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Author Topic:   Gas Tank Ground
fester posted 09-22-2001 05:09 PM ET (US)   Profile for fester   Send Email to fester  
I have a 1978 20 foot Outrage. The ground wire for the gas tank runs from the tank over the transom to a screw that is several inches below the water line. Is this how the tank should be grounded or is there a better way to ground it?
Thanks, Jeff
LarrySherman posted 09-22-2001 09:06 PM ET (US)     Profile for LarrySherman  Send Email to LarrySherman     
Fester, that screw used to go through a silicon bronze ground plate. It appears to be missing, it you do not see it. This is a special type of broze casting, that is very pourus, which creates a large surface area or the eletrical "bond" - thus it is commony called a "Bonding plate."
Jim Bennett posted 09-23-2001 07:58 PM ET (US)     Profile for Jim Bennett  Send Email to Jim Bennett     
I have 73 Outrage 21 with similar bolt,washer, nut assembly below waterline but nothing attached. Have been told that it is grounding assembly also. With my project boat, will be deciding whether to use or replace tank but am curious if that is proper i.e. best place to ground internal fuel tank?
ALAN G posted 09-23-2001 11:49 PM ET (US)     Profile for ALAN G  Send Email to ALAN G     
Larry Sherman is correct. I have a 75 Revenge and the ground plate (circular) has been there since new. ABYC codes normally call for all metal parts on boats to be bonded (electrically connected) to each other and finally to a water ground plate. On larger boats, you'll find everything bonded from aluminum rub rails to engine blocks to fuel tanks. Without going into too much detail, grounding can help prevent galvanic corrosion or stray current corrosion. If you want to know more, pick up a book on boat electric systems!
lhg posted 09-23-2001 11:57 PM ET (US)     Profile for lhg    
The silver dollar size round bronze ground plate on the Classic Whalers with built in gas tanks is for one reason only - grounding the filler on the tank and fuel system into the water.

That way, when you insert the gas nozzle into the tank filler you won't blow yourself up from a static electricity spark.

If you have lost the grounding plate, it should be replaced and reconnected. It has a heavy green wire leading to it from the withdrawl fitting on the tank.

Taylor posted 09-24-2001 02:12 PM ET (US)     Profile for Taylor  Send Email to Taylor     
Alan G mentioned bonding to a ground plate to reduce galvanic action. I believe that this ground plate on the stern is usually made of zinc, which is at one end of the scale in electronegativity, and thus serves as a sacrificial anode, corroding by galvanic action before something more important does. Certainly in larger boats I've seen this plate made of zinc on the transom. On my boat there is a zinc plate on the engine, and the trim tab is zinc, I assume this protects just the outboard in a smaller boat.

Could it be that the zinc's are missing on Fester's Outrage because they are corroded away?

lhg posted 09-24-2001 03:01 PM ET (US)     Profile for lhg    
The gas tank grounding plates on mine are definitely not "zinc", like the outboards use, but are some other porous material. I'll see if I can determine what it is. These must be sold in the boating catalogs, so there must be a description.

Incidentally, even "zincs" for outboard use are now out of date. About 5 years ago Mercury switched to aluminium sacrificial elements, which are 50% more effective. Now there is also a magnesium anode, for use in fresh water only. Magnesium is the "least noble" of the metals, followed by aluminum, then zinc.

bigz posted 09-24-2001 03:06 PM ET (US)     Profile for bigz    
Taylor yes the one on festers '78 20 Outage was zinc, we had the exact same set up on our '77 deep V Outrage.

As you and Allen say primary bonding to reduce galvanic corrison, yes it used by convention a green wire. The grounding of the tank filler was through the black wire attached to the filler neck then to the tank terminal then to the negative terminal of the battery which grounded on the motor.

Big mistake some folks make is to use the wire reinforcement in the filler hose to ground and or stuff the ground wire down the filler hose when installing it, either trick causes the USGC to get very irritable.

lhg posted 09-24-2001 08:41 PM ET (US)     Profile for lhg    
Tom - I have in front of me Whaler's construction diagrams of the 18, 22 & 25 Outrages, showing the gas tank grounding connections. Something must have changed from the filler plate grounding detail you are describing, as it does not go to the battery or engine, but rather, directly into the water.

A wire is connected to one of the fill fitting screws, which then is connected to a tab on the tank where the fuel enters. From there, back to the withdrawl fitting, the tank itself conducts the static current. Then, at the withdrawl fitting, a green wire attached to another tab on the tank goes to the transom exterior, below the water line, terminating the the sintered porous bronze disc fitting. The bronze fitting is essential for proper grounding and to disperse the charge into the water. Just a plain screw into the hull will not do.

