NASA / Clipper Echosounder - poor depth determination

[kas_1611]

Frustrating problem with the echosounder on our Hunter 23.5 that I would like to pick the community grey matter on to find some solutions.

The "pinger" is mounted to the hull (as per instructions in the manual) and is located just off the centre line about 2.5m from the stern. The cable has been run to the display avoiding interactions with other cables as much as possible to avoid interference.

Initially there were no depth readings, the display just showed "out". With a little fine tuning of the system setup I have been able to obtain depth readings which match what I expect for the state of tide and chart depth so the sensor is reading the bottom properly. The readings are fairly stable but we rarely get "DEPTH" displayed which indicates a reading is accurate so while the readout is good it is not reliable. We still get the occasional "out" showing there is a bad reflection or no signal but these are now less common.

This is while we are sat quietly bobbing around on our mooring.

As soon as we fire up the outboard then all hell breaks loose and the echosounder goes crazy. Instead of reading around 9.5m (which is about right) it jumped to 40m then randomly fluctuated from 40 to 50 to even 60m. This appears to be pure noise from the outboard as we were not motoring (still on the mooring) so can not be turbulence or bubbles. Altering engine revs had zero effect, in that it didn't get better at tickover or worse at high revs.

Switch off the engine and the echosounder swiftly settled back to 9.5m.

Anyone else had similar issues with their echosounder and what did you do to resolve the problem? Any ideas, including those that sound crazy, welcomed.

Cheers and cold beers

Keiron

[tkeithlu]

Sounds like electrical interference, meaning either the cable is not shielded, as in the shielding to the coax is not grounded, or the engine is produced gosh-awful sparking from the ignition. I suggest you approach which ever one seems more likely or easier to fix and see if it solves the problem.

If it's the engine, I would expect to hear bunches of static on the VHF radio.

If it's the shielding, I would expect to be able to create the problem by running an electric drill beside the cable.

[hpeer]

Dissenting view, It should read just fine at rest. There may be 2 problems but solve the first one first.

I’m assuming you have it mounted in the bilge to the hull, not a through hull. Make sure you don’t have something between the transducer and the water that could interfere with the signal. Maybe the bottom paint, or a coating on the inside, or even something in the make up of the hull.

If you can dismount it easily I would first try mounting it in a 5 gallon bucket and hold it over the side in the water. See how it reacts then. If good then crank up the motor, is the transducer effected? Does moving it away from the motor help?

Start with the very simplest install (bucket) to prove the system.

[kas_1611]

We are planing on checking the outboard to see if there is any stray electrical shenanigans going on there. Alternator and Sparkplug cap being the first points of call.

Will check the sensor cable connections and the ground but it is a new cable, factory made so I'd hope all was good but one never knows it might be a Friday 4:30 job.

May try the bucket of water test but it will have to be inside the boat and while the transducer is out of the mount will check the oil and for any gunk or debris. Not much we can do right now about hull paint as the boat is in the water but will also measure and mark the hull for future antifouling next time we pull her out.

I am pondering if we can reroute the sensor cable to avoid the power cables from battery to outboard completely. Currently these run very close to each other at one point but do not physically touch. All insulation is sound and the sensor cable is wrapped in silver (tin) foil, not that I expect that to actually do anything but wrapping the transducer in several layers did improve matters (hey maybe the conspiracy nuts actually are on to something )

Further thoughts and ideas very much welcomed

[tkeithlu]

Hmmm. I like the bottom paint idea, and the test method, if indeed the transducer is glued to the inside of the hull. But then, I've run several (I used to manage two boats that I did not own) doing exactly that, with a thick coat of International Trinidad. But hpeer speaks with wisdom - there seem to be two problems here - one is interference from the motor by some route or other, and the other a weak signal to begin with. If the bucket test doesn't lead to a solution, could it be a bad transducer is causing both? Or, could it be that the transducer to display head connection is corroded?

[kas_1611]

Hopefully it isn't the transducer as that is brand new, out of the box this year and only fitted a month or so ago. However the display unit is older so corrosion on the connection could be a possibility so will add that to the check list.

[JimsCAL]

Two things to consider: location of the transducer, and how it is glued to the hull.

Sounds like you may be near the keel which can cause issues. And a solid glue connection to the hull with no air bubbles is needed. I would try some different locations with the transducer in a plastic bag of water to see which works best. Some people have had good luck with silicone to attach the transducer, but I prefer epoxy.

[Halam]

A spark plug with an R in it has an extra resistor to suppress some of the RF noise. Give that a try.

I have read that NASA are good at helping on the phone if you just give them a call.

[kas_1611]

@JimsCAL - transducer is slightly starboard of the centre line of the boat and we have a centreboard rather than a weighted keel but will check there is nothing (obvious) that might be interferring with the pings. I believe the mounting unit is solidly attached to the hull but that could also be something we need to check. The mounting tube is filled with a special oil so the transducer itself is not physically "glued" to the hull.

@Halam - will check the spark plug for an R and install one of these if we don't have it.

Thanks for the ideas