Electrical Question

This is going to sound like a dumb question but here goes...

My mercury outboard is bolted directly to the aluminum transom, the motor is grounded directly to the battery, so the whole boat is now a ground for the battery. This doesn't seem right to me. Right for a car, but not a boat. Is the motor supposed to be mounted in a way that it is electrically insulated from the boat? I had a red wire short out to the boat seat and almost start a fire. Didn't know that the current would run through the boat back to the battery.

Anyway is this normal? If not, any way to fix it?

last year we was taking the boat out i had just put new pedestals in my tracker pan fish battery got shiffed and touch th battery post looked back as i was pulling the boat out and had a fire coming from the throttle cable as far as me and my buddy could tel this was the only place that was grounded out from th boat to motor replaced cable and moved battery and put it in a box

[quote=birdstrike;364494]This is going to sound like a dumb question but here goes...

My mercury outboard is bolted directly to the aluminum transom, the motor is grounded directly to the battery, so the whole boat is now a ground for the battery. This doesn't seem right to me. Right for a car, but not a boat. Is the motor supposed to be mounted in a way that it is electrically insulated from the boat? I had a red wire short out to the boat seat and almost start a fire. Didn't know that the current would run through the boat back to the battery.

Anyway is this normal? If not, any way to fix it?[/quote]

Great question. I don't know but I would think that any properly installed wires would be isolated via switches and circuit breakers in case of shorts. An odd short in the right place could be a problem I guess.

I don't see why the whole aluminum boat would be a ground? I just don't see the need. Maybe a boat mechanic can chime in....

This was the response I got from Mercury Marine :

"The scenario you described suggests a galvanic isolator. Please see your authorized dealer for assistance with this matter"

Galvanic Isolator, it that the Flux Capacitor's cousin? lol

[QUOTE=birdstrike;364494]Right for a car, but not a boat. Is the motor supposed to be mounted in a way that it is electrically insulated from the boat? I had a red wire short out to the boat seat and almost start a fire.
[/QUOTE]

Motor should be painted, but paint isn;t a great insulator. You could electrically isolate the motor with very thick electrical tape, the 4" wide stuff 3M makes, and the 3M 4200 sealant thats used around the mounting bolt holes and perimeter of interface to keep water out, but you'd probably end up still getting some low impedance paths thru the bond the mounting bolts make. Its not worth the effort. Having the same potential electrically can actually be a good thing when it comes to corrosion. If there is a minute electrical potential difference present between motor and hull, some electron flow could occur and create a small amount of corrosion or serve as a catalyst for crevice corrosion to form (between two similar metals) vice galvanic corrosion (between two dissimilar metals).

That all being said, I can't see any difference between a car and a boat when it comes to hull/chassis being at the 12V Return potential. If the wire shorted, it should have popped a breaker or fuse. The circuit overload protection is critical for safety and fire prevention. as are properly insulated and safely run wires and connections. Tight wires or wires subject to abrasion from vibration on sharp objects or surfaces are just asking for trouble over the life of the boat.

This was certainly not a stupid question.
Double check your breakers and fuses are properly sized for the "expected" load under normal conditions and try not to worry about it too much.

Thanks everyone. The wire that shorted was not run properly and that seems to be the real issue here. When it started smoking so close to the gas tank, it got my full attention! I can live with the current running through the boat hull, I just didn't expect to see this. I guess glass boats don't have this problem. Thanks again!

it has always been my understanding that you should never use an aluminum boat hull as a ground. Electrolysis will result. All electronics, lights, pumps, etc. should be grounded to the battery. The motor should be isolated via built in motor mounts. Not an expert, just my 2-cents worth

I'm going to check into a rubber mat between boat and motor and rubber grommets for the mounting bolts. Worth a try I guess, and might just do the trick.

[QUOTE=BSeybold;365081]it has always been my understanding that you should never use an aluminum boat hull as a ground. Electrolysis will result. All electronics, lights, pumps, etc. should be grounded to the battery. The motor should be isolated via built in motor mounts. Not an expert, just my 2-cents worth[/QUOTE]

I agree, never use hull as 12Volt Ground. My response was geared toward what might happen if the hull is at the same electrical potential as the ground, certainly not as a dedicated primary path for current to flow from +12 Volt source. always wire directly to battery.

[QUOTE=birdstrike;364494]This is going to sound like a dumb question but here goes...

My mercury outboard is bolted directly to the aluminum transom, the motor is grounded directly to the battery, so the whole boat is now a ground for the battery. This doesn't seem right to me. Right for a car, but not a boat. Is the motor supposed to be mounted in a way that it is electrically insulated from the boat? I had a red wire short out to the boat seat and almost start a fire. Didn't know that the current would run through the boat back to the battery.

Anyway is this normal? If not, any way to fix it?[/QUOTE]

The ground coming from the battery going to the motor goes to the starter. the starter is Isolated from the rest of the motor. Also the charging system on the motor is routed thru the lead depending on size and make whether it is a altonator or magnedo....All the wires are isolated from the motor frame

hmmmm,,,,,have ya noticed fish floating belly up around your boat when your out?

hmmmm? This just keeps getting more interesting. Thanks Jbyrd, that makes sense. I'll try to get to the bottom of it this weekend.