Jim Bennett posted 09-24-2001 09:52 PM ET (US)     Profile for Jim Bennett  Send Email to Jim Bennett     
Would appreciate if you could scan that diagram and e-mail to me. Am going to either refit or replace fuel tank in my 73 Outrage 21 and would like to see diagram for proper grounding. Would also appreciate your comments on my question in Repair Forum regarding dismantling bow rail. Thanks in advance.
bigz posted 09-25-2001 07:14 AM ET (US)     Profile for bigz    
Larry thank you, good points made me go back and check my files.

Your right about just the screw Larry! The "bonding disc" had either disintegrated or was ripped off, seemed to be apparent a screw would not suffice as a conductor. Either case a proper conductor should be installed or as shown below the engine can also be utilized.

The "grounding" wire USCG CFR 183.572, each metallic component of the fuel fill system and fuel tank which is in contact with fuel must be statically grounded so the a resistance between the ground and each metallic component of the fuel fill system and fuel tank is less than 100 ohms.
Grounding or bonding may be accomplished by connecting the metallic components electrically by running a wire from one component to the next, and so forth to the boats ground. Grounding can usually be accomplished by connection to the common bonding conductor or the engine negative terminal

I guess even though the "disc" on the '78 Outrage can be classed as a common bonding conductor on our Outrage the tank was also grounded to the negative lug on the engine battery. Maybe the dependence on just an external exposed bonding conductor might have been in question and a secondary (battery negative terminal) conductor was incorporated. The surveyor of the 20 Outrage was not "buying" that the external exposed green wire to the bonding conductor mounted on the transom as a sufficient means for grounding the fuel system. He said the internal black wire to the battery was ok, though I did have to take the stuffed down the rubber fill hose green wire from the filler neck out and properly ground to a screw terminal on the metal filler neck.

fester posted 09-25-2001 10:17 AM ET (US)     Profile for fester  Send Email to fester     
Thanks for all of the input. I spoke to the previous owner of my 1978 20' Outrage and he told me that there used to be a bonding plate on the boat which he removed several years ago and replaced with a screw. The current set up is that the gas tank filler cap has a wire that runs from the cap to a tab on the front part of the tank. On the back portion of the tank there is a second tab from which there is a wire that runs to the screw on the transom. I am going to install a new bonding plate on the transom of the boat.

In thinking about this, I usually fuel the boat when the boat is out of the water on the trailer. Thus, I would think that the bonding plate would have little effect when the boat is out of the water. What I am thinking of doing in addition to installing a new bonding plate is to also run a wire from the tab on the gas tank to the negative terminal of the battery. My intent is to provide some type of ground when the boat is being fueled out of the water. Do you think there are any problems with doing this.

Thanks, Jeff

LarrySherman posted 09-25-2001 10:38 AM ET (US)     Profile for LarrySherman  Send Email to LarrySherman     

That is a big NEGATIVE. Do not attach your bonding system to the negative battery terminal. You will be creating a perfect enviroment for massive electrolisis.


bigz posted 09-25-2001 11:35 AM ET (US)     Profile for bigz    
Excuse me Larry Sherman I don't follow your warning at all! Am I reading Pascoe's dissertation incorrectly? Please point me to the area you feel reflects this discussion on "grounding/bonding". That is the section stating that the approved practice for grounding fuel tanks by the USCG is a problem when using the engine as a the ground, or using the battery which is grounded to the engine in stead of running two wires to the engine ground since the engine ground is already tied into the battery ground.

Would be appreciated

LarrySherman posted 09-25-2001 02:32 PM ET (US)     Profile for LarrySherman  Send Email to LarrySherman     
Here it is. I posted the wrong link, but stand behind my statement. I do agree however, that the path from the engine, to the water to the bonding plate efficvly ties the bonding systems together.


One of the least understood aspects of a boats electrical system, and the most troublesome, is the proper method of grounding. That we often get questions of whether AC or DC electrical equipment should be grounded to the boat's bonding system is illustrative of this point. AC and DC grounding systems are two separate systems, for distinctly different reasons. If you don't understand these systems, you run the distinct risk of creating a disaster. Actually, there are four separate ground systems: DC ground, AC ground, AC grounding (or bond), and the vessel's bonding system. You can add to this lightning and HF radio grounds as well. Do you know the principles of each? Are you sufficiently confused to discourage you from doing your own wiring? I hope so. For unless you understand each thoroughly, you're headed for trouble.

The AC ground and grounding systems are "free floating," meaning that they do not ground on the vessel, but only to shore. The ground, or neutral, is a current carrying conductor, and is the source of many troubles because people do not regard it as such. The grounding, bond or green wire is the "safety" intended to channel current safely to ground in the event of a short circuit. Both of these circuits are capable of conducting current and can be the source of electrolysis when there are system faults with the dock or marina wiring. This is very easy to test for.

