I recommend reading this whole section to try and understand all the principles involved in doing this correctly.However you can click the links to access specific information quickly.
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It is a fact of nature that all metals when immersed in an electrically conductive fluid will react to each other.The least noble metal will corrode away .In order to stop galvanic corrosion we use sacrificial anodes which are usually zinc's or magnesium and magnesium alloy's.Most outboards or outdrives have multiple locations for you to attach anodes so either check with the dealer for placement or invest in a manual.If you have an aluminum or steel boat check with an expert for correct anode protection for the hull and anything else that is in the water.Make sure they know their stuff as it could be real bad to under protect or over protect your steel or aluminum hull.
Stray current corrosion
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Galvanic corrosion is a relatively slow process in general but stray electrical current corrosion can be hundreds of times more damaging.Any current especially DC positive current that can find a path through the bilge water or hull dampness to a metal fitting that is in contact with the outside water will corrode.Check boat wiring condition especially in the bilge area.If suspicious replace it.Make sure all DC devices have an on-off switch that is fitted on the positive or upstream side of the device.So switching the power coming to the device.
Ideally your boat should have a good bonding system and cathodic protection along with a galvanic isolator if you will be connecting to shore power in a marina.
Bonding and Cathodic Protection
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Cathodic protection is the practice of connecting all hardware in contact with the water to an external sacrificial anode(s). This is usually done by creating a common buss where we then connect all the underwater metals from inside the boat. Things usually connected to this buss would include things like through hull fittings, engines,generators, rudder shaft's, and anything else that is in contact with the outside water. That buss is then connected to an external anode via one of the bolts attaching the anode to the outside of the hull. Make sure all connections are good quality. If you have a bonding circuit tie it into the cathodic circuit for extra protection. Extra zinc's can also be fitted onto outdrives and outboard engines along with rudders, skegs and keels.
Bonding is the practice of connecting all major metal components on a boat that may be required to safely conduct electricity to the boats designated ground.
2)Rigging chainplates(for lightning protection) use 6awg marine grade wire when connecting between the chainplates and boats ground.Connect as directly as possible from the chain plate to the boat ground.
3)Deck fuel fills + fuel tanks must be bonded by law.
4)All metal body electrical appliances (Use same size wire as AC neutral or DC positive) to connect the appliances into the bonding circuit.
We also connect to all under water hardware together including our underwater sacrificial anode at a common buss then one wire connecting the common buss into the main bonding circuit( we recommend 10awg or 12awg wire for this).We should use a minimum of 6awg marine grade green wire for connecting the main highway of the circuit together going to the main boat grounding ( usually a metal keel or metal hull of boat or a special grounding plated sized and fitted) For bonding propellers we need to put a flexible strap across the shaft coupler for a better connection.We then need a spring loaded arm with a contact brush to be bolted to the shaft log and holding the contact brush onto the propeller shaft.We also need similar protection for our rudder shafts and logs.Shaft logs are the hardware used to keep the water from pouring in the propeller or rudder shafts.They are usually made of bronze or stainless steel and use a packing material which when compress will seal around the shaft so preventing water coming in.With a propeller shaft we usually want a small amount of water to come in the keep the packing lubricated and cool(unless you have a dripless system).Check with a professional for correct adjustment of the packing on your shafts.Usually for standard flax packing it's somewhere between 2 to 3 drops of water per minute with the engine running and transmission in gear.This will cool and lubricate the packing.
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If you connect to shore power in a marina you should invest in a galvanic isolator.As soon as you plug in you connect yourself to your neighbouring boats through the ground wire in your shore power cord.This creates a galvanic circuit between you and the other boats(not good if your neighbours have bad stray electrical currents on their boats,it will eat up your anodes fast)
The galvanic isolator interrupts low galvanic voltage/current but will connect larger amounts in the case or an appliance short that needs to be grounded.
An Isolating transformer will perform similar function.
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I don't need to tell you that lightning can be dangerous stuff.There are several theories' on how to best deal with or prevent a lightning strike. I have decided to discuss the one I think makes more sense.If lightning is a big problem in your area you might want to study the subject more and make a more informed decision.
I prefer the trusty old lightning rod.It serves as twofold.
1)To bleed electrons from the negatively charged water up through the lightning protection circuit to the lightning rod tip and into the surrounding atmosphere making it less likely for a strike.
2)If you get hit anyway it will provide a first point of contact.We then have to provide a low resistance path to ground ie(water).
For sailboats with aluminum masts
For sailboats with aluminum masts attaching a 1/4'' stainless steel rod shaped to a point with a good connection to the mast will do.The mast will conduct the lightning down.The top of the lightning rod should be 6'' above anything else on the mast and filed to a sharp point.At the base of the mast we have to connect directly as possible to a good ground ie external keel through a keel bolt or attaching to a metal centerboard.The hull of a steel or aluminum boat is an excellent ground.If you have neither of these a metal ground plate will have to be fitted to the outside of the hull bolted through to the inside somewhere close to the base of the mast.
It should be made of corrosion resistant materials such as ( copper,bronze or monel) and no less than 1 ft square.I don't recommend using a porous bronze sintered radio ground plate for this purpose as it will usually be destroyed after one hit.Some of the lightning may try travel down the rigging so we recommend connecting 6awg cable to the base of each chain plate going directly as possible from there to there boats ground.Our bonding circuit is also connected to that ground.
Sailboat with wooden masts
On a sailboat with a wooden mast run 4awg wire from the lightning rod all the way down the mast to the boats main grounding source.
For power boats we recommend having a short mast with a lightning rod attached.If the mast is metal use 4awg wire at the base of the mast to connect through to our grounding source.If you don't have a metal mast you will need to run 4awg wire all the way from the lightning rod to the grounding source.Your engines should also have a 4awg wire going bonding wire going directly to the grounding plate for bonding as well as extra protection against stray lightning current/voltage.
I hope this gives you some idea of the reasons setting your boats electrical/galvanic protection systems up with some prior planning.Make sure all connections you make in these systems are as high a quality as you can possible make.Have fun and remember it would be shocking if something went wrong with your boats bonding system.