I recently read where someone was suggesting that the battery should be directly connected (grounded) to an aluminum boat at the negative terminal. This is susposed to reduce the chances of electrolisis particularly in salt water. My understanding was that this was for the cranking and trolling batterys. Does anyone know if this is good advice? Thanks.
I expect your transom is also aluminum? The frame of an outboard is connected to battery negative, so when you clamp it on the transom, it's electrically connected to the boat as well.
Unless there's insulators in there somewhere, anyone?
You can test with a dc voltmeter. Place a lead on battery - and the other on the boat. I expect you'll read 0.0 volts.
Or, disconnect the + battery lead and test with an ohmmeter from battery - to the boat. it should show 0Ω.
Either method will tend to prove a connection between - and the boat. You could still add a wire but it'll be redundant.
I agree on the engine clamp bracket bringing the boat to the engine's potential. Besides, electrolysis would be between the engine and boat, not battery and boat.
Take my Merc for example. There is a zinc electrode on the clamp bracket (boat potential), and another on the lower unit just above the antivent plate (engine potential).
I realize that the batt - term connects to the engine block/frame which is connected to both zinc electrodes......or should be through the tilt bracket which is usually greased pretty well. But the clamp bracket is at the boat potential and any eddy currents would sacrifice them.........unless you have a peculiar setup where the engine clamps onto a wooden surface (non-slip) on the transom and the clamps are also on wood (or rubber insulated circular mounts) which would render the above useless and in that case, a separate connection to the boat would not be redundant, would be a necessity.
Never, ever ground the battery to the hull. That is exactly backwards.
If you ground the battery to an aluminum hull and put it in saltwater, you might be able to hear it corrode.
1730, I'm curious what you're basing your statement on?
Electrolysis takes place when there's 2 dissimilar metals in an electrolyte, like water, (with or without salt). Current flows from least noble (e.g. zinc) to more noble (e.g. aluminum).
In doing so, it creates oxygen on the zinc (-), which mixes with, and slowly consumes it. It also creates hydrogen on the aluminum, (+), protecting it.
In the case of an aluminum outboard and an aluminum boat, the metals are similar, so virtually no electrolysis takes place between them. (A ss prop is another matter, ss is far more noble than aluminum).
And, as mentioned, the outboard connected to battery - is clamped to the aluminum transom. So, electrically, the hull is connected to battery -.
Wire 2. Great response except for the SS prop. Yea verily the SS is more noble, but they both are in the same materials group listing (Group II which includes Alum alloys and SS) so there is no "dissimilar metal" situation to enhance corrosion as there would be between different metal groups like between magnesium (Group I) and copper (Group IV).
On your comment 1730V, every metal boat that has the engine clamped directly to the hull has the (opportunity for.....if contact points are clean, bright and tight) battery - tied to the hull if the battery is used for engine starting.
Yea verily the SS is more noble, but they both are in the same materials group listing (Group II which includes Alum alloys and SS) so there is no "dissimilar metal" situation to enhance corrosion as there would be between different metal groups like between magnesium (Group I) and copper (Group IV).
Ah, yes, listed in the same group, but definitely at a different potential. That's because the ss contains copper, chromium, nickel, iron, etc.
Have a look at the sacrificial anodes (or the drive itself) on an aluminum propped outdrive. Like new, even after several years.
Now check a drive with a ss prop, you'll see what I mean.
OMC used to coat their SST line of props with black teflon to limit the electrolysis. I plan to powder coat mine before spring.
I tried automotive clearcoat, it won't stay on.
I thought OMC coated their props with teflon because they rusted if you didn't don't know, just speculation). Every one that I saw with the Teflon worn off that sat for any length of time had rust pits in it.
That was the first days of SS props and apparently it took time for the industry to get the recipe right.....right #SS....like 304 or whatever; I think the nickel content was the big difference in whether it rusted or not.
But I couldn't understand the bronze diffuser ring on the Teflon coated props as it was bronze and bronze is a copper alloy, Group IV, of which SS is a member but there must have been some interaction between it and the alum alloy lower unit and that was before they went nuts with the Zinc sacrificial anodes.
My first association was a Johnny 125 back in '72 and I don't remember any Zinc including the trim tab which was alum; unlike Mercs Zinc.
"I recently read where someone was suggesting that the battery should be directly connected (grounded) to an aluminum boat at the negative terminal. This is susposed to reduce the chances of electrolisis particularly in salt water. My understanding was that this was for the cranking and trolling batterys. Does anyone know if this is good advice? Thanks."
This person is being told to add the hull to the ground plane. At least that is the way I understand it. Does that mean ground all the accesories to the hull too? If so, that can be a disaster. I've seen it.
Yes, outboards are directly connected to the hull via the clamp bracket. But outboards are directly connected (electrically-seperately) to the battery and are isolated from the hull-electrically. Many have specific instructions not to use the hull as a ground. The outboard makers want the connection to the battery-period.
Outboards are not the same type of aluminum as hulls. Outboards are a highly defined alloy of. Enough so to make them a dissimilar metal to an aluminum hull. Keeping them seperate (electrically) reduces the chances of galvanic action. Not eliminates-"reduces".
I do not want to experiment with this persons boat.
We can debate all day long as to the science of it. I do know, from years of experience, that keeping electricity out of an aluminum hull reduces galvanic action.
If the boat is moored in a "hot" (defined as stray current in the water) slip, all bets are off.
Understanding how electrolysis & galavanic corrosion happens is the key to avoiding it (or at least controlling it). Boat & motor manufacturers go to great lengths to try and use the nobilty table to their advantage and creating barriers but scratches in paint and seemingly infinite variations in alloy compisitions make the task near impossible - enter the sacraficial anode.
Electrolysis is a simple chemical reaction between 2 dissimmilar alloys, the common thread being a conductive electrolyte. That electrolyte could be anything from saltwater to a simple rain shower. You can't avoid it but you can minimise it by a good washdown after use and then dry storage.
Galvanic corrosion is a chemical process that occurs when an external power source is introduced such as a hot marina or a generators leaking ground connection or an alternator - this is the process that makes your battery create voltage, it is the reason your battery has a finite life. It is also a lot more destructive than electrolysis.
Back to the sacraficial anode - the anode sits very low on the table of nobility, it is screaming to the electrons "pick me pick me!" but if the electrons don't have a clear and easy path to the anode(s) they will pick someone else - such as an alloy rivet or lead sinker sitting in the bilge. So how do you make a superhighway to the anodes - grounding and bonding. Every single piece of electrical equipment on your boat needs it own dedicated ground wire directly to the battery '-', never use the hull as a common ground. Also, the motor, hull and battery need to be bonded exceptionally well.
So to answer your question, yes, the battery needs to be bonded to the hull but as already mentioned, this is already happenning via the fact that the motor is clamped/bolted to the transom. Just ensure it is bonded well.
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Grounding of Battery to Aluminum Boat
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