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Old 01-06-2018, 13:56   #16
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Re: Depth Sounders -- Beam Width

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Originally Posted by Cadence View Post
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.
I think the question was what is the REPORTED depth, not measured depth.

Everyone has his own preference. I have by long tradition kept the main depth sounder set to depth below the keel, and the forward looking sonar set to absolute depth (from the waterline). My FLS is presently not working, but I will try to get it going again this week.

Everyone here does understand, I presume, about setting depth offsets?
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Old 01-06-2018, 14:44   #17
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Re: Depth Sounders -- Beam Width

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Originally Posted by Dockhead View Post
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!
Fan beams are better for sailboats; they limit the fore and aft beam width and widen the starboard and port beam width. The narrower the beam the more it is affected by heel. The wider beams give you better reflection. For example if you use a transducer with a 7 degree beam you will likely not see any depth at all when you are heeled over. But with a 44 degree beam you will have an accurate reading at most any level of heel.

Lower frequency provide more penetrating power for seeing deeper bottoms. Higher frequency provide more detail but less depth.
NMEA transducers are typically 235 kHz
Analog depth finders transducers are typically 150 -200 kHz (200 kHz is more common)
Standard fishfinder transducers are typically dual beam (like 50/200 kHz)
CHIRP transducers have variable frequency

Tell me what interface you are using and what you are connecting it to and I can recommend a transducer. I have a few decades experience with transducers and the latest Airmar dealer catalog sitting in front of me.

One of these days when I find the energy I am going to write a book on transducers or maybe just a really long post.
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Old 01-06-2018, 14:59   #18
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Re: Depth Sounders -- Beam Width

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Originally Posted by KeelMe View Post
One of these days when I find the energy I am going to write a book on transducers or maybe just a really long post.
Please do and we could add it to the library as a reference book for eternity.

https://www.cruisersforum.com/forums/downloads.php
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Old 01-06-2018, 16:26   #19
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Re: Depth Sounders -- Beam Width

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Originally Posted by KeelMe View Post
Fan beams are better for sailboats; they limit the fore and aft beam width and widen the starboard and port beam width. The narrower the beam the more it is affected by heel. The wider beams give you better reflection. For example if you use a transducer with a 7 degree beam you will likely not see any depth at all when you are heeled over. But with a 44 degree beam you will have an accurate reading at most any level of heel.

Lower frequency provide more penetrating power for seeing deeper bottoms. Higher frequency provide more detail but less depth.
NMEA transducers are typically 235 kHz
Analog depth finders transducers are typically 150 -200 kHz (200 kHz is more common)
Standard fishfinder transducers are typically dual beam (like 50/200 kHz)
CHIRP transducers have variable frequency

Tell me what interface you are using and what you are connecting it to and I can recommend a transducer. I have a few decades experience with transducers and the latest Airmar dealer catalog sitting in front of me.

One of these days when I find the energy I am going to write a book on transducers or maybe just a really long post.
Thank you!

I have two choices of interfaces, or maybe three:

1. Straight connection to the N2K network, as per the failed DT800
2. Connection of an analogue, 200khz transducer to my Actisense DTS-2, which outputs 0183 which goes into my plotter which then converts the data to N2K (whew!).
3. Connection to the Sonar connection of my B&G Zeus Touch plotter.

But my main goal is to get simple depth into my network.

I have an Echopilot Gold forward looking sonar which is free standing (and not working at the moment, but I plan to revive it this weekend).
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Old 01-06-2018, 19:08   #20
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Re: Depth Sounders -- Beam Width

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Originally Posted by Dockhead View Post
Thank you!

I have two choices of interfaces, or maybe three:

1. Straight connection to the N2K network, as per the failed DT800
2. Connection of an analogue, 200khz transducer to my Actisense DTS-2, which outputs 0183 which goes into my plotter which then converts the data to N2K (whew!).
3. Connection to the Sonar connection of my B&G Zeus Touch plotter.

But my main goal is to get simple depth into my network.

I have an Echopilot Gold forward looking sonar which is free standing (and not working at the moment, but I plan to revive it this weekend).
Here are my recommendations based upon your three interfaces:

1. The DT800 is a great option. I would buy a tilted element to match the deadrise.
OR
The DST800 with the 44 degree fan shaped beam is excellent. This would also eliminate the need for the DST-2; now you would have Depth and Speed via N2k.

2. A D800B is the analog version of the DT800; minus temperature of course. You would be moving backwards though. I'd be working toward getting rid of the DST-2.

3. This is by far the coolest option. A B117-DT-BL would give you excellent bottom detection; the 50 kHz beam is 45 degrees and the 200 kHz is 12 degrees. Your plotter should output the depth and temp onto the network. I know the zeus2 will, not sure about the touch.
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Old 02-06-2018, 10:21   #21
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Re: Depth Sounders -- Beam Width

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Originally Posted by KeelMe View Post
Here are my recommendations based upon your three interfaces:

1. The DT800 is a great option. I would buy a tilted element to match the deadrise.
OR
The DST800 with the 44 degree fan shaped beam is excellent. This would also eliminate the need for the DST-2; now you would have Depth and Speed via N2k.

2. A D800B is the analog version of the DT800; minus temperature of course. You would be moving backwards though. I'd be working toward getting rid of the DST-2.

3. This is by far the coolest option. A B117-DT-BL would give you excellent bottom detection; the 50 kHz beam is 45 degrees and the 200 kHz is 12 degrees. Your plotter should output the depth and temp onto the network. I know the zeus2 will, not sure about the touch.
OK, thanks very much for that.

