducers that are, well, just transducers. The chartplotter (usually) or separate sonar module (less often) typically provide a depth output on NMEA, so yes, indirectly, you do get simple depth data. The catch is that the chartplotter or sonar module, as the case may be, has to be powered up and working.

Is that why I'm finding it so hard to buy one? I haven't noticed that boats are being fitted with something different, but then I don't notice everything

I have an Actisense DST-2 unit which I use to convert analogue speed pulses from my Airmar CS-4500 ultrasonic speed log. It is capable of dealing with an analog depth sounder as well (so long as it is 200hz -- like the analogue version of the DT800). Maybe I should just go that route? There is probably no overwhelming need for me to be using a native N2K transducer, which as we see is not necessarily completely reliable.

[GordMay]

Originally Posted by Dockhead

OK, but this is just to feed depth data to the network - not for imaging of the bottom. Simple depth transducer. I guess I want a narrow beam for that.

If I understand your application correctly, you want a wider beam xducer.

The advantage of a narrow beamwidth survey transducer is the ability to "see" into narrow valley shapes thus gaining a more representative definition of the bottom surface being surveyed.

This is contrary to the navigation use of an echo sounder which has a reasonably wide beamwidth where the returned signal within the beam is the ''shoalest'' or shallowest point within the beam, obviously of more interest for hull clearance.

[Cadence]

Originally Posted by a64pilot

Fishing thing, wider the beam the more fish finding coverage of course and the shallower the water it needs to work in, it’s just like a focusable flashlight, want to see any distance? Focus it to to tight beam. Fan is exactly what it sounds like.

Higher frequencies will break out better detail and bottom types etc, lower frequencies will work in deeper water. I don’t think I have that backwards.
Most bottom machines are dual frequency and I thought most xducers also. 75 and 200 Hz maybe?

It depends on the bottom machine and what you want to do with it. If you want real detail of the bottom, a chirp machine has to be seen to be believed, and they are so cheap I have a Raymarine Dragonfly for the dinghy.

However those photorealistic shots you see are in dead calm water, the machine doesn’t know the difference between the boat moving up and down in the waves or the bottom shape being really irregular.
Used to Dive old bridge spans in Panama City, the chirp on dead calm water would show them as if it was a picture, any wave action and you couldn’t even tell what they were, a bunch of wavy lines instead of straight girders.

I loved the flash light analogy.

[Maka]

Is your reported depth the depth from the transducer. or the depth from the waterline?
Could be several feet difference on a large full keel boat.

[Cadence]

Originally Posted by Maka

Is your reported depth the depth from the transducer. or the depth from the waterline?
Could be several feet difference on a large full keel boat.

Yep, but who cares from the waterline. Transducers are seldom mounted there.
On the other hand is someone's mounted at the extreme of their keel? A little common sense must prevail.