Electrically Isolated Prop

[Maine Sail]

You do not "need" to ground anything. On the 12 volt system, the negative on the battery is the ground. On the 110 volt system, the ground wire on the dock wiring is the ground. If you have a generator, the generator is the ground.

Unfortunately this is easier said than done as most all boats are shipped with a "ground return" system where the engine is the main ground point and the batteries are connected to the engines ground point. To convert to an "insulated return" ground system you will need to do some rewiring of the engine. Things like alts, starters, oil senders etc. all generally ground to the motor and if it is not "grounded" back to the battery you could have an incomplete circuit. To convert to and insulated return system you would need a specially designed & internally isolated alternator, starter, temp and oil sending units that all have dedicated and isolated separate ground circuits.

 

[Prime Time]

2 options for Grounding 12 V DC systems

I agree with Maine Sail. There are 2 different ways to go in terms of grounding the DC system. To the battery itself or to the ocean. There is no real need to ground to the ocean for the electrical system to work. It appears that most boats are grounded to the engine and isolating the engine would entail major rewiring of the electical system.

I think it is potentially dangerous to ground the AC circuit to the same ground as your DC circuit. It is very important to have a GFI in your AC system close to where the AC enters your boat to assure that the shore ground is functioning.

 

[Franklin]

The coupling seperates the two metals for a reason as I stated before. Look at it this way. As electricity leaves metal in seawater, it takes metal with it. The electricity will go out the metal with the least resistance, like the zinc on your shaft. The zinc though is designed to handle the very little trickle that is generated by having bronze and Stainless Steel touching each other in saltwater. It is not designed to handle any stray current from inside the boat. Most boat owners only put on zinc on (should but two) and rarely dive below to inspect.

If you have a 110 volt current leak into the ground, then that will eat the zinc in no more then a month at best but usually in a matter of days. So that would mean that for the next two weeks the prop will be eaten. You will be out motoring and then your prop will fall off.

You can get current from the dock in the ground but just let the other boats who ground it deal with the stray current. The stray current will not hurt you or the boat as long as you don't touch the stray current and a ground at the same time.

Even the best wired docks still have a little current in the 110 volt ground wire with all those isolators on the boats pushing current out at 2 volts.

Another common problem is people treating a boat like a car. On a car, the ground (the negative on battery and alternator) is connected to the engine which is in turn connected to the frame, so just about all metal on the car is a ground. So all they have to do is run a positive wire to stuff and then just the ground to the closest metal. On boats that's not a good idea. On boats, the ground should be an enclosed system. It's best to run a ground wire for each positive wire with the same guage. Using bus bars for grounds is ok but make sure it's a boat bus bar and not a house or car bus bar. The difference is that a boat bus bar does not touch the metal with what it bolts to, keeping the system safe.


So, summary, you don't want any electrical system connected to any metal exposed to saltwater as it may eat that metal and that could spell disaster. BUT, it is wise to run a wire from the mast to the keel bolt so that if you do get struck by lighting, it will flow through the boat without blowing a big hole in it.

 

[Bill Roosa]

is it any wonder we have corrosion problems at the dock

I think that you will find that there are some really good safety reasons why you should:
a) connect all the grounds to a common point (I think keel due to the lightning threat)
b) Do not cross connect the DC ground, AC ground, Radio ground, or lightning ground systems except at the common ground. That is to say don't count on the lightning ground to handle the DC current for the mast lights or use the AC ground wire to return DC current from a load. All DC returns must be independant of each other. The ground wires for lightning only connect to the system at the common ground, all AC grounds are seperate from DC and lightning grounds except at the common ground point. The radio ground should be isolated by capacators and the engine/DC ground should be "isolated" from the lightning by an inductor.
c) Maintain your AC ground at least bi-annually.
d) Insist that your shore power has a proper ground to the shore. This is a HUGE safety issue when the ground to the shore (dock side) is not working as it should. If your shore power ground is insufficient the only safety you have is your common ground to the sea. The only safety a swimmer has is the AC shore ground. There is also the issue of a genset or inverter having a "floating" ground which can get you into some shocking situations when boarding the boat from a dingy.

Grounds are a headach for sure but the safety issue is one that we must address as a community. If the guy in the next slip over has an electrical leak we all suffer.

 

[Hermit Scott]

You can still have an isolated DC system that includes the engine; that is isolated from any other ground other than the negative on the battery. The dc system will not run out of the boat but will run through the engine. I have never even heard of alternators or starters that don't use the frame/housing of the component as a ground. Even alternators that have a wire for ground, if you take it apart uses the frame as ground also. (I'm sure they may exist somewhere).

The DC is going to be isolated. The mast will be grounded to the keel. I Am not sure what to do with the AC though.

 

[Stu Jackson]

From Calder's Boatowner's Manual

These are diagrams for wiring AC (alternating current) on a boat. The neutral and ground are tied together on the DOCK side, never on the boat. There is another chapter on lightning protection and galvanic corrosion covering galvanic isolators and bonding straps. You appear to be asking about DC grounds (battery to engine), AC grounds (see thumbnail) and lightning protection grounding all in one question starting out with your flexible coupling.