There is only one point where the DC side is grounded, and that is at the battery. It, too, is a "free floating" system in which nothing is ever grounded to any metallic part of the vessel, most especially not the bonding system. Just like a car sitting on rubber tires, completely insulated from earth potential, the battery itself provides the negative potential.

The bonding system, also green wire, has nothing to do with electrical systems. Underwater metals are simply wired together to equalize differences in potential of different kinds of metal. Nothing should ever be grounded to the bonding system. Unfortunately, some people don't understand this and use it to ground electrical equipment, occasionally with disastrous results.



bigz posted 09-25-2001 03:42 PM ET (US)     Profile for bigz    
Thank you Larry, appreciated.

Still pondering your statement

creating a perfect environment for massive electrolysis

The negative battery terminal is used as a basis for grounding the electrical system much like an auto which grounds the battery to the frame in this case the outboard engine, the zinc or special alloy anode is used for bonding against both stray electrical current generated by the metal objects in contact with the water and also against lightening strikes.

Since we have acquired a boat which has both an extensive AC and DC system, I have had to brush up considerably on the AC portion as it deals with on the water and in the conjunction with DC circuits. Found the book Boatowner's Mechanical and Electrical Manual by Nigel Calder very useful particularly for wiring multi house batteries and proper use of the AC powered DC battery charger.

LarrySherman posted 09-25-2001 04:13 PM ET (US)     Profile for LarrySherman  Send Email to LarrySherman     

"There is only one point where the DC side is grounded, and that is at the battery. It, too, is a "free floating" system in which nothing is ever grounded to any metallic part of the vessel, most especially not the bonding system. Just like a car sitting on rubber tires, completely insulated from earth potential, the battery itself provides the negative potential.
LarrySherman posted 09-25-2001 04:43 PM ET (US)     Profile for LarrySherman  Send Email to LarrySherman     
I should clarify. when I rewired my boat, I used the article above as a basic guide. I have read Nigel's book, and think he is great, so no argument there.

My take on the article is that a differance in voltage potential may exist normally between the negative pole of your DC system and your bonding system. If you tie these systems directly together, you force them to equalize, which may actually increase your potential for electrolitic reaction.

So, in Jacqueline, the bonding system is compleatly seperate from the DC system.


lhg posted 09-25-2001 06:46 PM ET (US)     Profile for lhg    
I believe LarryS is completely correct, and this coincides with the way Boston Whaler sets up the boats, as I described above. THEY ought to know what they're doing. The gas tank ground is "stand alone" wiring on the Outrages.
fester posted 09-25-2001 07:08 PM ET (US)     Profile for fester  Send Email to fester     
Thanks for all of the valuable information. Your explanation of the grounding of the boat makes perfect sense to me. I really appreciate your input.
jimh posted 09-25-2001 11:53 PM ET (US)     Profile for jimh  Send Email to jimh     
In most outboard boats, I would assume that the negative lead of the battery is made common to the engine block at some point inside the engine cowling. When the lower unit of the engine is in the water, this connects the battery negative to the water via any conductive surface on the engine lower unit immersed in the water. Hopefully,
this immersed conductive surface will include specially installed sacrificial anode materials, often zinc but also magnesium or aluminum.

The green wire bonding the fuel system elements (tank, fill fitting) is nominally a 10-GA AWG conductor which leads over the transom to a bronze circular conductor that is located below the waterline on the transom. As lhg describes, the bronze fitting is about the size of a silver dollar (or slightly larger), and it is cast with a very porous surface to create as much contact area as possible.

The purpose of the fuel bonding system is to keep all the elements of the fuel system at the same potential, and to hold that potential as close to "earth" (or "water") as possible.

The bronze fitting is not intended to be sacrificial at all.

Most Whalers do not have 115-V AC wiring or shore power connections. If you do, you should pay special attention to the stray ground currents which can be induced from poor wiring practice in the marina and on other boats.

You can measure the "potential" (literally, potential means voltage) for galvanic corrosion by using a sensitive voltmeter connected to the engine chassis, i.e., the battery negative, and to a special "most-noble" electrode (generally silver or a salt of silver) that you immerse in the water.

In this measurement, you measure how many volts below the most-noble (least corrosive) electrode (the silver) your system is. The more voltage you measure, the more likely your sacrificial anode will be consumed.

This is what you want to happen: your sacrifical anodes are eaten away first, before you aluminum lower unit! The greater the voltage difference between your engine chassis and the silver electrode, the better your corrosion protection.


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