Grouning alum hull to battery

[AlienStarcraft]

1990 17' superfisherman. I noticed leaking rivets 5 in a row 3' from the stern, between keel and chine. Drilled out the rivets to replace with closed end 1/4" alum pop rivets (below gas tank no 'easy' access). Found the rivet holes completely eroded away. I've also found some small independent pin holes in the same area. No salt water use. I have zincs on hull and 1986 115 merc. Boat is trailered and stored in dry garage I checked for electrical for faults. Found 4.8 MV between neg. on battery and hull. I isolated it to the ignition circuit. Only when ign. key is on do I get the voltage- I'm testing in garage. Is 4.8 MV enough to cause the rivets to corrode? what else could cause this?. See example photo. Should I ground the hull back to the battery? I tested this and of course there is no more current as I've made a circuit. I've inspected the ign. switch. It is in a plastic housing mounted in a plastic panel and completely sealed with a factory plastic coating. Help!:facepalm:

 

[barato2]

Re: Grouning alum hull to battery

yes, i suspect 4.8 millivolts is enough to get galvanic corrosion going....the current leakage into the water in marinas will give less potential than that, and it's enough to get to corrosion demons at play. and galvanic corrosion is very much what that looks like. NO, i don't think you want to ground your battery to hull under any circumstances. still, galvanic corrosion should only be operative when you have the boat in the water...? and it sounds like this is happening when it's on a trailer in garage?

 

[AlienStarcraft]

Re: Grouning alum hull to battery

I'm measuring the voltage while on the trailer but the damage is most likely happening while in the water with the ignition on. I haven't tested in the water yet because I'm still repairing the damaged rivets and repainting. Don't really want to get wet until I fix the problem.:blue:

 

[elkhunter338]

Re: Grouning alum hull to battery

un hook the + battery lead and see if your voltage goes to zero. I think you will find most hulls are grounded to the battery neg. and and extra ground should not hurt.
Think about it an I/O boat has the battery ground to the engine block that is tied to the outdrive, which is bolted to the boat with bolts.
My fuel gauge sender units are tied to the hull with a ground wire, this looks factory.
If the hull is not tied to the battery you have a floating ground on the battery I would do some serious research on how to ground the hull of alum. boats. What I have said may not be the best advice. I am almost 100% my boat is tied to the battery (will have to verify) and I have no issues.
Does your battery drain and go dead when not in use?

 

[GA_Boater]

Re: Grouning alum hull to battery

When the key is on, what all is energized? Tach for example. Remove the boat side wiring harness from the motor and check the voltage. If it goes away, the trouble is on the motor end, if it's still there, boat side. Are any accessories grounded to the hull instead of a ground buss, which should be isolated from the hull. The hull should not be used as a ground buss.

BTW - Welcome to the Tinny Drydock.

 

[AlienStarcraft]

Re: Grouning alum hull to battery

I've already isolated the fault to the ign. circuit, as I removed all the accessory wires from the battery and had no fault. The tach and voltage meter do turn on with the ignition as I have reconnected all the positive wires back to the batter. I'll try disconnecting all the accessory wires again and see if the tack is eliminated.
elkhunter if I disconnect all + side on the battery there is no fault as there is no power source I do have a batter kill switch which stops the fault. My battery does not discharge because I use the kill switch. I'll try removing the lead to the coil relay, then the coil and see if that stops the fault. All accessories have been eliminated from the cause- but all are properly grounded back to the battery.

"rotting away one alum. molecule at a time."

 

[barato2]

Re: Grouning alum hull to battery

replacement ignition switches aren't all that expensive, and you live in a climate that seems ideal for moisture to get inside switch housings and cause havoc. just theorizing here, but would this be worth trying? i think the ign switch i recently purchased was <$30.

 

[elkhunter338]

Re: Grouning alum hull to battery

As stated don't use the hull as a known return. Make sure you have +/- wires returning to the fuse block. Fuse block should have +/- wires back to the battery. My boat hull has zero ohms to the battery -, outdrive is zero ohms to the boat. So all is tied together. But I do not use the boat as the return path.

 

[AlienStarcraft]

Re: Grouning alum hull to battery

As stated don't use the hull as a known return. Make sure you have +/- wires returning to the fuse block. Fuse block should have +/- wires back to the battery. My boat hull has zero ohms to the battery -, outdrive is zero ohms to the boat. So all is tied together. But I do not use the boat as the return path.

