Electrically Isolated Prop

[Hermit Scott]

I bought a book on sailboat electrics. There is a section on having the electrical system's ground ran to the engine, that in turn is connected to the prop shaft and into the water.
I have a rubber coupler in between the engine output and the prop shaft. I assume it isolates the engine from the prop shaft electrically, but I haven't tested it yet.
The book says you can have the prop isolated from the engine but then there must be another ground to the sea.
Is it ok to use the keel for the ground? Is there a special type of through hull that is just for this purpose?
Do you guys run your ground through the prop or another path?

I have decided to leave my boat on the hard for the winter like the yankees do. I am going to replace the through hulls and check out the rudder. It seems like a lot of rudders fall off at the worst possible moment.

 

[Don S/V ILLusion]

A flexible coupler, which I assume you are describing, serves a specific purpose unrelated to electrical grounding and does not serve to electrically isolate the prop and shaft. Rather than start a debate on the pros/cons of isolating the prop, please tell us specifically what you have and whether there is continuity between the engine ground and the shaft.

 

[Hermit Scott]

I'm pretty sure it does serve to electrically isolate the shaft. It works fine to correct misalignments, it stops current flow between the engine and shaft and also makes a great paper weight. It seems like it does more than what it is perceived to do.

I don't ask questions to start debates. I just want to know if I need to buy some type of through hull for the electrical system while I am buying the through hulls for the plumbing before I drive 3 hours to the boat yard.

There are different ways to do EVERYTHING, we all don't have to agree all the time. I just want to hear some people's take before I decide how to do it. I have never asked a question expecting people to be angry over it. My first question was what kind of boat is this, with a pic attached. There were paople that thought I was just faking a post about a boat.(I asked that question because I was buying my first boat) Why would they think that? Then I asked what kind of anchor do I need.(I asked that question because I was taking the boat I bought out in the ICW, the first time I had ever been on a sailboat, and I was single handing it for 40 miles). Then I asked if I could take a gun with me while cruising. That one got moved for obvious reasons to me now. That post had like a million threads to it, ended up debating the Bible. All I wanted to know was if I COULD carry. Is there religious implications of running the ground through the prop?

I just want to know what some guys or gals on here prefer and why. I wouldn't have to ask but the book didn't elaborate on the difference of the two electrical grounding methods.

 

[tommays]

If bolts are going through the coupling then i would thing it would be a circut

 

[Don S/V ILLusion]

Hermit
It's easy enough to verify - just check for continuity with a multimeter. If there is none and you want to have a grnd to water, any thruhull (or keel bolt) will work with a #10 wire as there should be essentially no current. The more important issue is why you are concerned given the consequences of installing multiple grounds where none exists and no problem(s) are evident. There are standards related to this issue but they too are generalities and not applicable in every circumstance.
Both for grounding as well as perceived lightning protection, there are mixed opinions and no definitive correct one for every situation. The author of the reference you mentioned probably knows that and intentionally made no definitive statement. I know this doesn't answer your question for which I apologize but to be clear, the answer very much depends on the current configuration as you NEVER want to have multiple grounds on a boat.

 

[Bill Roosa]

But Don?

Isn't that exactly what you have when you start isolating things, an set of non-common grounds.

Hermit
I'd also recommend the keel over a thru-hull or the engine/prop. A lightning strike WILL find a ground and it is best to not try and force it to go through a thru-hull or the prop and engine for that matter. I know there are different opinions out there but a lightning strike is not something you want bouncing around inside your cabin. Yes by grounding your mast you are taking a higher risk of being struck but you are also minimizing the risk to people in the boat. you can always get more equipment, you can't replace people! Take it straight down the mast and out the keel and be done with it.
Also be aware that all grounds are not necessarily capable of passing current. The ground for the radio only has to pass radio freq AC and so can be isolated with capacitors. You can do a similar thing with coils of wire (inductors) to fool a lightning strike to not go through your engine and prop.
West marine has a great discussion on this BTW.

 

[Don S/V ILLusion]

[FONT=&quot]Bill
I may not have been clear. By having multiple grounds, you run the risk of having multiple paths to ground, usually not of the same voltage potential thereby creating a problem where none previously existed. Not sure how else to describe it.
Regardless, on most boats you cannot isolate the DC ground from the underwater metal because the starter motor is bolted to the engine block which in turn connects to the propeller shaft and the water. So without the ground, an internal failure of the battery charger could put 120 or 230 volts AC on your DC ground and into the water.
[/FONT] [FONT=&quot]Although some through hulls (and maybe Hermit's prop and shaft) may appear to be isolated due to sitting in fiberglass and using non-metallic tubing, they quite often are not. For example the raw water cooling inlet for an inboard engine is in fact connected to the engine block by the water in the tubing which may or may not need bonding so the current flows though copper rather than the water and in/out of the through hull, again depending on whether multiple grounds exist.