I don't want the DST-800, because I get speed from an expensive ultrasonic CS-4500, which is why I have the DST-2. Although I might have to take it anyway because I can buy the DST-800 in the UK, but there doesn't seem to be a DT-800 in 0 degrees and NMEA2000 for immediate delivery anywhere in Europe

I also just got a rude surprise when I found out that my old ST60 depth transducer -- which I thought I would hook up to the DST-2 as a last result -- is 170khz , and thus incompatible with my 200khz DST-2

I would love to have some imaging of the bottom with the sonar function of my Zeus T, but I'm a little leery about depending on that more complex installation for something so basic as depth. OTH, I do have the forward looking sonar for backup depth data . . .. IF I can get THAT to work again . . . Also it's a fairly serious operation to pull the cable to the helm.

So I might be stuck with the D800B . . .
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Old 02-06-2018, 10:49   #22
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Re: Depth Sounders -- Beam Width

Not sure what your wanting, but another way may be by a Raymarine I50 depth display. Itís powered by the NMEA bus and of course also put data on the bus, so you donít have to run your plotter to put depth on the bus, but the plotter can also of course feed data to the bus, so you have redundancy.
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Old 02-06-2018, 11:28   #23
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Re: Depth Sounders -- Beam Width

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... ... but there doesn't seem to be a DT-800 in 0 degrees and NMEA2000 for immediate delivery anywhere in Europe
Is that Airmar Part 44-135-2-02? If so, jgtech.com in Poole seem to have them in stock at £299
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Old 05-06-2018, 02:06   #24
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Re: Depth Sounders -- Beam Width

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Is that Airmar Part 44-135-2-02? If so, jgtech.com in Poole seem to have them in stock at £299
I called John, actually, but he only has a 20 degree one, not a 0 degree one.


So in the event, I gave up on an NMEA 2000 transducer. Bizarrely, you simply can't buy one in Europe -- what must be the most common transducer in the world.

I bought a simple analogue D800, Raymarine branded, off the shelf from my local electronics guy, and spent a couple of hours pulling the cable and getting it hooked up.

I'm afraid it hasn't worked out so well. It works, but the reading jumps around quite a bit, most of the time. I can set a Triton on that depth display which shows 10 minute history, and you can clearly see the real depth in the trend line, but the simple number display is not very steady.

I don't know whether it's a bad unit, or the DST-2 is bad, or whether there is some kind of interference, but there we have it.
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Old 05-06-2018, 03:40   #25
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Re: Depth Sounders -- Beam Width

I have an Advansea depth sounder/instrument and that's all over the place too. Works better in deeper water but that's not really much use. So I've been looking to use one of the Airmar smart transducers.

Just a thought, could you mount a 20deg transducer looking forward? I suspect the beamwidth is greater than 20deg so should work OK and it might be useful looking ahead a bit to pick up sudden changes in depth.
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Old 05-06-2018, 03:47   #26
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Re: Depth Sounders -- Beam Width

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I have an Advansea depth sounder/instrument and that's all over the place too. Works better in deeper water but that's not really much use. So I've been looking to use one of the Airmar smart transducers.

Just a thought, could you mount a 20deg transducer looking forward? I suspect the beamwidth is greater than 20deg so should work OK and it might be useful looking ahead a bit to pick up sudden changes in depth.
AFAIK, the actual sensor in depth transducers is on a kind of gimbal so it always looks straight down. If you prevent it from gimballing straight down (by choosing the wrong tilt, for example), I believe you will get false depth data as the transducer measures DISTANCE, not depth per se. Distance to the bottom and depth are equal only if the sensor is pointed straight down.
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Old 05-06-2018, 03:50   #27
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Re: Depth Sounders -- Beam Width

I am an idiot.

My forward looking sonar is ALSO 200khz. Why in the (*&^%$$ didn't I think of that when I shelled out for a 200khz transducer?

When I disconnect the FLS, the depth transducer seems to work ok.

So I just wasted £130 and a couple hours work

I guess I'll leave this in place as I won't really need the FLS until I get to Iceland. Maybe I'll have a DT800 sent to me from the US, or brought by someone coming over. The DT800 (N2K version) is 235khz.
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Old 05-06-2018, 04:48   #28
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Re: Depth Sounders -- Beam Width

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AFAIK, the actual sensor in depth transducers is on a kind of gimbal so it always looks straight down. If you prevent it from gimballing straight down (by choosing the wrong tilt, for example), I believe you will get false depth data as the transducer measures DISTANCE, not depth per se. Distance to the bottom and depth are equal only if the sensor is pointed straight down.
I don't think transducers have moving parts such as gimbals, nor the means to sense level. I think the signal processing takes the first part of the pulse return that it sees, even if there is a spread of returns as that is the minimum depth/distance. I believe the need for tilt is to make sure the beam is centred when straight and level and still get useful returns when heeled/pitched.

I suppose my suggestion for looking ahead would possibly be problematic when the boat is pitching into heavy seas.

Sorry about your 200kHz saga!
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Old 05-06-2018, 04:55   #29
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Re: Depth Sounders -- Beam Width

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I don't think transducers have moving parts such as gimbals, nor the means to sense level. I think the signal processing takes the first part of the pulse return that it sees, even if there is a spread of returns as that is the minimum depth/distance. I believe the need for tilt is to make sure the beam is centred when straight and level and still get useful returns when heeled/pitched.. .
You may be right! I can only speculate as I don't have actual knowledge, but how would it work without a gimbal? You're heeled over 20 degrees, and your depth sensor has a 12 degree beam -- it wouldn't even see the bottom under the boat at all, if it doesn't gimbal to vertical!

Maybe someone knows for sure?
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Old 05-06-2018, 05:00   #30
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Re: Depth Sounders -- Beam Width

That's why, as has been discussed earlier in this thread, for a mono-hull sailing boat you need a wide beam transducer.
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