Should I be measuring ohms not voltage between the hull to batt neg.? Also i checked for voltage between the motor and the hull- nothing
Thank Elk hunter

 

[Grandad]

Re: Grouning alum hull to battery

I'm puzzled by the relatively extreme corrosion damage in isolated locations, especially on a trailered boat. I'm not a corrosion expert, but I would expect that corrosion would be less concentrated in a given spot and more relatively minor over a wider area/more rivets. I have some degradation/thinning of rivet heads below the waterline, though none of the 40 year old rivets have totally disintegrated (haven't lost any yet). Could your corroded rivets have been also more exposed to abrasion that might have kept their surfaces free of a protective oxide layer? Perhaps they were the only ones without a protective paint coating, so electrical activity was concentrated on them.

In regard to bonding the hull to the negative, I have done so on mine. As others have mentioned, it's almost impossible to avoid "grounding" the hull at some point via your engine because it is already bonded to the negative. In my experience as an electrician, I've seen more damage to unrelated components created by poor connections than by solid connections between them.

With a hull that's isolated from the negative, a meter should show zero volts and infinite ohms between the negative and hull.
With a hull that's bonded to the negative, a meter should show zero volts and zero ohms between the negative and the hull.
- Grandad

 

[heyyou325]

Re: Grouning alum hull to battery

I'm sort of with Grandad on this one. I have a 61 Crestliner that has been grounded to the hull since before I had it, and only one owner before me, he's dead so I can't ask him when it was done. No problems. It's an outboard. I also have a 74 Starcraft holiday 21 ft , and the engine has always been grounded to the hull. Had a few rivets leaking when I got it, but not due to corrosion. I've checked on a friends 18 ft starcraft and it is also grounded to the hull, so is another friends 21 ft alumaweld. Can't understand why this should be a problem.

 

[GA_Boater]

Re: Grouning alum hull to battery

Hull grounding and using the hull as ground causes current flow through the hull. Current flow is what causes galvanic corrosion. Yes, the motor ground and battery ground are common to the hull because it can't be avoided short of using plastic nuts and bolts. But the motor is grounded directly to the battery and any electrical should be directly grounded to the battery.

Trailering a boat does not stop galvanic corrosion, it just takes longer. Add salt (going by the presumption that Portland is located by the sea and zincs on the hull) with improper grounding, you see the result. When the boat is in salt water, it's being eaten away.

 

[AlienStarcraft]

Re: Grouning alum hull to battery

Thanks all for the help every one. I've traced the problem to the instrument panel. When the harness is disconnected, no voltage between hull to neg batt. Now to isolate the bad parts. either gas, tach. or voltage meter. I'll run a temporary new neg. wire the battery and should do the trick in finding the culprit gauge. I'm thinking that grounding the hull to the neg. battery ( bonding the proper term) will possibly create more opportunity for galvanic action but may stop the electrolysis. I will finish repainting the hull which will help send any stray electrical to the zinc. Grandad, thanks for the specifics on what voltage readings to expect. I believe I will try to get the hull isolated from stray currents as the best strategy.
Wish I could spell "grounding" right in the post title. :facepalm:

 

[GA_Boater]

Re: Grouning alum hull to battery

Electrolysis and galvanic action are the same thing. Salt is usually the cause. Has the boat been used in salt water? Are the zincs on bare metal or painted metal?

Don't run a new groun (sic)! You will mask the problem. Leave it the way it is and start disconnecting - It's the only way you will find it. Don't forget the fuel gauge sender.

 

[AlienStarcraft]

Re: Grouning alum hull to battery

Having done quite a bit of reading there seems to be a difference between electrolysis and galvanic action. I wanted to make sure I understood the cause of alum eroding away. What appears happening to me is electrolysis. See

There is a Difference.

Better than me repeating it. I now know zinc needs to be wire brushed to keep oxidization off if not stored in the water. I traced my electrical problem to a shorting ground wire and ran a replacement.Thanks all.
Alien- as in starcraft.

 

[barato2]

Re: Grouning alum hull to battery

hey thanks! your whole issue has helped vindicate my multiple-ground fetish...