I'm probably making this more confusing by my inability to describe it more clearly but I hope not...
[/FONT]

 

[Maine Sail]

The ocean water is in your engine via the raw water circuit anyway so by isolating the prop shaft with a flexible coupling you have not 100% electrically disconnected the motor. It is not a wise idea however to rely on just your raw water circuit for ground in case it becomes air bound..

There are ways to ground with a flexible coupling...

 

[Maine Sail]

I know this doesn't answer your question for which I apologize but to be clear, the answer very much depends on the current configuration as you NEVER want to have multiple grounds on a boat.

Don,

Having crawled around the bilges of hundreds of boats I have yet to see many boats with one single point or path of grounding. But most do have "connected" ground points via bonding etc.. Though of course some have multiple paths to gound and no-bonding. This is a bad scenario.

Masts are often grounded to keel bolts and some mast lights ground directly to the mast, the engines are grounds and have positive and negative electrical signals going to them. Many boats also have grounding plates that ground as well as the engines, and keels acting as grounds... In theory it all sounds good but in practice there are usually more than one path to ground on boats which is not the best practice.

It's actually fairly hard to have only one ground point but more often than not, and as it is ok per ABYC standard, I see the engine most often used as the main ground. If you have an engine, it is a ground if you add a grounding plate, it too is a ground as is any ground wire connected to a keel but all should be tied together.

The ABYC allows for all grounding systems to be interconnected:

"11.18.2.6. Combined DC Grounding and
Bonding Systems - The DC grounding conductors
may be combined with the following systems
providing all the requirements with respect to
conductor size are met for each system. (See FIGURE
18, FIGURE 19 and FIGURE 20 )

11.18.2.6.1. Lightning Protection

11.18.2.6.2. Cathodic Bonding

11.18.2.6.3. Static Electricity Grounding"

 

[Don S/V ILLusion]

Don,

Having crawled around the bilges of hundreds of boats I have yet to see many boats with one single point or path of grounding. But most do have "connected" ground points via bonding etc.

Absolutely!
As long as all separate grounds are of the same potential (by virtue of being bonded), the net effect is a single ground albeit at multiple points - quite "common" but also quite redundant and problematic as, if one bond corrodes or breaks, the resultant electrical differential between grounds can cause unintended consequences and is often the single largest contributor of so-called galvanic action or electrolysis on boats. In effect, these supposedly bonded multiple grounds create a battery effect if/when a bond fails and are typically not among the items on most peoples maintenance list.

For this reason, I tend to suggest not having multiple grounds unless the owner is very diligent

 

[kloudie1]

Hey Hermit.. I think you are correct.. the flex coupling does isolate the shaft and prop from the engine.. But you could put an ohm meter across it to be sure.. If it is, I think that the way to fix it is to use some flexible copper braid across the coupling.. from under the head on an aft bolt to under the head of a forward bolt.. it doesn't have to be big, since we are talking microvolts and microamperes.. Are the bolt heads drilled for aircraft safety wire? If so that could be used to connect the halves.. don't worry about balance, as long as the connection is small and close; the speed is low and the distance from the wire to the shaft center (of rotation) is small.. I believe Nigel Calder talks about this in his books. Good luck with it.

 

[Hermit Scott]

It is clear that a keel can be used for lightning protection. I was wondering if a keel could be used for AC/DC grounding? Or do I need a seperate grounding plate?
"No good can come from bonding underwater metal components that could otherwise be isolated" This he says is ok for onboard stray currents but if there are any stray currents in the water, such as a hot marina that the bonding wire completes a circuit from dissimlar metal components and CAUSES galvanic corrosion.
Does anyone use the keel as a ground for AC/DC ground and have the prop shaft isolated?

 

[Rich (P303)]

I'm pretty sure it does serve to electrically isolate the shaft. It works fine to correct misalignments, it stops current flow between the engine and shaft and also makes a great paper weight. It seems like it does more than what it is perceived to do.

I don't ask questions to start debates. I just want to know if I need to buy some type of through hull for the electrical system while I am buying the through hulls for the plumbing before I drive 3 hours to the boat yard.

There are different ways to do EVERYTHING, we all don't have to agree all the time. I just want to hear some people's take before I decide how to do it. I have never asked a question expecting people to be angry over it. My first question was what kind of boat is this, with a pic attached. There were paople that thought I was just faking a post about a boat.(I asked that question because I was buying my first boat) Why would they think that? Then I asked what kind of anchor do I need.(I asked that question because I was taking the boat I bought out in the ICW, the first time I had ever been on a sailboat, and I was single handing it for 40 miles). Then I asked if I could take a gun with me while cruising. That one got moved for obvious reasons to me now. That post had like a million threads to it, ended up debating the Bible. All I wanted to know was if I COULD carry. Is there religious implications of running the ground through the prop?

