Water ground for livescope

[retired chem teacher]

Installing a Garmin livescope on a Lund pro v bass.  Instructions say to ground to a water ground. Not real sure what a water ground is and what it does???   Or whether it is necessary???   Information I read is to attach a wire to the negative terminal on the engine which to me is the negative terminal of the starting battery???  I am using a separate/isolated battery to power the livescope and other electronics. Do I still attach this to the negative terminal of the motor/starting battery or elsewhere???

Any help would be appreciated as I'm "Lost in electronics "

[fishinwrench]

It is merely a jumper wire which creates continuity from the ground of your FFS unit to the negative terminal of your engine cranking battery.   

And NOT the trolling motor ground. 

Isolating those two ground circuits helps to eliminate feedback noise (distortion) when operating the trolling motor. 

[bfishn]

I know little about Livescope, but the "water ground" instruction is valid for trolling motors and associated electronics. It simply means connecting to your starting battery ground, as your outboard case is "ground" just like a car. When the big motor is in the water, that's a water ground. If you plan to 'scope with the motor up, you might need to go further if the 'scope requires it. 

The main reason that I know of for this is electrolysis. If there's even a slight potential difference between the big motor ground and the TM ground, current will flow thru the water between the two, and whichever is relatively positive will sacrifice surface material... not good. I always ran a jumper from my 24V TM ground to my starting battery ground and never had any issues. 

[bfishn]

Wrench has a lot more recent experience than I. Listen to him.

[fishinwrench]

14 minutes ago, bfishn said:

I always ran a jumper from my 24V TM ground to my starting battery ground and never had any issues.

It is best NOT TO.  

Keeping those two circuits as isolated from each other as humanly possible will result in a better overall system. 

Almost impossible on an aluminum boat.....but not entirely impossible..... A rubber inner tube gasket between the trolling motor and the boat hull, and using MOTORGUIDE mounting hardware can accomplish the task. 

[retired chem teacher]

2 hours ago, fishinwrench said:

It is merely a jumper wire which creates continuity from the ground of your FFS unit to the negative terminal of your engine cranking battery.   

And NOT the trolling motor ground. 

Isolating those two ground circuits helps to eliminate feedback noise (distortion) when operating the trolling motor. 

So if I am powering live scope unit with a separate battery I would still ground unit to the engine cranking battery?

[fishinwrench]

2 hours ago, retired chem teacher said:

So if I am powering live scope unit with a separate battery I would still ground unit to the engine cranking battery?

You would ALSO ground the unit to the cranking battery.   

You are not depending on the cranking battery to supply the ground connection for the FFS circuit.    You are merely connecting the negative (-) side of the FFS circuitry to the engine..... which is in contact with the water at all times.  

They could have easily included a ground into the transducer harness, to serve the same function, but they didn't.....or haven't YET. (probably in an attempt to eliminate/lessen RF noise distortion from the trolling motor) 

[jdmidwest]

23 hours ago, fishinwrench said:

It is best NOT TO.  

Keeping those two circuits as isolated from each other as humanly possible will result in a better overall system. 

Almost impossible on an aluminum boat.....but not entirely impossible..... A rubber inner tube gasket between the trolling motor and the boat hull, and using MOTORGUIDE mounting hardware can accomplish the task. 

I always thought the rubber wings on those mounts were for vibration reduction.  Learned something.

They always tear out after a while on the jet boat and I have sandwiched them to the aluminum underside with a big fender washer and lock nut grounding the trolling motor to the boat I guess.  Will have to look into that when it warms up and put a rubber gasket under that fender washer.  Need to find a rubber washer to do the trick.

[fishinwrench]

1 hour ago, jdmidwest said:

I always thought the rubber wings on those mounts were for vibration reduction.  Learned something.

They always tear out after a while on the jet boat and I have sandwiched them to the aluminum underside with a big fender washer and lock nut grounding the trolling motor to the boat I guess.  Will have to look into that when it warms up and put a rubber gasket under that fender washer.  Need to find a rubber washer to do the trick.

Actually they were originally designed to operate like a drywall anchor, to fasten the unit when you can't get to the underside for attaching a washer & nut.   

That was fine back before trolling motors weighed 200 pounds 🙄

Isolating from grounding to the hull wasn't a THING back then. 😅   And still isn't....unless you're a new-age punk who's all concerned about RF interference on your Forward Facing Sonar unit.  🙄

[jdmidwest]

11 hours ago, fishinwrench said:

Actually they were originally designed to operate like a drywall anchor, to fasten the unit when you can't get to the underside for attaching a washer & nut.   

That was fine back before trolling motors weighed 200 pounds 🙄

😅 
🙄

Or, when you use them on the river and hit rocks, trees, and shoals all of time.  I have had to hold onto foot pedal to keep all from going overboard.  That rubber goes bad pretty quick and loses grip, hole gets wallered out, then all lets go.