Grounding to OB motor

[WizardIII]

Hello all, I have searched grounding threads here and have found useful info. I have a descent knowledge of electronics so a lot I already knew.
Here's what I have:
18" fiberglass boat w/1977 Evinrude 85hp. I have replaced the original metal Tempo saddle tanks with Moeller plastic. No problems. Have the senders grounded to neg buss under dash, everything works. I'm going to ground the metal filler tubes to the filler hose brackets on the tanks and then my thought was back to the motor.
The original metal tanks (gauges on the tank) had a very heavy wire (cable) running back both sides to the mounting bracket on the original motor (1960 Starflite 75) for grounding. I ***-u-med the grounding was between the motor and the water. My '77 has the same bolt holes in the same place so i just figured......
But I can't see how this would ground to the water. When I check with my meter I don't have continuity between the mounting bracket and the swivel bracket and also not between the swivel bracket and the motor block.
Any thoughts?
Thanks, Steve

 

[Silvertip]

Re: Grounding to OB motor

Why the concern about grounding to the water??? The engine block is a ground point because the large negative battery cable connects there. The ONLY central ground point on a boat is the NEGATIVE battery terminal. There certainly are other points to ground things (like the negative buss at the helm, the engine block, or a negative buss at the stern) but the end result is they all terminate at the negative battery post.

 

[WizardIII]

Re: Grounding to OB motor

Thanks for the reply.
I'd seen mention of the water being ground although I realize it is through the engine block/gear case. I also understand that this would be tied to the neg battery terminal (which to my mind tells me it's all tied to the water). I was confused about the original grounding from the metal tanks to the engine mounting bracket since there is no continuity between that bracket and the engine block.
It seems there is no use for those 'grounding' cables so I will yank them and simply connect the filler tube, tank flange and sender ground to the neg buss at the helm.
Thanks again and please let me know if I've missed something.

 

[j_martin]

Re: Grounding to OB motor

Thanks for the reply.
I'd seen mention of the water being ground although I realize it is through the engine block/gear case. I also understand that this would be tied to the neg battery terminal (which to my mind tells me it's all tied to the water). I was confused about the original grounding from the metal tanks to the engine mounting bracket since there is no continuity between that bracket and the engine block.
It seems there is no use for those 'grounding' cables so I will yank them and simply connect the filler tube, tank flange and sender ground to the neg buss at the helm.
Thanks again and please let me know if I've missed something.

You missed ESD (Electro Static Discharge) and safety.

Bond all tank and filler elements with reasonably heavy wire (14G or better) to the central grounding point, either the battery negative or engine frame ground. Don't rely on a loaded electrical ground.

 

[Silvertip]

Re: Grounding to OB motor

The ground buss at the helm is NOT the place for these grounds (bonding wires). Do it right.

 

[WizardIII]

Re: Grounding to OB motor

Yes of course, ESD. I guess that's why the original thick wires running back from the old metal tanks to the mounting bracket.
So is bonding the filler tube & tank flange back to the mounting bracket 'sufficient'? Again, there is NOT continuity (bonding) between the mounting bracket and the engine block....???
Doing it right is naturally of prime importance.
Thanks again.

 

[j_martin]

Re: Grounding to OB motor

Yes of course, ESD. I guess that's why the original thick wires running back from the old metal tanks to the mounting bracket.
So is bonding the filler tube & tank flange back to the mounting bracket 'sufficient'? Again, there is NOT continuity (bonding) between the mounting bracket and the engine block....???
Doing it right is naturally of prime importance.
Thanks again.

There should be a bonding strap on the engine. Maybe it's missing. In addition to ESD, it's needed for proper operation of the sacrificial anode(s) and corrosion control.

 

[WizardIII]

Re: Grounding to OB motor

Thank you for that; it makes sense. I will be pouring over the service manual and parts catalog looking for a (possibly) missing bonding strap.
This is a lake boat ( for now) and I didn't think I needed anodes until I hit salt water.....???

 

[j_martin]

Re: Grounding to OB motor

Any polution that makes the water even slightly conductive will cause galvanic current between the steel or stainless parts and the aluminum, corroding the aluminum, unless there's zinc available to corrode. Yes, you need the anode(s) in fresh water also. On that one the trim tab is probably zinc for that purpose.

 

[dingbat]

Re: Grounding to OB motor

Yes, you need the anode(s) in fresh water also. On that one the trim tab is probably zinc for that purpose.

In freshwater you use magnesium anodes.

 

[j_martin]

Re: Grounding to OB motor

In freshwater you use magnesium anodes.

Shoot me a part number for a magnesium sacrificial anode for my XR4.

 

[WizardIII]

Re: Grounding to OB motor

Thanks for the reminder on that dingbat. I knew I had this book marked for a reason:
http://www.boatzincs.com/
j_martin, as far as my 'continuity' issue I have found no bonding strap missing but I woke up thinking that since this motor has been newly painted possibly some metal parts are not making contact.
Does that seem feasible?

 

[j_martin]

Re: Grounding to OB motor

Heck I don't know, I'm a merc V6 man.

It's possible that there is a zinc on the clamp and also on the LU, and then an electrical ground connection would not be needed. I don't know of any application except red neck trailers that rely on a moving joint for electrical continuity. Both unreliable, and any electricity, even static going across a bearing of any kind erodes it.

On the original question, master ground bond is to the engine block on anything.

 

[WizardIII]

Re: Grounding to OB motor

I've seen ground straps on later models but my '77 85hp Evinrude didn't have one or any stock anodes. I'll add one or two and continue my research on the J&E forum.
Thanks for all the replies.