I just want to know what some guys or gals on here prefer and why. I wouldn't have to ask but the book didn't elaborate on the difference of the two electrical grounding methods.

Yes... there is a religious impact of running a ground through prop and carrying a concealed weapon!

I have the same set-up. My prop is isolated from the engine via shaft saver. I carry. Do I need to ground the prop?

I am serious. Really.

 

[kloudie1]

Man, Scott that can open a big can of those wiggly things ! I think that the current builders codes (ABYC?) call for all that to be connected together .. In the past, that was not the case.. My ’85 Hunter has the DC negative grounded to the keel but the AC green wire ground is not connected to the keel. You can find out if yours is easily by putting your ohmmeter across a keel bolt and the engine block and then across the keel bolt and the “boat” green wire .. of course with the dock cord disconnected. . There is a test where ya put a sensitive AC voltmeter on the green end at the end of your dock cord with the cord disconnected and the other voltmeter lead in the water .. if ya read ANY AC voltage you are probably better off not connecting AC green to the neg batt and lightning ground. My dock has an AC leak to ground (probably a result of our hurricane friends Gustave and Ike blowing up the power system) that measures 0.218 VAC at my slip. So to avoid rapid zinc loss, I left the original Hunter wiring alone and did not connect Green to the batt neg and the keel. I know that installing a galvanic isolator would allow me to connect the green to the other grounds, but I have not decided yet to bite that bullet. I think that the guidance all says that the wires should not be tied together except at one common point; in other words, if connected, the green should run from the AC panel to keel bolt on a separate wire and the bat neg from the panel should do the same and be tied to the same point on the keel to avoid stray currents.. I hope this doesn’t muddy the waters too much !

 

[RAD]

Shaft isolation

OK now I want to chime in
I installed a shaft saver a few years back and it does isolate the shaft couplings!!! and I installed a brush from a ground plate to the shaft and after 2 seasons of finding a whole community of barnacles and what ever all over my shaft,prop and strut (and I do protect the metal with the best and thats another subject) I decided last season to remove the brush and what ya know there was hardly any barnacles,I read in a few books on the subject including Charles Wing that you could isolate the shaft and I also know that Barnacles are attracted to electricity.
I've been meaning to ask my boat neighbor how his boat was when it was hauled cause were in a public marina and there maybe stray current from neighboring boats

 

[Hermit Scott]

Kloudie, running the wires sounds like a good way to bridge the flex coupler. Another way that was told to me is to have brushes that rub the shaft if you want to ground through the shaft.
I live 3 hours away from the boat yard, extra money is reserved for Christmas, so I won't get down there til Jan. So I won't know for certain if the flex coupler is isolating or not. However I am 99% sure it is. In that case now that I know some boats are grounded through the keel, I will ground the mast, the DC(engine), and the AC through the keel. I will leave the prop isolated, as well as all the through hulls isolated.

 

[Franklin]

Definately do not ground it to the keel or any other thru-hull metal.

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.

Connecting the 110 volt ground to the prop will only serve to really mess up your prop if you do need to ground something as the zinc will disappear in no time and then eat that prop. AND...it will ground so slowly due to the very high resistance, that it still doesn't protect you from getting shocked if you happen to touch a ground and the stray voltage.

Notice that I'm not talking about lighting. I am talking about the 12 volt system and the 110 volt system. For lighting, connect the mast to a keel bolt. Do not connect the mast to anything else like the 12 volt ground or 110 volt ground.

 

[Franklin]

If bolts are going through the coupling then i would thing it would be a circut

If it's like mine, it doesn't. The tranny will bolt to the rubber with 4 bolts and the shaft will bolt to the rubber with 4 different bolts. No electricity can go through.

 

[Hermit Scott]

Franklin,
The previous, previous owner of my boat was an electrician. Everything is done to code. Even the water pump wires are brown. Everything crimped, soldered, labeled.
He had the prop isolated. There was no water ground for the AC. I wondered why a guy that was so meticulous with everything else would leave out the water grounds. I guess he agreed with you.
The coupling is like yours; 4 bolts out of the rubber to the tranny, 4 bolts out of the rubber to the coupling, with metal plates on each side.
RAD what do you mean you protect it with the best. If you are talking about a coating do tell. I posted a question about that a month or so ago. My conclusion on that is that I would just leave it bare and do alot of diving to keep it clean. I plan on leaving my boat in the water for at least 2 or 3 years when I put it back in.

 

[kloudie1]

Isolated Prop

Scott, I am not familiar with the shaft saver coupling.. It seems that it would be easy for them to run a braid wire inside the elastomer, connecting the nut blocks inside... before they mold the elastomer.. but I don't know if they do this or not..?? I have seen the shaft brushes talked about but I've not seen a set in practice.. The ones I have seen pictures of looked a little delicate. Interesting how many knowledgable folks disagree on this grounding point !