A Catalina 30 most likely has Marelon thru hull seacocks, so the thru hulls have no reason to be bonded together. It appears that your concern, Hermit, is how to tie the three items mntioned above all together. Bill's and Maine Sail's comments are appropriate.

I recommend that you consult some reputable technical sources in addition to the responses on this thread, because, I for one, wouldn't ever dream of telling you what to do with the circuits on your boat (except for charging systems and 1-2-B switches:dance.

You mentioned you consulted a book, but it didn't answer your question. What book?

 

[Bill Roosa]

Hermit, consider the following

a lightning strike hits your mast at the radio antenna. The bolt travels down your radio antenna coax into the radio and from there to the DC electrical system. The entire boat is now energized and only God knows where it will find a ground. Hopefully not through you or your crew. The common thinking is that since the engine is such a large piece of metal that the lightning will be encouraged to go through it due to the massive induced voltages that occur when that much amperage starts moving around. Even if the lightning travels down the lightning circuit to the keel there will still be massive voltages induced in all the wires that are running close to parallel to the lightning. Do you really want to have thousands of volts grounded into your batteries?
or
You fire up the genset while at anchor and one of the AC devices shorts. Without a ground to the sea you have "hot" outlets or the box that is shorting is hot because there is nowhere for the juice to go.

Grounds are safety devices. Yes the DC system requires a ground and it can be a floating one but you are accepting risk in not providing a circuit to the sea.

For the record, I am kinda concerned at the direction this thread is taking. There seems to be a focus on protecting the equipment from damage when the real purpose is protecting the people in the boat, at the dock and swimming near the boat.

 

[Hermit Scott]

Don Casey Sailboat electrics simplified. The edition is about 10 years old.

 

[Bill Roosa]

An excelent primer on grounding

Provided by west marine of all places

http://www.westmarine.com/webapp/wc...sor/10001/-1/10001/MarineGroundingSystems.htm

 

[Hermit Scott]

The book says tie them all together and run them to a grounding plate, leaving the prop isolated using a flexible coupling and leaving all metal through hulls and anyother metal going through the hull isolated. I wanted to know if you could use a keel as a grounding plate. The book says tying the mast to the keel is ok(for lightning protection) because a lightning bolt that travelled that far will not be stopped by some bottom paint. But if the keel is encapsualted to have a grounding plate for all grounds; lightening, DC, AC. It talks about using capacitors to isolate things and the RF circuit.
So to be clear, the engine is grounded directly to the grounding plate, the mast is grounded directly to the grounding plate and the AC is grounded directly to the grounding plate. No other metal that goes through the hull is tied to this grounding plate.
It says all stays and chain plates should have seperate grounding wires running to the grounding plate for the reason Bill mentions above.

 

[Franklin]

The DC is going to be isolated.

great.

The mast will be grounded to the keel.

Great

I Am not sure what to do with the AC though.

Leave it just the way the builder built it. AC uses the ground from the dock. Don't do anything unless previous owner messed with it. You're whole AC system should be grounded to a "Green" wire which goes back out the boat via the power cord. The marina grounds that wire....and of course, all the other boats who ground it to the keel or engine, will also ground it for you.

 

[Franklin]

sorry Bill

I think that you will find that there are some really good safety reasons why you should:
a) connect all the grounds to a common point (I think keel due to the lightning threat)
b) Do not cross connect the DC ground, AC ground, Radio ground, or lightning ground systems except at the common ground. That is to say don't count on the lightning ground to handle the DC current for the mast lights or use the AC ground wire to return DC current from a load. All DC returns must be independant of each other. The ground wires for lightning only connect to the system at the common ground, all AC grounds are seperate from DC and lightning grounds except at the common ground point. The radio ground should be isolated by capacators and the engine/DC ground should be "isolated" from the lightning by an inductor.
c) Maintain your AC ground at least bi-annually.
d) Insist that your shore power has a proper ground to the shore. This is a HUGE safety issue when the ground to the shore (dock side) is not working as it should. If your shore power ground is insufficient the only safety you have is your common ground to the sea. The only safety a swimmer has is the AC shore ground. There is also the issue of a genset or inverter having a "floating" ground which can get you into some shocking situations when boarding the boat from a dingy.

Grounds are a headach for sure but the safety issue is one that we must address as a community. If the guy in the next slip over has an electrical leak we all suffer.

but you got it WRONG! If you run the AC green wire to the keel bolt, and the DC negative to the keel boat, and the mast to the keel bolt, all three are one complete cuircit which you don't want to do.

As a matter of fact, you don't want ANY electricity flowing to any metal underwater UNLESS it prevents a hole being blown in your hull and that's only a lighting strike. Any other grounding there will lead to corrosion at those metals.

