Depth gauge/transducer issue help

[Captain Bill]

For years I have had a problem with my Raymarine ITC5/DST800 depth indications. Originally my boat came with a depth only transducer connected to a ST60 display which worked fine. I updated my electronics in 2014 and replaced the original depth sounder with the combination mentioned above and displayed on an I70 MFD. When I installed the new transducer I used the same hole in the hull for the new transducer as the original. The depth reading has never been stable. Typically the display will read the true water depth, then count down to zero and back up to the true water depth. This seems to happen at random times in deep or shallow water. Note that I have a catamaran. I also added a depth/fishfinder transducer to the port hull when I updated my electronics in essentially the same position on the hull connected directly to my c97 MFD. That depth reading is absolutely solid and reliable. Both transducers are approximately 2ft (.6m) below the waterline and approximately 2ft above the bottom of the keel so I run both displays with a 2ft offset so that the display reads depth under the keel. The transducers are mounted about 1 ft outside of the keel. I have the ability to record the NMEA 2000 sentences on the bus from the ITC5 and have found that the depth sentence from the ITC5 does not go to zero but to 0.6m with and offset of -0.6m which is displayed as 0 when the problem occurs. The I70 is correctly displaying the sentence so the I70 is not part of the problem. The sentences are displayed below were adjacent to each other in the NMEA 2000 dump:


$AIDPT,1.9,-0.6,,*6B
$AIDPT,0.6,-0.6,,*65



The correct depth is 1.9m. It seemed odd to me that it would fail to exactly the offset depth rather than 0 or some random number. I reviewed all of the documentation and noted that the installation instructions said to place the transducer in front of the keel, but since my boat has low aspect ratio keels I don't really have access to the bottom except very far forward of the keels or very far aft near the engine(s). The position of the original transducer +- 1ft fore aft is really the only place I can put a transducer. Since the port transducer in approximately the same position was not having the problem I started studying the difference between the original transducer, the port hull transducer and the DST800. What I found is that the port transducer is a dual frequency 200/50kHz while the original was a 200kHz and the DST is a 235 kHz. The port transducer has a beam width of 44 degrees at 200kHz and 12 degrees at 50kHz while the DST800 has beam width of 44 degrees and the original transducer had a beam width of 13 degrees. Doing some measurements and a little trig showed that the beam of the DST800 intersected with the keel near the bottom as did the 200kHz beam of the Port hull, but the narrower beam of the original and the 50kHz beam of the port transducer does not. Apparently the c97 uses the 50kHz beam for depth and the 200kHz beam for fish finding. I worked with Raymarine support on this and another problem with my wind transducer for about 2 weeks. I finally came up with the theory that the DST800 was getting interference from a reflection off of the keel and sent that to them with a much longer dump of the NMEA 2000 depth sentences than I presented here. Since then crickets(2 weeks ago).



I am hoping that someone with a bit more knowledge than I have about these devices can tell me if I'm on the right track or chasing a wild goose with my theory. I think I have a solution to my problem but would like another opinion before spending a bunch of money to fix the problem.



Since I want to keep the STW transducer and water temperature and I cannot relocate the transducer to an appropriate spot, I am looking at installing a P79 transducer with a 14 degree beam width shooting through the starboard hull. The hull is solid glass below the waterline so it should not present any issues on installation.



Whoops, I left something out. Both the ITC5 and the DST800 have been replaced without solving the problem.

[cal40john]

Unplug the power to the depthmeter without problems and see if the one with problems now works.

[Captain Bill]

Originally Posted by cal40john

Unplug the power to the depthmeter without problems and see if the one with problems now works.

I've already done that. I can use the I70 and the starboard transducer without even applying power to the c97 and it causes no change in the behavior.

[Kelkara]

I was going to suggest that you double-check that your DST800 is really 235KHz ... The airmar site says that the analog version (the one you would use with an ITC5) is available in both 200KHz and 235KHz versions ... if you have a 200KHz version then the two transducers could be interfering.


However if you have already tested it with power to the other transducer turned off this isn't the problem ... but if it turns out you do have two 200KHz transducers then I'd double check anyway that the power to the other one really is off when you think it is.


As for failing to the depth offset ... I expect that is deliberate, a zero reading on the display looks more like a fault and more likely to grab your attention.

[Captain Bill]

I have disconnected the starboard transducer and it reads --- not 0, but maybe if it gets a properly formatted depth sentence it defaults to 0. This is the kind of information I would have liked to have gotten from Raymarine support if they hadn't stopped responding.



I've even gone so far as to remove the fuse from the c97 so I'm pretty sure there was no power going to the port transducer.

[Kelkara]

Originally Posted by Captain Bill

Since I want to keep the STW transducer and water temperature and I cannot relocate the transducer to an appropriate spot, I am looking at installing a P79 transducer

If you can't solve the problem and do try this route, be aware that the analog P79 is a 200/50KHz unit just like your portside transducer ... the P79s NMEA2000 smart transducer is a 235Khz device.

[Emmalina]

I had so many problems with the i70 and DST800 they are now in the spares drawer ! Have a Raymarine Dragonfly now and epoxied the transducer to the hull been perfect for 4 years now. Don't think its a Praymarine product it works so well LOL

[Captain Bill]

Just an update. I found that Defender still sells some older P79 single frequency transducers (200KHz) that were made for the Raymarine analog instrument line. The new version and the only one currently offered from Raymarine is a dual frequency model that has the same beam width problem as the DST800. I bought one of these older models and got it installed 2 days before Raymarine support finally got back to me and suggested a P79 might solve my problem. A week after the install and several hundred thousand captured readings there is not one spurious reading.



One problem however is that now that I disconnected the depth sounder side of the DST800 the speed and temperature side no longer work at all. The ITC5 led says there are no speed or temperature transducer connected. The wiring diagram I got for the DST800 says there is no nothing in common between the two circuits except the shield ground which is connected to the proper pin on the ITC5. I put this question to Raymarine support, and I'll probably get a response in a few weeks.