 

[Hermit Scott]

The AC on my boat has been completely redone including the panel. There currently is no ground to the sea only through the shore power cable.

 

[Franklin]

You fire up the genset while at anchor and one of the AC devices shorts. Without a ground to the sea you have "hot" outlets or the box that is shorting is hot because there is nowhere for the juice to go.

It goes back to the genset. Electricity always goes back to the creator. Take the electricity you get from your local provider. They don't provide a return circuit, so, by grounding it, the return circuit is the ground. It will eventually make it back to that big generator that created the current. Now with a generator, you don't need to run it into ground to return it because the source is on the boat.

Grounds are safety devices. Yes the DC system requires a ground and it can be a floating one but you are accepting risk in not providing a circuit to the sea.

SURE!!!! so I guess we are all at risk in our cars. Nothing there is grounded to anything other then the source...battery and alternator.

 

[Franklin]

The AC on my boat has been completely redone including the panel. There currently is no ground to the sea only through the shore power cable.

exactly the way it's supposed to be. As you said, the previous owner was a good electrician.

 

[Joseph Shirley]

My shaft & prop are grounded and bonded to the rest of my ground system by the use of a shaft brush. It is composed of a bronze contact bonded to a stainless steel strap that is spring loaded to keep the contact against the shaft as the shaft rotates. The SS strap is connected with a wire to the engine. I believe it's the best and easiest solution.

Here is a source

www.fisheriessupply.com/online/sear...partial&Ntx=mode+matchallpartial+rel+Inactive

This one has a graphite contact and a bronze strap but works the same

Good luck


Joe S

 

[Stu Jackson]

Bill, thanks for that great grounding article

One of the best I've ever seen. Much appreciated, and very pertinent to Hermit's issues and questions. And reflects positively on your concern about the direction this post was taking. Seems almost pretty straightforward now.

Only thing I haven't seen is a discussion about isolated shafts, like in Hermit's original question. From reading the material, I would suggest that the zinc on the shaft should protect the prop vs. the shaft and the engine takes the DC grounds.

His other question was about A.C. grounds and this, too, has been included in the material.

Great thread. Hermit, thanks for brining up this important subject.

 

[Hermit Scott]

The article was very relevant. However I am still not clear why the motor should be grounded to the keel and then have a capacitor to block DC. DC is the only type of current running through the motor block.
Why not do what Frankilin recomends and not connect the DC system to the water?

Also on a somewhat related subject. I had a prop shaft made at a local machine shop I regularly use, but they don't make shafts for boats. They just did it for me because they know me.
It really turned out to be a pain. The shaft is a little too long. There is about 2" between the back of the cutlass bearing and the prop. This makes it possible for me to put a zinc in that place but too avoid prop instability, Maine Sail recommended having this space less than an inch. Where do you guys put your zincs in relation to the prop?

 

[Franklin]

Maine Sail recommended having this space less than an inch. Where do you guys put your zincs in relation to the prop?

I agree with Main Sail, the more the space, the more leverage the prop has to wear out the bearing. Mine is about 1/2 an inch and I put two zincs right behind the bearing. I like the idea Bennys have with bolting the zinc on to the end of the shaft, but you can only do one so I would still put another on the shaft.

I would say that because the boat's previous owner was an electrician, I will stick with what he did verses a bunch of icons on a web page. I got a lot of my information from an electrician. I never knew how much I didn't know about something as simple as a ground until I talked to him and he explained how grounds work in the marine and land environment.

I used to drop a wire zinc over the size of the boat that was tied into the boat's powerchord plug to ground my 110 system. He explained to me that even though it is tied in directly to the green wire and also touching the bottom, it still wasn't much of a ground because of the resistance. He said if the dock didn't have it's wires grounded then it would eventually provide an escape of the current in the green wire, but it would be a slow drain. Because of that, I still keep that wire for when I go to a marina I don't trust, but I know my marina's ground is good so I don't use it anymore.

So if you don't trust the marina's ground, then splice in a zinc wire into the green wire right where the powercord plugs into the boat and then throw that wire overboard. That will give you the safety you are looking for without putting your prop, thru hulls and keel bolts at risk. Those should be the last thing you want current going through.

I hope this helps some. I've given all the information I could. If you still don't believe me, then I can't help you then as I've tried my best.

BTW: Just because some books says it's to be done a certain way doesn't make it the best way and I bet there are almost always conflicting books out there. You just have to weigh the source and be open minded about the information. Me, I'll take an electrician and a boat maker who's made tens of thousands of boats over a period of 30 years over some guys who wrote a book and worked on a few of them.

 

[Hermit Scott]

Franklin, I agree with everything you say, except maybe about the generator. I have never really understood the importance of the default ground in a house, unless there is a GFCI. I built the apartment I live in in the loft of my shop. I ran the electric and ran all the default grounds together but never got around to actually grounding it to the I-beam superstructure of the building. Maybe if I understood the importance in a house I could understand the importance in a boat.